Study on the Structural Styles of Pyay Oil Field-2015

STUDY ON THE STRUCTURAL STYLES OF

PYAY OIL FIELD

MSc (THESIS)

KYAW ZIN OO

DEPARTMENT OF GEOLOGY

UNIVERSITY OF YANGON

MYANMAR

MAY, 2015

UNIVERSITY OF YANGON

DEPARTMENT OF GEOLOGY

MSc (THESIS)

STUDY ON THE STRUCTURAL STYLES OF

PYAY OIL FIELD

KYAW ZIN OO

M2 GEOL – 38

MAY, 2015

i

ABSTRACT

The Pyay oil field, located in the north western part of Pyay Embayment and to

the south of Pyay city, is one of the Tertiary sub-basins of the Central Myanmar Basin.

Generally the area lies 150-450 ft above mean sea-level in a northwest-southeast

trending topographic terrain. The surface geomorphic expression and drainage patterns

correspond to the sub-surface of structural features seen in the 2D/3D seismic sections.

The area is comprised of three major anticline structures, namely, the Namayan

monocline, Pyay North and Pyay South anticlines, which are bounded by a west-

verging major thrust, i.e., the Pyay thrust, in the west flank. The area is

compartmentalized by east-west trending cross faults. All of the three structures are

asymmetrical anticlines with the gentle east-flank and the steep west-flank double

plunging to the north and south, and are separated by two saddles with 346° trending

mean fold axes. Both of Pyay North and Pyay South folds have disharmonic features

within the core of anticlines in the Lower Miocene Pyawbwe Formation. The area was

probably uplifted in the north, started form the northern tip of Pyay South anticline. It

is evident by the facts that some hydrocarbon producing sands (eg. 4,200 ft sand or base

of the Middle Miocene Obogon Formation) of Pyay South anticline are exposed in the

Pyay North anticline, and the whole thickness of Irrawaddy sediments are absent in the

Pyay North anticline. The Pyay thrust is generally shallow in Namayan monocline and

gradually deepen towards the Pyay South anticline. The depth cross-sections clearly

show the geometry of Pyay thrust and stratigraphic sequences. The Pyay thrust is

observed to have bifurcated and terminated in the Pyawbwe Formation in the Pyay

North and Pyay South anticlines. One of the bifurcated thrust sheet is probably

terminated near the formation boundary of the Pyawbwe and Kyaukkok formations.

The other thrust sheet is terminated as nearly vertical in the Pyawbwe Formation with

no significant displacement.

The stereographic projection results and joint strike rose-diagram indicated that

the compressional stress direction is from 255° and the evidences of active deformation

features are observed on the Pyay thrust suggesting an active northeast-southwest

compression, and thrusting is believed to have formed after folding. Time of folding is

accounted for during Plio-Pleistocene period (i.e., during deposition of the Irrawaddy

Formation and the tectonic inversion period in the Central Myanmar Basin), due to the

conglomerate beds in the upper Irrawaddy Formation observed in the Pyay South

ii

anticline. Hydrocarbon occurs mainly in the supra-thrust (east-flank) and the sub-thrust

(west-flank) structures of the two anticlines (Pyay South and Pyay North), where the

Pyawbwe, Kyaukkok, Obogon and Irrawaddy formations are formed. The potential

source rocks are probably derived from the shale and mudstone units of the Upper

Oligocene Okhmintaung Formation and the Lower Miocene Pyawbwe Formation.

Trapping mechanism is commonly observed as the fault bounded closures at the crest

of anticlines, and are sealed by the intraformational shale of the Obogon Formation.

iii

ACKNOWLEDGEMENTS

First and foremost, I would like to express my sincere gratitude to my supervisor

Dr. Day Wa Aung, advisors Dr. Tun Naing Zaw and Dr. Myo Thant, Department of

Geology, University of Yangon, for their guidance and various suggestion on this

thesis.

Besides my supervisor and advisors, I would like to thanks to all members from

the Research Committee of the Department of Geology, University of Yangon, for their

permission to carry out this research.

My sincere thanks to Resource and Environment Myanmar (REM Co. Ltd.) for

gave me a great chance to join the Master Degree in University of Yangon and also

thanks goes to MPRL E&P Pte. Ltd., for offering me internship opportunities in their

new projects.

I am indebted to my many colleagues (Geology & Geophysics team) from

MPRL E&P Pte. Ltd., especially thanks to Dr. Eloi Dolivo and Dr. Aung Zayar Myint

for their valuable discussions, training on subsurface geological interpretations with

Pyay oil field project.

I would like to show my gratitude to Dr. Saw Mu Tha Lay Paw (Ko Jar Muu)

for his technical supporting for field supervising with his own fund and also thanks to

U Thein Win for shearing his regional geological knowledge and supporting field

equipment during this research.

I owe my deepest gratitude to Dr. Kyaw Lin Oo, U Soe Thura Tun and U Than

Htut for their reading and very detailed corrections on all chapters for final defence and

also thanks to Dr. Lin Thu Aung for their valuable discussion on this research.

Thanks are extending to Mak Pho Sayartaw, Inn Ngu Sayartaws and all local

peoples from the Pyay oil field environs for their helps in various ways on this research.

Last but not the least, I would like to thanks my family; my father, my mother

and my two younger sisters for their unconditional support, especially emotional

through my Master Degree.

iv

TABLE OF CONTENTS

Page

ABSTRACT i

ACKNOWLEDGEMENTS iii

TABLES OF CONTENTS iv

LIST OF FIGURES vi

LIST OF TABLE

xii

CHAPTER I INTRODUCTION 1

1.1 General Statement 1

1.2 Location and Size 2

1.3 Accessibility 2

1.4 Topography 3

1.5 Drainage and Watershed Lines 4

1.6 Purpose of Investigation 7

1.7 Method of Investigation 8

1.7.1 Preliminary study 8

1.7.2 Fieldwork 8

1.7.3 Subsurface data interpretations 8

1.8 Previous Works 9

CHAPTER II

REGIONAL GEOLOGIC SETTING

11

2.1 Geologic and Tectonic Setting 11

2.2 Stratigraphy of Pyay Embayment 12

CHAPTER III STRATIGRAPHY OF THE PYAY OIL FIELD 17

3.1 General Statement

Stratigraphy of Exposed Formations in Pyay Oil Field

17

3.2 17

3.2.1

3.2.2

3.2.2

3.2.3

Pyawbwe Formation 19

Kyaukkok Formation 19

Obogon Formation 20

Irrawaddy Formation 20

v

3.3 Sedimentary Structures 21

CHAPTER IV STRUCTURAL INTEREPERTATONS, RELATIONS

AND HYDORCARBON POTENTIAL

24

4.1 General Statement 24

4.2 Namayan Structure 24

4.2.1

4.2.2

4.2.3

4.2.4

Image interpretations 24

Field geological observations 24

Namayan mud volcanoes 26

Subsurface structural interpretations 27

4.3 Pyay North Structure 29

4.3.1 Image interpretations 29

4.3.2 Field geological observations 30

4.3.3 Subsurface structural observations 34

4.4 Pyay South Structure 38

4.4.1 Image Interpretations 38

4.4.2 Field geological observations 38

4.4.3 Subsurface structural interpretations 41

4.5 Structural Relations 47

4.6 Pyay Thrust 49

4.7 Cross Faults 51

4.8 Time of Deformation 51

4.9 Hydrocarbon Potential 52

CHAPTER V CONCLUSIONS AND SUGGESTIONS 54

5.1 Conclusions 54

5.2 Suggestions 55

REFERENCES 57

APPENDIX

vi

LIST OF FIGURES Page

Figure 1.1 Location map of the study area. Sources, IHS Energy; Myanmar

Basin 2004 and Google Earth 2013.

2

Figure 1.2 (A) Altitude of 10 ft interval topographic nature of Pyay oil

field, (B) interpreted map of Pyay oil field (cited from Eloi;

2015).

4

Figure 1.3 Major drainage patterns of the study and surrounding area. The

description of (a, b, c, d & e) are shown in Figure 1.4. Sources,

Google Earth 2014 and 1:250,000 topographic map).

6

Figure 1.4 Various drainage patterns in study area showing location of (a)

Dendritic (b) Trellis (c & d) Fine dendritic to sub-angular

pattern (e) Parallel channel networks. Sources, Google Earth

2014 and 1:63,360 topographic map).

7

Figure 2.1 Tectonic sketch map of Myanmar and surrounding regions

(left), showing the major structural elements of Pyay

Embayment bounded by Kabaw fault in the west, CVL and

major strike-slip Sagaing fault in the east as modified from

Pivnik et al., (1998), Kyi Khin & Myitta (1999), Soe Thura Tun

& MGS (2007) and Bertrand & Rangin (2003). Gravity map of

MOGE (1996) (right) showing the high and low anomalies by

anticlinal folds and synclinal fold axes of the Pyay Embayment.

13

Figure 2.2 Regional geological map of the Pyay oil field, modified form

Bender (1981), Soe Thura Tun and MGS (2014).

14

Figure 2.3 Generalized stratigraphy of Pyay Embayment, modified from

Wandery (2006) and Bender (1983).

15

Figure 2.4 (A) Reflection seismic profile two-way travel time (s) section

and (B) interpreted depth cross-section (Modified form Eloi;

2015) showing the depocenter thicken westward as evidenced

16

vii

by Miocene units thickening in the Pyay oil field. Okhmintaung

Fm (Oligocene), Pyawbwe Fm (L Miocene), Kyaukkok Fm (M

Miocene), Obogon (U Miocene), Irrawaddy Fm (Plio-

Pleistocene) and Alluvium (Quaternary). Line location shown

in Figure 2.2 and tow-way travel time to depth converted

function are using check-shot velocity (Appendix).

Figure 3.1 Stratigraphic column for Pyay oil field based on measured

section at Kyet Pyu Taung Chaung, from MOGE (1966).

Measured stream section location is shown in Figure 1.3.

18

Figure 3.2 Nodular shale structure observed in the Pyawbwe Formation,

near east bank of Ayeyarwaddy River, (Location N

18°45'53.08" E 95°12' 58.81" Elev. 144 ft), (Facing-NE).

21

Figure 3.3 Hard band and concretion in thick bedded sandstone layer of the

Kyaukkok Formation, (Location N18°46'7.84" E 95°14'3.04"

Elev. 185 ft), (Facing-N).

22

Figure 3.4 Small scale rounded shale concretion in sand shale alternation

of the Obogon Formation (Location N 18°34'41.47" E

95°16'35.32" Elev. 265 ft), (Facing-E).

22

Figure 3.5 Convolute structure of sand layer occurred in the Obogon

Formation, (Location N 18° 44' 45.1" E 95° 15' 23.3" Elev. 300

ft), (Facing-E).

