Chemostratigraphic Approach: A Tool to Unravel the ... · A Tool to Unravel the Stratigraphy of the ... Well 1 Well 3 N 0 km 20 SCALE ... correlation in the subsurface. INTRODUCTION

SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2016

ABSTRACT

The Permo-Carboniferous Unayzah Group is generally lacking

in high resolution biostratigraphic control and fails to produce

a stratigraphic correlation using lithostratigraphy, due to its

large similarities with sandstones encountered in both the

Devonian and the Silurian sections.

This study strives to propose correlation schemes for the

Permo-Carboniferous Unayzah Group in central Saudi Arabia

and to define the Unayzah Group and basal Khuff clastics

(BKC) boundaries based on chemostratigraphic analysis.

A total of 1,521 core and cutting samples from 15 wells

Chemostratigraphic Approach: A Tool to Unravel the Stratigraphy of the Permo-Carboniferous Unayzah Group and Basal Khuff Clastics Member, Central Saudi ArabiaAuthor: Dr. Mohamed Soua

Arabian Sea

Gulf of Oman

Gulf of Aden

Arabian G ulf

UAE

Red Sea

MeccaArabian Shield

SUDANSAUDI ARABIA

AR

ERITREA

Kahf

Ghawar

Riyadh

T

Study Area

Permo-CarboniferousUnayzah Province

Province V

Province I

Province IIProvince III Province IV

Well 6

Well 5

Well 4

Well 1

Well 3

N

0 20km

SCALE

Well 2

Well presented in this paper

Well studied in this project

0 300

km

N

Fig. 1. Map displaying the locations of the studied provinces and wells2.

FALL 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY

were subjected to geochemical analysis. A correlation scheme was developed based on specific changes in elemental ratios dealing with glaciogenic, fluvial, eolian and coarse-grained alluvial sediments. These changes occurred in the following key element ratios: zirconium/niobium (Zr/Nb), niobium/ura-nium (Nb/U), (rubidium + cesium)/lanthanum ((Rb+Cs)/La), aluminum/(calcium + magnesium + potassium + sodium) (Al/(Ca+Mg+K+Na)), (zirconium*hafnium)/(niobium*tantalum) ((Zr*Hf)/(Nb*Ta)), (zirconium*hafnium)/niobium ((Zr*Hf)/Nb) and zirconium/(niobium*tantalum) (Zr/(Nb*Ta)).

This study shows that the Ghazal, Jawb, Wudayhi and Tinat members are well characterized chemostratigraphically, being associated with distinct chemozones, and so facilitate correlation in the subsurface.

INTRODUCTION

The Unayzah was first described as a formation in the subsurface of Saudi Arabia in the Hawtah region1, Fig. 1. Ferguson and Chambers (1991)1 documented and defined a threefold lithostratigraphic subdivision of the section in three members, which they labeled, in ascending order, Unayzah C, Unayzah B and Unayzah A. Many subsequent studies adopted this subdivision3, 4. A new member was recognized and defined in many wells located in central Saudi Arabia5 as occurring stratigraphically between the Unayzah B and the Unayzah A members. Generally, the Unayzah occurs within

the Arabian TMS AP5 mega-sequence6. Biostratigraphic dating of the Unayzah is based mainly on

palynological analyses in the subsurface7-9. Price et al. (2008)10 upgraded the Unayzah section to

Group status and subdivided it into the Juwayl and the Nuayyim formations, Figs. 2 and 3. The Unayzah C was re-named the Ghazal member and given a Bashkirian-Gzhelian (Pennsylvanian) age; the Unayzah B was renamed the Jawb member and attributed to the Asselian to early Sakmarian — earliest Permian — age; and finally the Unayzah A was subdi-vided into the Wudayhi and Tinat members and provisionally attributed, based on sparse palynological evidence, to the late Samarian to Kungurian (early Permian) stage.

Previous publications on the Unayzah Group have fo-cused mostly on sedimentological and biostratigraphical analyses, highlighting the challenges in producing regional correlation schemes. The purpose of this article is to assess the Unayzah Group from a chemostratigraphic point of view and strive to produce a regional correlation scheme throughout central Arabia.

