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
bedding data indicated as the trend of axis and plunge angle 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
bedding data indicated as the trend of axis and plunge angle of
the Pyay North anticline (left).
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
Figure 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).
44
Figure 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 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
1.1 General Statement
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
Watershed area (Drainage basin)
Watershed area (Drainage basin)
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
3.1 General Statement
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
4.1 General Statement
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
4.3.1 Image interpretations
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
the Pyay North anticline (left).
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).
4.3.3 Subsurface structural interpretations
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
shown in Figure 4.7).
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
4.4.1 Image interpretations
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).
4.4.3 Subsurface structural interpretations
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
shown in Figure 4.19).
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)