23

Figure 3.6 Large scale cross-bedding exposure observed in the Irrawaddy

Formation (N 18° 31' 57.6" E 95° 17' 13.7" Elev. 260 ft),

(Facing-NE).

23

Figure 4.1 Interpretations of Namayan monocline (A) DEM image, (B &

C) Google earth images.

25

Figure 4.2 Generalized geological map of the Namayan monocline,

modified from MOGE (1965) and stereographic projection of

25

viii

bedding data indicated as the trend of axis and plunge angle of

the Namayan monocline (left).

Figure 4.3 Namayan mud volcanoes 26

Figure 4.4 Uninterpreted (up) and interpreted (below) east-west seismic

profile (TWTS) across the Namayan monocline (Line location

is shown in Figure 4.2).

27

Figure 4.5 Uninterpreted (up) interpreted (below) north-south seismic

profile (TWTS) through the Namayan monocline (Line location

is shown in Figure 4.2).

28

Figure 4.6 Image interpretations of Pyay North anticline showing the south

plunging character and surface fault traces of Pyay thrust using

the 3D view of DEM image (left) and Google Earth image

(right).

29

Figure 4.7 Generalized geological map of the Pyay North structure,

modified from MOGE (1965) and stereographic projection of


the Pyay North anticline (left).

31

Figure 4.8 Vertical to overturned beds of sand shale interbedded Obogon

Formation showing the surface fault trace of Pyay thrust at the

west-flank of the Pyay North anticline (N 18°33'14.04" E

95°15'32.57" Elev. 275 ft), (Facing-S).

32

Figure 4.9 Thrust fault traces of shale vertical beds outcrops observed in

west-flank of the Pyay North anticline (N 18°33'9.36" E

95°15'31.91" Elev. 295 ft).

32

Figure 4.10 East hading low angle oblique normal fault in conglomerate

beds, Irrawaddy Formation (N 18° 30' 18.2" E 95° 17' 00.8"

Elev. 530 ft), (Facing-S).

33

ix

Figure 4.11 Low angle oblique normal fault observed in the Obogon

Formation (N 18°36'13.78" E 95°15'45.95" Elev. 291 ft),

(Facing-NE).

33

Figure 4.12 South east hading low angle oblique normal fault in Irrawaddy

Formation (N 18°35'13.00" E 95°17'14.96" Elev. 298 ft),

(Facing-SE).

34

Figure 4.13 Interpreted 3D view of three dip lines seismic profiles across

the Pyay North anticline where line A, B & C showing the

structural styles of disharmonic folding between Middle and

Lower Miocene formations (KK/PY Fm) (Line locations are

shown in Figure 4.7).

35

Figure 4.14 Uninterpreted (up) and interpreted (below) strike seismic

profile section (TWTS) from the west-flank of the Pyay North

anticline showing the northeast-southeast striking and south

hading of high angle oblique sense normal faults are penetrated

from the Irrawaddy Formation to Miocene age of Pyawbwe,

Kyaukkok and Obogon formations (Line location is shown in

Figure 4.7).

36

Figure 4.15 Time-slices interpretations on (A) 1.2 s, (B) 1.6 s and (C) 2.5 s

below the subsurface images showing the structural styles of the

Pyay North anticline. Black square shape and red triangle heads

are indicated that the fault hading. Most cross normal faults are

hading to the south. Major thrusted anticline caused by east-

west compression.

37

Figure 4.16 Image interpretations of Pyay South anticline, 3D view of DEM

image (left) and Google Earth image (right).

38

Figure 4.17 The conjugated faults ("V-shape") developed by the different

hading of adjacent high angle oblique two normal faults are

39

x

observed in the Irrawaddy Formation (Loc. N 18° 26' 13.0" E

95° 18' 26.4" Elev. 368 ft.), (Facing-N).

Figure 4.18 Nearly equal spacing vertical joint-sets occurred in the

conglomerate beds of Irrawaddy Formation, (Loc. N 18° 29

02.1 E 95° 17' 24.9" Elev. 465 ft), (Facing-SE).

40

Figure 4.19 Generalized geological map of the Pyay South anticline

(Modified from MOGE, 1965) and stereographic projection of



41

Figure 4.20 Interpreted 3D view of three dip lines seismic profiles across

the Pyay South anticline where line A,B & C showing the

structural styles of folding, major thrust geometry, cross faults

and some flower structural patterns. Significant structural

feature is the disharmonic folding between the Pyawbwe and

Kyaukkok formations (Line locations are shown in Figure

4.19).

43


profile (TWTS) from the east-flank of the Pyay South anticline

showing the northeast-southeast striking and different hading of

high angle oblique sense normal fault developed in the Miocene

formations caused by northwest-southeast extensional feature

(Line location shown in Figure 4.19).

44


profile (TWTS) from the west flank of anticline showing the

northeast-southwest striking and southeast hading high angle

en-echelon normal faults developed in Miocene formations

caused by northwest-southeast extension and also right-lateral

shear sense (?) (Line location shown in Figure 4.19).

45

xi

Figure 4.23 Time-slices interpretations on (A) 1.2 s, (B) 1.6 s and (C) 3.5 s

below the surface images showing the structural styles of Pyay

S anticline. Major thrusted anticline caused by northeast-

southwest compression and right-lateral shear sense. The

secondary normal cross fault caused by northwest-southeast

extension. The square shapes and triangle heads are indicated

that the fault hading.

46

Figure 4.24 Generalized geological map of the Pyay oil field (center). The

stereographic projection of Namayan (Figure a-b), Pyay North

(Figure c-d), and Pyay South anticlines (Figure e-f) showing the

trend of fold axes and plunge angles. Joints strike-rose diagram

(Figure g) and combination of compression directions from

Figure a-g indicates the mean fold axis and compression

direction of the whole area (Figure h).

48

Figure 4.25 Generalized geological map (left, modified from MOGE; 1965)

and depth cross-sections (right) are based on 2D/3D seismic

data, wells data and surface geology where Namayan monocline

(A_A'), Pyay North anticline (B_B'), and Pyay South anticline

(C_C') showing the geometry of stratigraphic units, Pyay thrust

termination characters and decreased vertical displacement

towards the south.

50

xii

LIST OF TABLE Page

Table 3.1 Stratigraphic sequence of Pyay Oil Field 17

1

CHAPTER I

INTRODUCTION


Pyay oil field was discovered in April 1965 by Peoples Oil Industry, POI (now

MOGE). Since then, a total of 183 wells have been separated and drilled on both Pyay

North (Pyay N), Pyay South (Pyay S) and Namayan structures. Among them, the Pyay

N and Pyay S structures have been currently producing but the Namayan structure is

not yet proven and, that needs to be explored in future. The present study will therefore

concentrate on the structural styles and apparently more hydrocarbon prospective area

of the Pyay oil field over already discovered and accumulations.

Many previous studies were made on the regional geology, stratigraphy,

tectonics and deep test exploration wells for improved hydrocarbon production of the

Pyay oil field. However, there is not detailed structural analysis on the Pyay N, Pyay S

and Namayan structures. The main objective of this study is to analyze the structural

styles among these structures by using the surface geology integrated with selected

seismic profiles and well data to unravel the structural of Pyay oil field.

1.2 Location and Size

The study area (Pyay oil field) is located in the northwestern margin of the Pyay

(Prome) Embayment, a sub-basin of the Central Myanmar Basin, in Bago Division,

Pyay District, to the south of Pyay city. It is approximately 260 km to the north Yangon

and the nearest cities are Pyay, Shwedaung and Paungde. The Pyay structure, a NNW-

SSE trending anticline, approximately 48 km long and 8 km wide, comprises of three

structures, i.e., the Pyay N, Pyay S culminations and Namayan structure.

Pyay oil field area boundary is covered by 380.7 km2 and also demarcated by

four corner points as shown in Figure 1.1 but the actual hydrocarbon producing area is

covering about 50 km2 along a narrow area. All of the Pyay N, Pyay S and Namayan

structures lie in the southern part of Pyay city.

2

1.3 Accessibility

The Pyay oil field is situated about 260 km to the NNW of Yangon, on the

Yangon-Magway highway road and Yangon-Pyay railway line, located on the east bank

of Ayeyareaddy River. The study area can be easily accessed by car, railway and boats

throughout the year and accessibility is very good Figure 1.1.

Fig. 1.1 Location map of the study area. Sources, IHS Energy; Myanmar Basin 2004

and Google Earth 2013.

INDEX

CB – Chindwin Basin SB – Salin sub-Basin PE – Pyay Embayment ADB – Ayeyarwaddy Delta Basin

Study area corner points location

A. 18° 48' 30" 95° 12' 30" B. 18° 48' 30" 95° 16' 30" C. 18° 21' 00" 95° 16' 00" D. 18° 21' 00" 95° 21' 00"

N

MYANMAR

3

1.4 Topography

The Pyay fold, like most of the folds of the Central Myanmar Basin from

Letpando to Chauk, Yanangyaung, Mann or Htaukshabin for instance, is a fold whose

most intense deformation occurred during the late Pliocene-Pleistocene Figure 2.1. This

intense recent deformation consequently strongly affects the present topography. Rivers

not only erode the softest lithologies, but also soft zones intensely fractured by faulting.

The following sections (Figure 1.2) show surface observations which may reveal the

sub-surface image of the complex Pyay fold. Further on, the research will examine how

is the surface topography relates to the sub-surface structuration: for instance does the

Pyay N and Pyay S topography correspond to the Pyay N and Pyay S subsurface image?

The study area lies in hilly terrains between 150-450 ft above sea level from

north to south. The topographic contour map of Figure 1.2 is generated from a DEM

image with a 10 ft interval. Contouring shows a steep topography of westerly oriented

slopes versus mild dipping easterly slopes indicated the area is east dipping. Pyay N,

Pyay S and Namayan structure of topographical elevation and alignments are not quite

different but a significantly different topographic profile between Pyay N and Pyay S,

which are separated by saddle (?).

Pyay N and Namayan, located at some 350 ft above sea level where consistence

of three narrow ridges separated by valleys and parallel from north to south alignments

indicative of different formations or rock units. From west to east, near the east bank of

the Ayeyarwaddy River, the narrowest outcrop shows a sharp topography and may

suggesting a fault location. The two longest outcrops, probably two different harder

lithological units or formations, gradually narrow from the north to terminate

southwards that may approach the saddle between Pyay N and Pyay S.

Pyay S, elevation at 300 ft above mean sea level, is bounded to the west by a

sharp steep slope indicative of a faulted area, with outcrops gently dipping eastwards

away from the east of the fault.

4

Fig. 1.2 (A) Altitude of 10 ft interval topographic nature of Pyay oil field, (B)

interpreted map of Pyay oil field (cited from Eloi; 2015).

1.5 Drainage and Watershed Lines

The Ayeyareaddy River runs from north to south along the western part of the

study area. The main streams (Chaung) in the study area are Po Wa Chaung, Ka Din

Chaung, Na Ma Yan Chaung, Kyat Phyu Taung Chaung, Ka La Chaung, Kyaun

Chaung, Chaung Ma Gyi Chaung. The drainage patterns of the area are commonly east-

PYAY

Mud volcano Crestal axes Drainage axes Oil & Gas Field

N

A B

Town/villages

5

west trending because of the north-south drainage divide along the topographical

ridges. There are also dams or reservoirs fed by some streams, including the Shwedaung

dam covering 1.3 km2 in the area Figure 1.3. Detailed drainage patterns show some

degree of regularity in their tributary orientations. Several distinct drainage patterns

occur in the study area, Figure 1.4 illustrate their diversity. A dendritic drainage pattern

is occur in the northern part of study area, and develops where channel has evolved on

the relatively uniform regional slope. This drainage network may lack major structural

controls Figure 1.4 a. A trellised drainage pattern develop in the central part of the area

where they suggest a strong structural control upon the streams. Because the channel

align themselves parallel to structural in bedrock with minor tributaries coming in at

right angles. These character may cause by fault control or bedrock of joints nature

Figure 1.4 b. A very fine dendritic to subangular pattern is commonly occur in the area

with short and steep-sided tributaries, narrow V-shaped gullies are the most common

drainage patterns where multiple factors influence channel formation or where drainage

has also evolved on a relatively uniform regional slope Figure 1.4 c, d. Parallel drainage

patterns occur in the southern part of the area caused by generally steep slopes Figure

1.5 b. Due to a strong control by the steep slopes, the streams are swift, straight and

short, with very few tributaries, and all flow in the same direction. In this area, the

parallel pattern may indicate the presence of major fault zone (Pyay thrust), mainly in

the west side that cuts across an area of steeply folded bedrock Figure 1.4 e. A drainage

divide is a topographic feature (boundary line of continuous elevated land surface)

which forms a boder between separate watershed area or drainage basins. It is a

geological boundary that physical separates the drainage of one drainage basin (area

drainage by a river or lake) from that another drainage basin (Akhtar et al., 2013).

Figure 1.3 show the drainage divide lines and watershed areas. The present

drainage divide follows the highlands terrain and generally passes through the Namayan

in the east, Pyay S and Namayan structure in the center. Obviously, the drainage divide

line is not continuously from the Namayan, Pyay N to Pyay S structures which was

stopped at the end of Pyay N by shifting to the west. The position and trending of

divides is strongly affected by tectonic forces direction (east-west), which indicate the

different subsurface structures (e.g., folding), rasie mountains trend (north-south) and

alter drainage patterns (from the north-south primary drainage system to secondary

east-west short triburies).

6

Fig. 1.3 Major drainage patterns of the study and surrounding area. The description of

(a, b, c, d & e) are shown in Figure 1.4. Sources, Google Earth 2014 and 1:250,000

topographic map).

(a)

(c)

(b) (d)

(e)

Watershed area (Drainage basin)

Inn Ma Reservoir



Kyet Pyu Taung C

N

7

2 km

2 km

2 km

1 km

N

Fig. 1.4 Various drainage patterns in study area showing location of (a) Dendritic (b) Trellis

(c & d) Fine dendritic to sub-angular pattern (e) Parallel channel networks. Sources, Google

Earth 2014 and 1:63,360 topographic map).

1.6 Purpose of Investigation

The present study is focused on the structural styles of the Pyay oil field from

surface to sub-surface by reviewing the regional geology of Pyay oil field prior to

understand the tectonic history of the Pyay Embayment. Although many previous

workers have made extensive research works in the Pyay oil field, the structural nature

(b) (c) (d)

(e)

(a)

2 km

8

and relationship between the Pyay N and Pyay S structures (separated by a saddle?) is

still unclear. This study will:

– describe the geometry of folds, faults and lineaments of the Pyay oil field.

– compare the surface and subsurface structural nature of the Pyay oil field

– correlate the structures and stratigraphy of Pyay N, Pyay S and Namayan

structures of the Pyay oil field.

– make implication for the timing and tectonic history of major folding, major

faulting, cross faults and their possible mechanisms.

– attempt to conclude the hydrocarbon prospect of the Pyay oil field.

1.7 Method of Investigation

This study proceeded along the following steps from desktop review of past

work, existing data to fieldwork and back to desktop for integration.

1.7.1 Preliminary study

The extensive literature survey was make to cover the regional and structural

geology of the study area. Data including Remote Sensing from Google Earth 2014,

topographic maps and SRTM imagery which are interpreted to produce the structural

images of the Pyay oil field.

1.7.2 Fieldwork

Preliminary and extensive field traverses along the whole area, especially to

check rock types, structural data were run especially across on structure using a Brunton

compass, hammer, a GPS unit and various accessories combine with topographic maps,

old geological maps and field studies. Dip and strike data were measured to identify

fold and fault features combined with a surface fracture map of the area. Structural

geology, tectonics and stratigraphy of the area were checked based on the regional

geological map, lineaments mapping, DEM (Digital Elevation Model), well logs and

2D/3D seismic data.

1.7.3 Subsurface data interpretations

Based on seismic sections and well logs correlations of representative

subsurface geological cross-sections have been made at suitable locations (i.e. along

9

the strike and dip of the structure). Gravity data were extracted from available MOGE

gravity maps to generate a gravity anomalies map to check for anticlines and synclines

from high and low gravity anomalies (Figure 2.1). The major subsurface geological

formation boundaries, folds and faults were interpreted from 2D dip (E-W), strike (N-

S) 6.0 second seismic sections and also 3D seismic cube. Wells were selected to

calibrate the seismic interpretation and construct cross-sections. Because of the higher

definition of the logs, well data are more accurate than the seismic data to locate the

depth of formation boundaries and define fault throws.

1.8 Previous Works

The area is mainly composed of Tertiary sediments and Quaternary alluvium.

Tertiary rocks of Myanmar had been studied by many MOGE and oversea geologists

since 1896. Theobald (1896), the pioneer of Myanmar Tertiary geology, distinguished

two units, the “Pegu Group" and the "Fossil Wood Group”. Nummulitic clastic rocks

of the former group were deposited in a marine environment overlain by clastic rocks

with abundant fossil wood under a more continental environment.

Aung Khin and Kyaw Win (1966) carried out researched the geology and

Hydrocarbon prospects of the Myanmar Tertiary Geosyncline. In addition, they

studied its Cenozoic paleogeography in 1967.

Clegg (1938) correlated the Pegu system at Pyay, Kanma, Thayetmyo and Minbu

area. The Pyay oil field was discovered in April 1965 by Peoples Oil Industry, POI

(now MOGE).

Khin Aung Than (1990) studied the sedimentology facies and ichnology of the

Pyay and Myanaung-Kyangyin area for his thesis.

Chinnery Assets Ltd. (2004) acquired a total of 335 line km 2D seismic and 188

km2 3D for the Pyay oil field.

Lin Thu Aung (2014) studied the active tectonics of the central Myanmar belt

between latitude 17° N to 22° N area for his Ph.D dissertation.

MOGE (1965-1968) reported on geological, structural mapping, gravity and

seismic survey of the Prome Hills Structure.

MOGE (1975) reported on Burma geological Note 561, position review note on

Pyay oil field.

10

Noetling (1898 and 1901) revised the Theobald’s Pegu system and introduced the

Prome and Yenangyaung units.

Pascoe (1912) divided the Pegu series (Oligocene – Miocene) of Prome (Pyay) and

Kama area and lower Myanmar and also stated the existence of “Red Beds”

between the Peguans and Irrawaddians.

Vredenburg (1920 -1922) sub-divided the Pegu system in Tertiary basins of

Myanmar into various stages based on paleontology.

11

CHAPTER II

REGIONAL GEOLOGIC SETTING

2.1 Geologic and Tectonic Setting

The large depocenter located between the Rakhine Yoma into the west and Shan

plateau into the east are called the Central Myanmar Basin (CMB) (GIAC; 1996-1999).

The CMB is divided into several sub-basins along its nearly 1100 km length (Figure

2.1). These subbasins are the Irrawaddy Delta, Pyay (Prome Embayment), Salin,

Shwebo, Chindwin, and Hukaung basins from south to north (Bender; 1983, Pivnik et

al., 1998, GIAC; 1999). These en-echelon basins (Bender; 1983) may have formed as

a series of pull-apart basins since the early Eocene as the Bruma plate moved northward

relative to the Asia plate (Pivnik et al., 1998, GIAC; 1999). The CMB is characterized

by strike-slip movement of the Burma plate against the Shan Plateau along the north-

south striking Sagaing fault (Gross; 1985, Win Swe; 1972). The north-south running

Kabaw fault delimits the Pyay Embayment from the Rakhine Yoma to the west (Win

Swe; 1972). At the east, the boundary between the Pyay Embayment and the Bago

Yoma, is marked by the Inner Volcanic Arc including the Late Miocene-Quaternary

volcanic of the Central Volcanic Line (Bender; 1983, Kyi Khin and Myitta; 1999). The

Pyay Embayment and Salin subbasin are separated by a multiphase deformation of

20°N uplift (Bender; 1983, Pivnik et al., 1998). The Pyay-Aunglan earthquake (24th

August, 1858) indicated as the Pyay thrust is still active since the Pliocene-Pleistocene

(Chhibber; 1934, Lin Thu Aung; 2014). In general, three structural storeys can be

experienced in Pyay Embayment: 1) pre-Oligocene compressive structural elements

and wrench faulting (Gross; 1985), 2) Miocene extensional faulting, grabens and

igneous intrusions (Maung Thein; 1973, Gross; 1985, Pivnik et al., 1998), and 3)

Pliocene-Pleistocene deformations by thrust or reverse faulting, inversion of normal

faults and extrusion of volcanics (Pivnik et al., 1998).

Roughly parallel, north-south trending folds, often vergent to the west and with

a wavelength of few kilometers can be observed in many places within the Neogene

sediments of the Pyay Embayment Figure 2.2. The gravity anomalies map (MOGE;

1966) reveals the Pyay oil field is located on the northwestern margin of Pyay

Embayment and NNW-SSE orientation of the major syncline is steadily widen towards

12

the south (Bender; 1983, Le Dain et al., 1984) up to 100 km in 18° 20' N. The

depocenter dominantly thicken westward (Figure 2.4), towards the transition with the

Indo-Burma ranges (Mitchell; 1979, Bertnard; 2003). Generally CMB contains over 15

km thickness of the Eocene to Plio-Quaternary sediments (Curray et al., 1979, Le Dain

et al., 1984, Pivnik et al., 1998, GIAC; 1999, Bertrand et al., 2003) and might reach 17

km in southern part (Mitchell; 1975, Le Dain et al., 1984). Specifically for Pyay

Embayment, the depth of magnetic basement is rather deep in the range of 15 km which

rises towards the east between the 17° N and 23°50' N (Bannert et al., 2011). Generally,

Tertiary sedimentation of Pyay Embayment continental depositional systems prograded

southwards over marine depositional environments (Bender; 1983, Maung Thein; 1973,

Wandrey; 2006), but the continental sedimentation prevails since the Late Miocene in

the whole basin system (Pivnik et al., 1998, GIAC; 1996-1999). Mixed marine and

fluvial sedimentation took place until the late Miocene or Pliocene, when fluvial

sediments of the Irrawaddy Formation began accumulating throughout the basin

(Pascoe, 1964; Bender; 1983, Pivnik et al., 1998). The outcrops of the small bodies of

ultramafic rocks, mostly along the Kabaw fault zone (Bannert et al., 2011) and Dolerites

rocks outcrops (about 20 miles east of Zegon) which intruded into Miocene units

(Chhibber; 1934, Win Swe; 2012) could be the basement of Pyay Embayment.

2.2 Stratigraphy of Pyay Embayment

Generalized stratigraphic sequences and approximated thickness ranging in age

from Eocene to Holocene are shown in Figure 2.2 for Pyay Embayment. The upper

Eocene of Taunggalay limestone and Yaw shale were deposited in the western part of

Pyay Embayment and Irrawaddy Delta basin (Gross; 1985, Wandrey; 2006). Chhibber

1934 also described the Eocene age of sandstone and black shale alternation of Negrais

Group exposed in the west of Thayet in the north to Pyay, Pathein and Cape Negaris

(a.k.a Mawtin Zoon) in the south.

The lower Pegu Group of Kyaukpon or Shwezetaw Formations consists of

shallow marine interbedded clays, shales, and sandstones overlain by the Padaung

Formation (Wandrey; 2006). Then, the unconformity is marked by the Okhmintaung

sandstones and the upper Pegu group of Pyawbwe Formation (Chhibber; 1934 and

Wandrey; 2006).

13

The upper Pegu Group or Miocene units of Pyawbwe, Kyaukkok and Obogon

formations are more extensively exposed especially in the Pyay and Mayaman oil field.

Deposited in alluvial environments, on the unconformable top of Pegu Group of

nonmarine massive sandstones, clays, and conglomerates Quaternary Irrawaddy Group

(Chhibber; 1934 and Win Swe; 2012). Total sedimentary thickness exceeds 12 km

(Wandrey; 2006) and also 17 km (Mitchell; 1975, Le Dain et al., 1984) in Pyay

Embayment.

Fig. 2.1 Tectonic sketch map of Myanmar and surrounding regions (left), showing the major

structural elements of Pyay Embayment bounded by Kabaw fault in the west, CVL and

major strike-slip Sagaing fault in the east as modified from Pivnik et al., (1998), Kyi Khin

& Myitta (1999), Soe Thura Tun & MGS (2007) and Bertrand & Rangin (2003). Gravity

map of MOGE (1996) (right) showing the high and low anomalies by anticlinal folds and

synclinal fold axes of the Pyay Embayment.

N

HB – Hukaung basin SB – Salin basin PE – Pyay Embayment ADB – Ayeyarwaddy

Delta Basin CVL – Central Volcanic

Line

EXPLANATION

Anticline Syncline Thrust Fault Strike-slip Fault Spreading Center Gas Show Low Gravity

L

14

Fig. 2.2 Regional geological map of the Pyay oil field, modified form Bender (1981), Soe

Thura Tun and MGS (2014).

Quaternary

Miocene-Pleistocene

Miocene Oligocene

Eocene (Molasse) Eocene (Flysch) Cretaceous Jurassic Paleozoic (Mergui Group) Carboniferous Mogok metamorphic Mesozoic (Granite) Cretaceous-Eocene (Ultrabasic) Strike-slip (Active) Thrust/Reverse (Active) Thrust/Reverse (Inactive)

LEGEND

N

15

Fig. 2.3 Generalized stratigraphy of Pyay Embayment, modified from Wandery (2006)

and Bender (1983).

PE

RIO

D

EP

OC

H

GR

OU

P

F OR M A T IONA PPR OX IM A T E

T HIC KN ESS ( f t .)

Holocene - P leistocene

Pliocene

1475 - 2950

1215 - 1970

1510

> 3930

- Eocene

3445 - 8200

1280 - 4100M iocene

3935 - 8530

785 - 820

Irraw

addy

T ert iary

Jaint

ia

Oligocene

Pegu

QuaternaryAlluv ium

Irrawaddy

Obogon

Ky aukkok

Py awbwe

Okhmintaung

Padaung/Tiy o

Shwezetaw/ Ky aukpon

Yaw

Pondaung

Taunggale

Kanbala

EXPLANATION

Unconformity Oil production Gas production Source rocks Shale Sandstone Siltstone, shale and sandstone Shale, siltstone and sandstone Limestone

16

Fig

. 2.4

(A

) R

efle

ctio

n s

eism

ic p

rofi

le t

wo

-way

tra

vel

tim

e (s

) se

ctio

n a

nd (

B)

inte

rpre

ted d

epth

cro

ss-s

ecti

on

(M

odif

ied f

orm

Elo

i; 2

015)

show

ing t

he

dep

oce

nte

r th

icken

wes

twar

d a

s ev

iden

ced b

y M

ioce

ne

unit

s th

icken

ing i

n t

he

Pyay

oil

fie

ld.

Okhm

inta

ung F

m (

Oli

goce

ne)

,

Pyaw

bw

e F

m (

L M

ioce

ne)

, K

yau

kk

ok F

m (

M M

ioce

ne)

, O

bogon (

U M

ioce

ne)

, Ir

raw

addy F

m (

Pli

o-P

leis

toce

ne)

and A

lluviu

m (

Quat

ernar

y).

Lin

e lo

cati

on s

how

n i

n F

igure

2.2

an

d t

ow

-way

tra

vel

tim

e to

dep

th c

onver

ted f

unct

ion a

re u

sing c

hec

k-s

hot

vel

oci

ty (

Appen

dix

).

0.00

0

1.00

0

2.00

0

3.00

0

4.00

0

5.00

0

0.00

0

1.00

0

2.00

0

3.00

0

4.00

0

5.00

0

250.03

00.035

0.0400

.0450.05

00.055

0.0600

.0650.07

00.075

0.0800

.0850.09

00.095

0.0100

0.01050

.01100

.01150

.01200

.01250

.0- E

P93-E

-13

00.013

50.014

00.014

50.015

00.015

50.016

00.016

50.017

00.017

50.018

00.018

50.019

00.019

50.020

00.020

50.021

00.021

50.022

00.0

SP:

010

000

2000

030

000

4000

049

793

Offse

t: 0.00

0

1.00

0

2.00

0

3.00

0

4.00

0

5.00

00

1.0

2.0

3.0

4.0

5.0

A

A'

Two-way Travel Time (s)

17

17

CHAPTER III

STRATIGRAPHY OF PYAY OIL FIELD


Stratigraphical nomenclature of the Pyay Embayment is the same which is use

in the CMB by Aung Khin and Kyaw Win, 1969. In the Pyay oil field, at the latitude of

Namayan, Pyabwe Formation (Lower Miocene) to Irrawaddy Formation (Plio-

Pleistocene) but to the south of Shwedaung latitude (current hydrocarbon producing

Pyay North area), only Kyaukkok Formation (Middle Mocene) to Irrawaddy Formation

are exposed. Southwards (Pyay South current hydrocarbon producing area) only

Irrawaddy Formations are exposed.

The present study is using the Pyay well-1 (N 18° 34' 29.2", E 95° 15' 57") for

collection and reviewing the stratigraphy in the research area. Pyay well-1 penetrated

only the Miocene formations from top to bottom 1840 ft of Kyaukkok and Obogon

formations, 910 ft of Pyawbwe Formation in the supra-thrust and 800 ft of Kyaukkok

and Obogon formations, overlying 1050 ft of Pyawbwe Formation in the sub-thrust.

The formation boundaries are based on Paleontological examinations and well-logs

interpretations. Stratigraphic succession of the area resulted from the well-1 is

compared with previous MOGE workers measured from the stream section which

exposed in the Pyay oil field Figure 3.1 and Burma geological note (No. 561, 1975) in

Table 3.1.

3.2 Stratigraphy of Exposed Formations in Pyay oil field

Table 3.1 Stratigraphic sequence of Pyay oil field

Well 1 (Supra-T)

1990

Well 1 (Sub-T)

1990

Stream Section

1966

Burma Geological

Note No. 561, 1975

Holocene Quaternary - - - -

Plio-Pleistocene Irrawaddy 1850 - 510 > 5200

Miocene (Upp) Obogon 3085 2500

Miocene (Mid) Kyaukkok 3815 2600

Miocene (Low) Pyawbwe 950 > 1080 350 > 2500

Thickness in feet

Age Formation

1200 950

1

1

18

Fig. 3.1 Stratigraphic column for Pyay oil field based on measured section at Kyet Pyu

Taung Chaung, from MOGE (1966). Measured stream section location is shown in

Figure 1.3.

Mid

dle

Sh

ale

Un

it (

239

0 f

t)

Upp

er S

andst

on

e

Un

it (

67

5 f

t)

Low

er S

andst

on

e U

nit

3900

4800

3600

4200

4500

5100

5400

5700

6000

6600

6900

OB

Fm

(3085

ft)

6300

2700

2100

1800

900

300

0

600

1200

1500

2400

3000

Lowest exposed horizon

Low

er A

lter

nat

ion

Unit

(85

0 f

t)

Feet

3300

(350 f

t)

KK

Fm

(3815 f

t)

7200

IRR

Fm

(510 f

t)

8100

Feet

Lithological descriptions

IRR Fm (Plio-Pleistocene)

OB Fm (Upper Miocene)

KK Fm (Middle Micoene)

PY Fm (Lower Miocene)

Alluvium

No exposure

Alternations

Shale/Clay

Silt

Sand

Gravel

3600

PY

Fm

7200

19

3.2.1 Pyawbwe Formation

This formation is very poorly exposed in the Kyetphyutaung stream section.

The Pyawbwe Formation can be photogeologically traced down to the east of Kyet Pyu

Taung where it is probably wedged out by the thrust.

The lower boundary of the formation is not exposed and the upper boundary is

taken at the base of a conspicuous hard ridge forming by a sandstone. The

biostratigraphic boundary extends much higher up into the overlying Kyaukkok

Formation, in the Kyet Pyu Taung section.

The Pyawbwe Formation consists of dark grey clay bluish weathering greyish

yellow, soft, nodular, locally bedded, mostly structureless, rich visible macro-fauna,

including shell fragments of Lamellibranches and Gastropods, and calcareous

concretions, occasionally hard, slightly sandy and gypsiferous, occasionally marly,

with conformable boundary to overlying Kyaukkok lower boundary. The microfauna,

shell fragments of lamellibranch and gastropods, and calcareous concretions content of

the Pyawbwe formation indicates that during the deposition of this formation, deeper

marine conditions prevailed in the area (MOGE; 1966).

3.2.2 Kyaukkok Formation

The Kyaukkok Formation is exposed and extends to the east of Namayan, and

the south of it, where the formation is cut out by the cross-fault at the latitude of

Butlegon village (N 18° 37' 50" E 95° 14' 40"). South of this cross fault, the Kyaukkok

Formation is not exposed in the Pyay oil field Boundary.

The upper boundary of the Kyaukkok Formation is taken at the top of a

comparatively hard sandstone unit forming the base of massive alternation of shale and

sandstones. The Kyaukkok Formation can be tentatively subdivided into 3 units:

i. Upper Sandstone Unit 625 ft

ii. Middle Shale Unit 2290 ft

iii. Lower Alternations Unit 850 ft

Lower Alternations unit consist of yellowish brown, hard, calcareous, fine

grained sandstone and bluish grey, moderately hard, laminated silty shale. The exposed

rocks have a sand-shale ratio of 40%.

20

Middle Shale unit consists of bluish grey, soft to moderately hard, thin -bedded

to nodular, silty and micaceous in places.

Upper Sandstone unit consists of yellowish brown, fine grain massive,

micaceous and hard fossiliferous Sandstone with occasional grey shale bands. Fossils

include lamellibranch shells and gastropods.

With deposition overtaking the rate of subsidence, or possibly with some uplift,

the area gradually shallows and during the Kyaukkok deposition, shallow marine

conditions prevailed over the whole area. This shallowing is not uniform over the whole

area. It is evident that Kyet Pyu Taung area attain minimum rate of emergence as

indicated by the lowest deeper marine beds, while elsewhere shallow marine conditions

prevailed (MOGE; 1966).

3.2.3 Obogon Formation

The formation is marked by ridges separated by valleys and low areas. The

formation exposed poorly and sporadic especially in the Latitude of east of Shwedaung

dam and current hydrocarbon producing Pyay North area, near Paung Gyok village

Figure 4.7.

The Obogon Formation consists of alternation of sandstone yellowish brown,

soft, fairly bard, fine to medium grained, occasionally gritty, ferruginous with rare

fossiliferous bands and shale, grey, soft, laminated micaceous and frequently silty. A

spot current bedding analysis indicates that the deposits come from probably north-

westerly direction. The estuarine/fluviatile conditions which prevailed during the

Obogon deposition (MOGE; 1966).

3.2.4 Irrawaddy Formation

This formation is widely exposed and mostly occupies in the east and current

hydrocarbon producing Pyay South area of the Pyay oil field. Part of the base of

Irrawaddy Formation is measured in the Kyet Pyu Taung Stream section and it consists

of bluish grey to grey, soft, massive-thinly bedded silty shale underlying the soft,

loosely consolidated, pebbly and massively current bedded medium to coarse grained,

yellowish sandstone. Sandstones are ferruginous and bear silicified fossil wood

fragments.

21

Continental conditions make their first appearance during the Obogon

deposition and becomes dominant during the Irrawaddian (MOGE; 1966).

3.3 Sedimentary Structures

Sedimentary structures of small to large cross-bedding (Fig 3.6), wavy

lamination, rounded and disk-shaped concretions (Fig. 3.3, 3.4) are observed in the

stratigraphic unit especially in the Irrawaddy Formation (Plio-Pleistocene). Obogon

Formation consists of small concretions and convolute sand layer (Fig 3.5). Miocene

units of Pyawbwe, Kyaukkok and Obogon formations are rare sedimentary structure

due to the poor exposure. Concretions are mostly found in very thick sand layers and

some have as 2 to 3 ft in diameter. The cross-bedding has inclination of 15° to 30°

towards southward 200°.

In the Irrawaddy Formation, the sandstone layer is overlain by younger muddy

sandstone layer and angular-unconformity. The angle of older sandstone layer is 35°

and towards southeast 120°.

Fig. 3.2 Nodular shale structure observed in the Pyawbwe Formation, near east bank of

Ayeyarwaddy River, (Location N 18°45'53.08" E 95°12' 58.81" Elev. 144 ft), (Facing-

NE).

22

Fig. 3.3 Hard band and concretion in thick bedded sandstone layer of the Kyaukkok

Formation, (Location N18°46'7.84" E 95°14'3.04" Elev. 185 ft), (Facing-N).

Fig. 3.4 Small scale rounded shale concretion in sand shale alternation of the Obogon

Formation (Location N 18°34'41.47" E 95°16'35.32" Elev. 265 ft), (Facing-E).

2 ft

23

Fig. 3.5 Convolute structure of sand layer occurred in the Obogon Formation, (Location

N 18° 44' 45.1" E 95° 15' 23.3" Elev. 300 ft), (Facing-E).

Fig. 3.6 Large scale cross-bedding exposure observed in the Irrawaddy Formation (N

18° 31' 57.6" E 95° 17' 13.7" Elev. 260 ft), (Facing-NE).

2.5

ft

24

CHAPTER IV

STRUCTURAL INTERPRETAIONS, RELATIONS AND

HYDROCARBON POTENTIAL


Structural interpretations are mainly focus on the structural styles of the current

hydrocarbon producing area of Pyay North, Pyay South anticlines and also on the new

prospect of Namayan monocline (Figure 1.1). Observations from a traverse field

geological data with satellite image interpretations, regional geological observations,

well logs data and geophysical data interpretations are integrated to produce a structural

images and hydrocarbon potential of the Pyay oil field.

4.2 Namayan Structure

4.2.1 Image interpretations

The area consists of three narrow and parallel north to south outcrops indicated

that the different formations or rock units. Near the east bank of the Ayeyarwaddy

River, from the west to east and narrowest outcrop shows a sharp topography

suggesting a surface fault traces. The two longest outcrops, probably two different

harder lithologies units are gradually narrow from the north to southwards and

terminated at the Latitude of Shwedaung. Google Earth images (Figure 4.1 B & C)

clearly show the dextral strike-slip features of topographic patterns near the Shwedaung

reservoir.

4.2.2 Field geological obsrevations

The area is covered by the complete Miocene units of Pyawbwe, Kyaukkok

and Obogon formations and Plio-Pleistocene Irrawaddy Formation from west to east.

The monoclinal eastern limb of the structure is exposed, whereas the eastern limb being

thrusted and covered by the alluvium is observed along the Pyay, Shwedaung town and

near the Butlegon village. The dips vary from the 80° to 20° and gradually gentle to the

east in the Irrawaddy Formation (Figure 4.2).

25

Fig. 4.1 Interpretations of Namayan monocline (A) DEM image, (B & C) Google earth images.

Fig. 4.2 Generalized geological map of the Namayan monocline, modified from MOGE

(1965) and stereographic projection of bedding data indicated as the trend of axis and

plunge angle of the Namayan monocline (left).

(B)

(B)

(C)

(C)

Shwedaung Reservoir

N

Fig. 4.2

04°/150°

12°/162°

Fold Axis

Poles to planes

Number of bedding

n

(A)

26

4.2.3 Namayan mud volcanoes

Active mud volcanoes found in Pyawbwe Formation at the east of Namayan. It

comprises altogether 10 mud volcanoes. A number of small-size active mud volcanoes and

mound-like features are present, which have high from near ground level to maximum 6 ft.

Diameters of the observed craters range from 3 inches to 2.5 ft. They spew out liquid mixed

with fine silt or clay bubbles and sometimes frequently in order to emit mud-water and

hydrocarbon gases (Figure 4.3). The volcanoes are apparently lie within the Pyawbwe

Formation or these are porbably formed along the fault plane of the Pyay thrust, which

forms the contact of boundary between the Pyawbwe and Kyaukkok formations (MOGE;

1966).

Fig. 4.3 Namayan mud volcanoes

18°

45'

100 ft

Largest Crater

Largest Crater

Largest Crater

Smallest Crater

Smallest Crater

N

2.5 ft

-

- 95° 13'

N

27

4.2.4 Subsurface structural interpretations

The Namayan monocline reveal the Pyay thrust geometry (Figure 4.4), the

presence of steeply east dipping bed along its east side, the fold was found above

(supra), an east dipping high-angle thrust fault and also below (sub) the thrust fault.

Sedimentary layers within the sub-thrust monocline’s forelimb dip up to 40° to the east,

in contrast to those gently dipping lessthan 5° to the west within the backlimb (Figure

4.4). Neraly northwest-southeast oriented about 10 km long and 5 km wide. The south

plunging character is suggested by the sterographic projection evidence (Figuure 4.1)

and also seismic evidence. But the seismic profile expression (Figure 4.5) is strongly

indicated as the north plunging characters of the structure.

Fig. 4.4 Uninterpreted (up) and interpreted (below) east-west seismic profile (TWTS)

across the Namayan monocline (Line location is shown in Figure 4.2).

Tow

-Way

Tra

vel

tim

e (s

)

2.0

1.0

3.0

4.0

2.0

1.0

3.0

4.0

App

rox

imat

ed F

orm

atio

ns

Bou

nd

arie

s

2.0

1.0

3.0

4.0

2.0

1.0

3.0

4.0

EAST WEST

IRR Fm.

OB Fm.

KK Fm.

PY Fm.

IRR Fm.

OB Fm.

KK Fm.

1 km

Shwedaung-1 (Projected)

TD-6500 ft

28

East-west striking of south hading faults and conjugated faults are developed

probably in the steeply dipping Obogon Formation at supra-thrust or the east-flank

(Figure 4.4). In the west-flank or sbu-thrust zone is not easy to interpreted the

subsurface structures because of the sedimentary layers are make up by only Miocene

formations especially the Pyawbwe shales. But the sub-thrust Pyawbwe shales are more

favour to accumulate the hydorcarbon than the supra-thrust due to the structural

developed as the monocline closure make up by forelimb of Shwedung syncline.

Fig. 4.5 Uninterpreted (up) interpreted (below) north-south seismic profile (TWTS)

through the Namayan monocline (Line location is shown in Figure 4.2).

Tow

-Way

Tra

vel

Tim

e (s

)

2.0

1.0

3.0

4.0

SOUTH NORTH

2.0

1.0

3.0

4.0

0

0

App

rox

imat

ed F

orm

atio

n B

oun

dar

y

2.0

1.0

3.0

4.0

0

2.0

1.0

3.0

4.0

0

Shwedaung-1 (Projected)

TD-6500 ft

OB Fm.

KK Fm.

1 km

29

4.3 Pyay North Structure


Both of DEM and Google Earth images significantly show the nearly north-

south striking and south plunging anticline features of the Pyay North anticline. The

black color dotted alignments indicated that the different lithological boundaries by the

different morphological features and vegetation patterns. Obviously, the surface fault

traces (red color thick line) can be interpreted by sharp topographical features at the

west of the area (Figure 4.6).

Fig. 4.6 Image interpretations of Pyay North anticline showing the south plunging

character and surface fault traces of Pyay thrust using the 3D view of DEM image (left)

and Google Earth image (right).

Inn Ma Inn Ma

2 km

Sharp

N

N

18

° 3

5'

-

- 95° 17'

30

4.3.2 Field geological observations

The area is covered by the north-south striking Miocene units of Kyaukkok and

Obogon formations overlain by the Irrawaddy Formation (Plio-Pleistocene) (Figure

4.7). The exposure of these formations are ascending order to the east by the age.

Among the Irrawaddy Formation occupies most of the Pyay North anticline especially

in the east. These formations boundary contacts are not easy to separate by the only

field data due to the poor exposure, especially in the Kyaukkok and Obogon formations.

Observation of field measurement bedding data suggested that the Pyay North

anticline is asymmetrical fold which gentle east-flank and steeper west-flank. The

gentle east-flank of the anticline is generally composed of 10° to 40° dips which are

sometimes decrease to nearly 5° from the northern part to the south of anticline. And

the steeper west-flank is composed of 25° to 65° dips angle (sometime nearly vertical)

that enclosed by thrust fault (Figure 4.7). The axis of the anticline can be traced by the

satellite image interpretations, dips and strike data of field observations where the axis

of the fold is observed in the Obogon Formation. The axis of anticline is striking ~350°

and the northern end of axis also plunged to the north (16°) in Obogon Formation near

the Pa Aing village and the southern end of anticline axis is plunging about <10° in the

Obogon Formation (Figure 4.7).

In the west-flank, sand and shale alternation of north-south striking vertical to

overturned beds are observed in the Obogon Formation along the western foot hills

slope of topography (Figure 4.8 & 4.9) which are suggested the surface fault traces of

Pyay thrust location or fault zone. It is also indicated that the nearly east-west

compressional direction.

In the east-flank, the minor cross faults are also observed in the Irrawaddy

Formation especially in the conglomerate beds (Figure 4.10 & 4.11). Northeast-

southwest striking and east hading cross faults are commonly observed in southern and

central part of the anticline. Some northwest-southeast direction with low angle (< 45°)

and west hading faults are also observed in north plunge of Pyay North anticline (Figure

4.12). The all observations of cross-faults could not mapped on present geological map

(Figure 4.7) because of these faults are very small but they may represent the general

orientation of lineaments and major cross-faults of the subsurface structures.

31

Fig. 4.7 Generalized geological map of the Pyay North structure, modified from MOGE (1965)

and stereographic projection of bedding data indicated as the trend of axis and plunge angle of


N

16°/360°

08°/174°

Fold Axis

Poles to planes

Number of bedding

n

32

Fig. 4.8 Vertical to overturned beds of sand shale interbedded Obogon Formation

showing the surface fault trace of Pyay thrust at the west-flank of the Pyay North

anticline (N 18°33'14.04" E 95°15'32.57" Elev. 275 ft), (Facing-S).

Fig. 4.9 Thrust fault traces of shale vertical beds outcrops observed in west-flank of the

Pyay North anticline (N 18°33'9.36" E 95°15'31.91" Elev. 295 ft).

Facing NE

Facing E

Facing NE

33

Fig. 4.10 East hading low angle oblique normal fault in conglomerate beds, Irrawaddy

Formation (N 18° 30' 18.2" E 95° 17' 00.8" Elev. 530 ft), (Facing-S).

Fig. 4.11 Low angle oblique normal fault observed in the Obogon Formation (N

18°36'13.78" E 95°15'45.95" Elev. 291 ft), (Facing-NE).

34

Fig. 4.12 South east hading low angle oblique normal fault in Irrawaddy Formation (N

18°35'13.00" E 95°17'14.96" Elev. 298 ft), (Facing-SE).


Based on the integrated seismic and well data interpretations, the Pyay North

anticline is asymmetrically on gentle east-flank and steeper west-flank. The axis is

generally north-south striking and also double plunging closure. The north plunge angle

(16°) is steeper than the south plunge approximately 08° (Figure 4.7). The amplitude of

the folded features are quite different in the each subsurface units. The monoclinal

character of fold layer is start developed in the Lower Pyawbwe Formation and

gradually changed to thrust-bounded anticlinal fold near the contact of Pyawbwe and

Kyaukkok formations at the core of anticline. The fold is well developed in the Obogon

and Irrawaddy formations by the thrust related and the west-vergent asymmetrical

anticline. The anticline axis is more or less parallel with thrust plane orientation about

40° to the west. Along the crest of the fold is structurally complex due to the related

longitudinal thrust zone is much closed from the crest and the whole fold is

compartmentalized by east-west cross-faults on the both flanks (Figure 4.13).

In the seep west-flank, a bounding longitudinal thrust fault and significant of

northeast-southwest striking and southeast hading extensional cross faults are also

developed in the west-flank. Some of these faults are penetrated from the surface to the

deeper part of Miocene Formations. This is proved by Figure 4.14 which suggested that

35

the tectonic history of extensional cross-faults are developed after the Irrawaddy

deposition time (Plio-Pleistocene). This flank is also linked with the Shwedaung

syncline to the west (Figure 4.15).

In the east-flank, also southeast hading and northeast-southwest trending

extensional normal faults are developed in the Miocene Formations. The structural

styles are probably same with the west-flank. But the significant stratigraphic variations

between east and west flanks in which the Miocene Formations are thickening in the

east-flank than the west-flank. This is interpreted as the local tectonic reason because

of the normal stratigraphic thickness of Miocene Formations are thicker to the west

from the Pyay Embayment depocenter (Figure 2.4).

Time-slices interpretations of Figure 4.15 (A) 1.2 s, (B) 1.6 s and (C) 2.5 s below

the subsurface images are indicated that the major structural patterns of the Pyay North

anticline, Pyay thrust and cross normal faults.

Fig. 4.13 Interpreted 3D view of three dip lines seismic profiles across the Pyay North

anticline where line A, B & C showing the structural styles of disharmonic folding

between Middle and Lower Miocene formations (KK/PY Fm) (Line locations are


1.0

2.0

TW

TS

3.0

4.0

Well-62 Well-1 Well-20

TD 1675 ft

TD 1980 ft

TD 1722 ft

App

rox

imat

ed F

orm

atio

n B

oun

dar

ies

IRR Fm.

OB/KK Fm.

PY Fm.

OKH Fm?

36

Fig. 4.14 Uninterpreted (up) and interpreted (below) strike seismic profile section

(TWTS) from the west-flank of the Pyay North anticline showing the northeast-

southeast striking and south hading of high angle oblique sense normal faults are

penetrated from the Irrawaddy Formation to Miocene age of Pyawbwe, Kyaukkok and

Obogon formations (Line location is shown in Figure 4.7).

2.0

1.0

3.0

4.0

SOUTH NORTH

2.0

1.0

3.0

4.0 1 km

TW

TS

App

rox

imat

ed F

orm

atio

n B

oun

dar

ies

IRR Fm.

OB Fm.

KK Fm.

Tow

-Way

Tra

vel

Tim

e (s

) PY Fm.

37

Fig. 4.15 Time-slices interpretations on (A) 1.2 s, (B) 1.6 s and (C) 2.5 s below the

subsurface images showing the structural styles of the Pyay North anticline. Black

square shape and red triangle heads are indicated that the fault hading. Most cross

normal faults are hading to the south. Major thrusted anticline caused by east-west

compression.

(A)

(B)

(C)

N

38

4.4 Pyay South Structure


DEM and Google Earth images can be interpreted the major structural features

of the Pyay thrust by abrupt changed topography in the west. In accordance with the

interpretation of probable strike of formation by the linear alignments of topography,

the anticline is developed from the north of structure (Pyay North anticline) and

plunging towards the south (Figure 4.16).

Fig. 4.16 Image interpretations of Pyay South anticline, 3D view of DEM image (left)

and Google Earth image (right).

4.4.2 Field geological observations

The area is exposed only the Irrawaddy Formation and recent alluvium (Figure

4.19). The Irrawaddy Formation consists of a large number of highly unconsolidated

yellowish-reddish brown and white color sand bodies are intercalated with clay or marl

and gritty to conglomerate beds are tightly ferruginous.

Field measurement structural geological data suggested the Pyay South

anticline is asymmetrical fold which gentle east-flank and steeper west-flank. Dip of

Inn Ma

N

4 km

18

° 2

9'

-

- 95° 19'

39

10° to 35° were measured on the east-flank and west-flank dips are ranging 25° to 45°

at the Latitude of near 18° 26' of the structure. Both flanks of the dips are gradually

decrease to south at the Latitude 19° 25' and the stereographic projection is which

indicates the structure is double plunging to the north and south (Figure 4.19). The

anticline axis is mapped from the sporadic field data and combined with image

interpretation. It is striking approximately northwest-southeast (~340°) direction and

disappear in the alluvium cut-off by the soutweheast-northwest cross fault at the

Latitude of near 18° 23' (Figure 4.19).

Minor folds and other geological structures are difficult to observe in surface

due to the whole area covered by the loosely unconsolidated Irrawaddina sands and

recent alluvium. For this reason, even the major structural features of image interpreted

thrust/reverse fault was not detected in field but the minor faults and joints sets are

observed in Pyay South anticline.

Conjugate normal faults (Figure 4.17) form at a range within 100 ft where the

two normal faults cross each other, the faults are commonly interpreted to accommodate

extension by simultaneous slip on the crossing faults (David et al., 2000).

Fig. 4.17 The conjugated faults ("V-shape") developed by the different hading of

adjacent high angle oblique two normal faults are observed in the Irrawaddy Formation

(Loc. N 18° 26' 13.0" E 95° 18' 26.4" Elev. 368 ft.), (Facing-N).

~100 ft

NW SE

3 ft 1 ft

40

Typically, vertical joints-sets exposure is observed in the conglomerate beds

(Figure 4.18). The joints-sets (joint-1) are striking southeast direction (140°) and

inclined nearly vertical in which spacing between 30-50 ft. and the secondary joints-

sets (joint-2) are crossing north-east direction (60°) and inclination is also nearly

vertical. This joint-sets exposure also suggestion the nearly northeast-southwest

extensional features of the Pyay South anticline.

Fig. 4.18 Nearly equal spacing vertical joint-sets occurred in the conglomerate beds of

Irrawaddy Formation, (Loc. N 18° 29 02.1 E 95° 17' 24.9" Elev. 465 ft), (Facing-SE).

41

Fig. 4.19 Generalized geological map of the Pyay South anticline (Modified from

MOGE, 1965) and stereographic projection of bedding data indicated as the trend of

axis and plunge angle of the Pyay North anticline (left).


Based on the 2D/3D seismic and wells data interpretations, the Pyay South

anticline is asymmetrically on gentle east-flank and steep west-flank and the axis is

striking about 340° and also double plunging anticline (Figure 4.19 and 4.23). The

plunge angle is <10° in which the south is more plunge than the north. Particularly, the

fold is developed disharmonically in the subsurface of Pyawbwe Formation and also

fold character is different on north and south plunges from the crest. Clearly, the any

N

06°/348°

08°/162°

Fold Axis

Poles to planes

Number of bedding

n

42

folded features have not found in approximated Okhmintaung Formation. The fold is

start developed by the symmetrically anticline only on the crest and south plunge but

the north plunge is developed by the monoclinal fold in the Lower Pyawbwe Formation.

Then, the fold is obviously changed from the symmetrical to thrust related asymmetrical

anticline by disharmony near the contact of Pyawbwe and Kyaukkok formations at the

core of anticline. Finally, the fold is completely developed in the Obogon and Irrawaddy

formations by the thrust-bounded and west vergent asymmetrical anticline. The

inclination of axis is nearly parallel with the angle of thrust plane about 45° to the west.

And the crest zone is structurally complex due to the related longitudinal thrust zone is

much closed from the crest and the whole fold is compartmentalized by east-west cross

faults on the both flanks. Along the axis line or the crest zone (Culmination) is

structurally more compelx and highly deformed due to the related major high angle

thrust fault is very near from the west-flank.

The east-flank of anticline consists of two parallel east-north-east striking

conjugated normal faults developed in the Obogon Formation and cross each other in

the Kyaukkok Formation (Figure 4.21). These faults are commonly interpreted to

accommodate northwest-southeast extension by simultaneous slip on the crossing faults

(David et al., 2000).

The steep west-flank is more complex than the east-flank due to the major thrust

fault zone is dominant. The northeast-southwest striking en-echelon parallel faults are

common and predominantly southeast hading features with approximately 50° dip

angle. Although all faults are high angle normal characters, the vertical movement

senses were different each faults on seismic expression. These faults are developed only

in the Miocene Formation and not penetrated the formation boundary of Irrawaddy and

Obogon formations contact and Irrawaddy unconformity. This may indicated as these

en-echelons faults are developed before the Irrawaddy deposition time (Plio-

Pleistocene). Another interpretation or consideration of these en-echelon faults are

developed by not only the northeast-southwest extension but also contains the dextral

strike-slip sense like the Namayan structure.

43

Fig. 4.20 Interpreted 3D view of three dip lines seismic profiles across the Pyay South

anticline where line A,B & C showing the structural styles of folding, major thrust

geometry, cross faults and some flower structural patterns. Significant structural feature

is the disharmonic folding between the Pyawbwe and Kyaukkok formations (Line

locations are shown in Figure 4.19).

2.0

1.0

3.0

4.0

TW

TS

IRR Fm.

PY Fm.

OB/KK Fm.

~ 2.2 km ~ 4.5 km

App

rox

imat

e F

orm

atio

n B

ou

nd

arie

s

Well-50 PSC-2 Well-2

TD 9020 Ft

TD 12335 Ft

TD 6500 Ft

44

Fig. 4.21 Uninterpreted (up) and interpreted (below) strike seismic profile (TWTS)

from the east-flank of the Pyay South anticline showing the northeast-southeast striking

and different hading of high angle oblique sense normal fault developed in the Miocene

formations caused by northwest-southeast extensional feature (Line location shown in

Figure 4.19).

2.0

1.0

3.0

4.0

SOUTH NORTH

2.0

1.0

3.0

4.0

1 km

IRR Fm.

OB/KK Fm.

PY Fm.

OK Fm. (?)

App

rox

imat

ed F

orm

atio

n B

oun

dar

ies

2.0

1.0

3.0

4.0

Tow

-Way

Tra

vel

tim

e (s

)

45

Fig. 4.22 Uninterpreted (up) and interpreted (below) strike seismic profile (TWTS)

from the west flank of anticline showing the northeast-southwest striking and southeast

hading high angle en-echelon normal faults developed in Miocene formations caused

by northwest-southeast extension and also right-lateral shear sense (?) (Line location


4.0

1.0

SOUTH NORTH

3.0

2.0

4.0

1.0

TW

TS

1 km

IRR Fm.

OB/KK Fm.

PY Fm.

OK Fm. (?)

App

rox

imat

ed F

orm

atio

n B

oun

dar

ies

3.0

2.0

2.0

1.0

3.0

4.0

Tow

-Way

Tra

vel

tim

e (s

)

46

Fig. 4.23 Time-slices interpretations on (A) 1.2 s, (B) 1.6 s and (C) 3.5 s below the

surface images showing the structural styles of Pyay S anticline. Major thrusted

anticline caused by northeast-southwest compression and right-lateral shear sense. The

secondary normal cross fault caused by northwest-southeast extension. The square

shapes and triangle heads are indicated that the fault hading.

(A)

(B)

(C)

N

47

4.5 Structural Relations

The Pyay oil field structure lie along the trend of the Pyay anticline. As shown

in Figure 4.26, the structure begins in the Namayan monocline with the northwest-

southeast trend and farther south it little bends westward continues to the Pyay North

and Pyay South anticlinal features of the areas in which the Pyay thrust zone is most

prominent in the west. The anticline is trending southward, slightly asymmetrical, with

the steeper west-flank links with the Shwedaung syncline and gentle east-flank towards

the Pyay Embayment depocenter, and are separated by two saddles to each structures.

The Namayan monocline, Pyay South and Pyay North structures are doubly plunging

anticlines, and terminated in the alluvium at the southern end of Pyay South anticline.

The Namayan monocline is exposed the Pyawbwe, Kyaukkok and Obogon formations

(Miocene) and Irrawaddy Formation (Plio-Pleistocene). The Pyay North anticline is

exposed the Kyaukkok, Obogon and Irrawaddy formations, and the southern part of

Pyay South anticline being covered by only Irrawaddy Formation. About 10 km apart

from the each crest of Namayan, Pyay North to Pyay South structures are probably local

upraises to the north (Namayan monocline) started from the Pyay South anticline

(Figure 4.26). Because of the some hydrocarbon producing sands (4200 ft sand or base

of the Obogon Formation) of Pyay South anticline are exposed in Pyay North anticline,

and the whole thickness of Irrawaddy Formation (1850-5200 ft) is absent in the Pyay

North anticline that may indicated as the uplifted features. The relation between the

anticlines of Pyay South, Pyay North and Namayan monocline are observed by the

intervening cover of poor exposure, but there is said to be subsurface evidence that

Pyay South, Pyay North and Namayan form a continuous anticline in Miocene

Formations, but that a saddle must lie between them in the Plio-Miocene Formations

(Figure 4.26).

The attitude of formations exposed in the Pyay oil field is shown by means of

structure contours (Figure 4.26) and the structural cross-sections on Figure 4.25; and

also show some features of the subsurface structures. These faults are based on 2D/3D

seismic interpretations, but for the black solid lines are used to represent the contours

and also projections of subsurface faults at the present horizon (base of Obogon

formations or 4200 ft sand) of the contoured bed (Figure 4.34). The area along the fault

represented by stippling indicates the part of the fault plane between the displacements

is great enough or where the dip of fault plane is low enough to be shown on map of

this scale.

48

Fig. 4.24 Generalized geological map of the Pyay oil field (center). The stereographic

projection of Namayan (Figure a-b), Pyay North (Figure c-d), and Pyay South anticlines

(Figure e-f) showing the trend of fold axes and plunge angles. Joints strike-rose diagram

(Figure g) and combination of compression directions from Figure a-g indicates the

mean fold axis and compression direction of the whole area (Figure h).

(a)

(b)

(c)

(d)

04°/150°

12°/162°

16°/360°

08°/174°

06°/348°

08°/162°

(e)

(f)

(h)

(g)

N

Fold Axis Poles to planes Number of bedding

n

49

4.6 Pyay Thrust

The surface fault trace of Pyay thrust is 172 km length (Lin Thu Aung; 2014)

from near Kanma town (19° N) to the southern end (18° 20' N) of Pyay oil field (Figure

2.2). Pyay thrust is the most prominent fault in the Pyay oil field and is more or less

parallel with the axis of the Pyay anticline (Figure 4.24). Sharp topography, nearly

vertical to overturned bedding, short and straight drainage features (Figure 4.8 & 4.9)

are the surface fault traces of Pyay thrust in the Pyay oil field area. Most of the area is

covered by loosely Irrawaddian sand and poor exposure of Miocene Formation (Figure

4.24) where no mesoscopic indicators are observed in the present study. The foot wall

block is believed to be major evidence of present day uplifting due to the occurrences

of colluvial deposits (Holocene?) along the west bank of Ayeyarwaddy River while the

small alluvial fan and associated scarps are observed on the hanging wall block along

the Pyay anticline (Lin Thu Aung; 2014).

In the subsurface, the present study can interpreted only for the Pyay oil field

from Pyay city (18° 50' N) to the southern end of Pyay oil field (Figure 4.24). From the

interpretations of at least 4 to 6 second cover two-way traveltime 2D seismic profiles

(TWTS) and 3D cube results, Namayan monocline, Pyay North and Pyay South

anticlines are due to the propagating of Pyay thrust. At the Namayan monocline (Figure

4.5) and North plunge of Pyay North anticline (Figure 4.13 C) where the Pyay thrust is

terminated in the east flank of the bedding plane between below the surface. Thrust

geometry is complicated and bifurcated in the whole fold of Pyay South anticline

(Figure 4.20) and south plunge of Pyay North anticline (Figure 4.13). The first one is

same geometry with the Namayan monocline and another one is vertically going down

to the deep (thrust zone). The present study proposed the Pyay thrust geometry is pure

thrust features in the Namayan monocline, high angle thrust or reverse fault in the Pyay

North anticline and also Pyay South anticline (Figure 4.25).

The present study propose the Pyay thrust terminates in the Southern end of

Pyay oil field (Pyay South anticline). Changes in the amplitude of the anticline may

preserve profile compression as the thrust displacement decrease towards the fault

termination (Figure 4.25). Depth cross section from the seismic profile interpretations

and well data information of the vertical displacements or throws are generally up to

6000 ft in Namayan monocline, 5000 ft in Pyay North anticline, 3000 ft in Pyay South

anticline and decreased towards the south of Pyay oil field (Figure 4.25).

50

Fig. 4.25 Generalized geological map (left, modified from MOGE; 1965) and depth

cross-sections (right) are based on 2D/3D seismic data, wells data and surface geology

where Namayan monocline (A_A'), Pyay North anticline (B_B'), and Pyay South

anticline (C_C') showing the geometry of stratigraphic units, Pyay thrust termination

characters and decreased vertical displacement towards the south.

N

51

4.7 Cross Faults

Generally, northeast-southwest directed cross-faults are common in Pyay oil

field. Although the surface geological observation cannot proved for cross faults in

present study, 3D seismic interpretations of time-slices images (Figure 4.15 and 4.23)

reveals the compartmentalized cross faults patterns on both of Pyay North and Pyay

South anticlines. South-east dipping en-echelon (Figure 4.22) normal faults are

commons and some north-west dipping fault have been observed (Figure 4.21). These

two sets of steeply dipping fault most likely form conjugate set that indicates

northwest–southeast oriented extension. Due to the en-echelon normal faulting with

southeast dipping from north to south, the sediments thickness are thicker to the south

especially in Miocene units (Figure 4.14 and 4.22).

4.8 Time of Deformation

The Irrawaddy Formation, which is assigned to the Pliocene-Pleistocene period,

is involved in the folding of Pyay anticlines (Figure 2.4), the main part of which

preceded in the deposition of the conglomerate beds (upper part of Irrawaddy

Formation) in the Pyay South anticline (Figure 4.18). The amount of erosion that has

occurred in the Pyay North anticline seems sufficient to place the beginning of the

folding somewhere in the Irrawaddy Formation deposition time, and it may have been

contemporaneous with deformation in other areas (eg. Myanaung and Htantabin folds)

of the Pyay Embayment. Stereographic projection of bedding data and joint strike rose-

diagram indicated that the compressional stress direction of folding from 255° (Figure

4.24 h). The active features of Pyay thrust (Lin Thu Aung; 2014) north-south along the

Pyay oil field suggested as the northeast-southwest compression is still active and

deformation is believed to be after folding (Aung Din, MOGE; 1965). Complicated

cross faults deformations have been generally interpreted two time; before Irrawaddy

Formation deposition faulting or Plio-Pleistocene deformation and after deformation.

The present study interpreted that the surface cross-faults or very shallow faults are

accounted after the Plio-Pleistocene folding or Pyay thrust deformation (Figure 4.2).

Below the subsurface of ~1.5 TWTS or more deeper part of faults (especially in

Miocene Formations) are occurred before the Pliio-Pleistocene folding or thrusting

because of these faults have not penetrated the Irrawaddy unconformity and Formation

(Figure 4.21 and 4.22).w

52

4.9 Hydrocarbon Potential

Oil and gas occurs mainly in supra-thrust (east-flank) and sub-thrust (west-

flank) structure of Pyawbwe, Kyaukkok, Obogon and Irrawaddy formations. The

source rock potential for the Pyay oil field is probably Lower Miocene (so-call

Pyawbwe shale) and need to consider the Oligocene age of Okhmintaung Formation.

Commercial accumulation of oil is found in the sub-thrust Kyaukkok formation and gas

in Pyawbwe Formation. In the Pyay South anticline, Kyaukkok, Obogon and Irrawaddy

formations are found to be productive whereas in Pyay North anticline only Kyaukkok

Formation are proved to be hydrocarbon bearing. The structure rises to the north and

producing sands of Pyay South anticline is exposed in the Pyay North anticline at Pyay

well no.1 (Figure 4.26).

Almost all of the hydrocarbon bearing sand layers are trapped by fault bounded

closure at the crest and plunge of the anticline and seal by shale layer of Obogon

Formation. Generally, 31 hydrocarbon bearing sand with a total net pay thickness about

600 ft in Pyay South anticline and 10 hydrocarbon bearing sand with total net pay

thickness about 180 ft is also present in Pyay North anticline (MOGE; 1979). Most of

the sand layers are development and situated in supra-thrust position in both of Pyay

South and Pyay North anticlines. The sub-thrust sand or deeper Pyawbwe Formation

can be prospect in the both of Pyay South and Pyay North anticlines. To the farther

north, the deeper part of Pyawbwe Formation in Namayan monocline can also be new

prospect for the Pyay oil field (Figure 4.26).

53

Fig. 4.26 Seismic and integrated wells data interpretation of N-S strike line A-A' along

the Pyay oil field (left). Generalized structural contour map (right) of the base of Obogon

Formation or current hydrocarbon producing 4200 ft sand (highlighted yellow horizon).

0

1

2

3

4

5 Tow-way traveltime (s)

Approximated Formation Boundaries

Pyaw

bwe

Fm

A'

A

NA

MA

YA

N

PY

AY

NO

RT

H

PY

AY

SO

UT

H

PS

C-2

TD

-12

33

5

50

TD

-65

00

1

TD

-90

20

5 k

m

N S

hw

edaaun

g-1

(P

roje

cted

)

TD

-65

00

Okh

min

taun

g Fm

Kyau

kkok

Fm

O

bogo

n Fm

54

CHAPTER V

CONCLUSIONS AND SUGGESTIONS

5.1 Conclusions

1. The Pyay oil field structure includes three prominent structural features, i.e.,

Namayan monocline, Pyay North and Pyay South anticlines from north to south,

forming northwest-southeast trending and east dipping thrust bounded major

anticline.

2. All structures are double plunging, and disharmonic folding have observed in the

Lower Miocene Pyawbwe Formation and the crest of the anticline is broken by the

Pyay thrust.

3. Both of the gentle east-flank and steeper west-flank are compartmentalized by east-

west cross-faults and are showing normal sense and dipping to the southeast.

4. The Namayan structure reveals the Pyay thrust geometry as evident by the presence

of steeply east-dipping beds along the east-flank.

5. Active mud volcanoes are observed in Pyawbwe Formation at located to the east of

Namayan structure. The mud volcanoes are apparently lie within the Pyawbwe

Formation or that are probably formed along the fault plane of the Pyay thrust,

which forms boundary between the Pyawbwe and Kyaukkok formations.

6. The structural deformation could have started at the Namayan monocline with the

northwest-southeast trend and it is bending westward to the further south, and

continues to the Pyay North and Pyay South anticlines, which are separated by two

saddle structures.

7. Generally, 10 km apart from the crest of Namayan to Pyay North and also to Pyay

South structures was probably uplifted to the north, started from the southern tip of

the Pyay South anticline. It is evident by the facts that some hydrocarbon producing

sand (4200 ft sand or base of the Obogon Formation) of the Pyay South anticline

are exposed in Pyay North anticline.

8. Timing of deformation is accounted for the Pliocene-Pleistocene period which is

evident by the conglomerated beds in upper Irrawaddy Formation, mainly exposed

in the Pyay South anticline. The stereographic projection results and joints strike

rose- diagrams have indicated that the compressional stress direction is from 255°

and the presence of active deformation features of the Pyay thrust along the Pyay

55

oil field, indicating that the northeast-southwest compression is still active and

thrusting is believed to be formed after the folding.

9. Three east-west depth cross-sections across the Pyay oil field structure give the

displacement of Pyay thrust from north to south, i.e., ~6,000 ft in Namayan

monocline, ~5,000 ft in Pyay North anticline, ~3,000 ft in Pyay South anticline, and

the displacement value decreases towards the south of Pyay oil field. The Pyay

thrust is observed to have bifurcated; the shallow one thrust sheet is probably

terminated near the boundary of the Pyawbwe and Kyaukkok formations and the

deeper one is terminated as nearly vertical in the lower part of Pyawbwe Formation

with no significant displacement.

10. Oil and gas occur mainly in both supra-thrust (east-flank) and sub-thrust (west-

flank) structures of the Pyay North and Pyay South anticlines, which are composed

of Pyawbwe, Kyaukkok, Obogon, Irrawaddy formations.

11. Almost all of the hydrocarbon bearing sand layers are trapped by fault bounded

closures at the crest and plunging nose of the anticlines, and are sealed by

intraformational shale layers of the Obogon Formation.

12. The potential source rock of the Pyay oil field is probably Lower Miocene Pyawbwe

Formation. The Oligocene Okhmintaung Formation could also be consider as a

potential source rock.

5.2 Suggestions

1. The surface structural expression and the subsurface 3D seismic interpretation of

the east-west trending cross-faults are very different, especially on the sub-thrust

structure (west-flank) because the west-flank is covered by alluvium. The surface

major east-west cross-faults are referenced from previous geological map of MOGE

(1965). However the present study could not observe any major cross-faults on the

surface. Therefore, the correlation of cross-faults between the surface and

subsurface are plausible in the present study. It is also unable to correlate between

the west-flank faults and east-flank faults because the crest is broken by the Pyay

thrust. If the cross-faults of the two flanks are able to correlate (i.e., continuous or

different faults), timing of deformation on cross-faults and timing of hydrocarbon

migration can be estimated.

56

2. Stratigraphic correlation between east-flank and west-flank of the three structures

(Namayan monocline, Pyay North and Pyay South anticlines) are crucial to better

understand the reservoir continuity for the future exploration activities in the studied

area.

3. The present study can only offer and estimated source rock potential of Pyay oil

field (i.e., Pyawbwe Formation). For that reason, detailed geochemical analysis on

the potential source rocks are also recommended for the future studies.

57

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APPENDIX

APPENDIX 1

Time to depth converted chart using velocity check-shot of the Pyay well no.121 and

Mann well no.636 are as follow:

The Geographical Information System (GIS) of the present study is used the following

parameters:

Projection Datum Distant/Area Units

Latitude/Longitude WGS 1984 Kilometer/Square Kilometer

0

1500

3000

4500

6000

7500

9000

10500

12000

13500

15000

16500

18000

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

Dep

th (

ft)

Two Way-travel Time (s)

Mann Well_636

Pyay Well_121

Linear (Mann Well_636)

Linear (Pyay Well_121)

Documents

Study on the Structural Styles of Pyay Oil Field-2015