GEOLOGICAL SETTING

The study area is situated in the central part of Saudi Arabia to the southeast of the Arabian shield. During the Carboniferous, the Arabian Plate experienced one of the major tectonic events in its history due to the collision of Gondwana and Laurussia,

Bas

hkiri

an -

Gzh

elia

n

CA

RB

ON

IFE

RO

US

PE

RM

IAN

Pen

nsyl

vani

anE

arly

Mid

dle

OSPZ 3

KSA

Palynostratigraphy

OSPZ 1

OSPZ 2

cba

OSPZ 4

OSPZ 5

OSPZ 6

Melvin &Sprague, (2006)

Khuff

Unayzah Amember

Unayzah Bmember

Unayzah Cmember

Unnamed middleUnayzah member

Unayzah Lithostratigraphy

Age

Capitanian

Roadian

Wordian

Kungurian

Asselian

earlySakmarian

Artinskian

Ferguson &Chambers, (1991)

McGillivray &Husseini, (1992)

Al-Husseini,(2004)

Khuff

Unayzah A

Unayzah A

Red Siltstone

Unayzah BUnayzah B Unayzah Bmember

Unayzah C

Unayzah C

LowerUnayzah A

Sub-member

Unayzah Cmember

Upper UnayzahA Sub-member

Base Khuff

Khuff

Senalp & Duaiji,(2001)

UnayzahFormation

JawbFormation

HaradhFormation

KhuffFormation

Ash-ShiqqahFormation

C1

P4

P3

B

A

lateSakmarian

P2

B

A

Khu

ff Fo

rmat

ion

Nua

yyim

For

mat

ion

Juw

ayl F

orm

atio

nU

nayz

ah G

roup

Khuff D

Basal KhuffClastics

TinatMember

WudayhiMember

JawbMember

GhazalMember

Akh

dar G

roup

Un

ayz

ah

Fo

rma

tion

Al Laboun, (1982)

Basal KhuffClastics

Basal KhuffClastics

OmanLithostratigraphy

Saudi Aramco Saudi Aramco Stephenson et al. (2003)

Price et al. (2008)

UN

AY

ZA

H G

RO

UP

Juwayl Formation

Nuayyim Formation

Ghazal

Jawb

Wudayhi

Tinat

Basal KhuffClastics

Fig. 2. Summary of the historical stratigraphic nomenclature of the Unayzah Group.


and the closure of the Rheic Ocean, resulting in the Hercynian deformation event11, Figs. 4a and 4b.

In the subsurface, the Permo-Carboniferous clastic succes-sion gently dips westward and has been developed between the pre-Unayzah unconformity (PUU), which marks the base of the Group, and the pre-Khuff unconformity (PKU), which defines the top. In the studied wells, the Ghazal member (Unayzah C) consists of glaciogenic patch, channelized, in-cised valley fill sequences on the major erosional surface of the PUU1-3, 13. The Jawb member (Unayzah B) consists of a coarse-grained sandstone section showing massive debris flows and braided channel-fill sediments14 that filled irregular surface structures during a rift system following the Hercynian oro-genic event15.

The Tinat member (Unayzah A) is generally interpreted as fluvial, from braided fluvial to flood plain deposits1, 3, 14.

A siltstone member occurring generally between the Tinat and Jawb members approximately 60 m thick has been de-

scribed by Wender et al. (1998)16. It has been defined also as the lower Unayzah A sub-member by Al-Husseini (2004)2, while Melvin and Sprague (2006)5 labeled it the unnamed Middle Unayzah member. Price et al. (2008)10, in their amended nomenclature, defined this interval as the Wudayhi member.

The Khuff formation is predominately carbonates, but typically includes a basal clastic unit — a basal Khuff clastics (BKC) member — consisting of sandstone alternating with marls, shales and some limestones.

MATERIAL AND METHODS

Chemostratigraphy

A total of 1,521 core and cuttings samples were analyzed in the present study. The samples were washed, sieved and mag-netized. Representative cutting fragments were selected and

Limestone

Dolomitic Limestone

Calcarenite

Wakestone

Shale-Claystone

Conglomerate

Siltstone

Siltstone irregular

Dolomite

Sandstone Bedded

Sandstone

Argellaceous Dolomite

Silty Clay, Mudstone

Oil Reservoir

Gas Reservoir

Oil and/or Gas Reservoir

Source Bed

Prospective Source

Prospective Reservoir

Anhydrite

Gypsum

Salts

Basement

Volcanics

E X P L A N A T I O N

PUU: Pre-Unayzah UnconformityPKU: Pre-Khu� Unconformity

Unayzah A

Unayzah B

Sudair

Unayzah C

Upper Lower

Lower

Upper

Tinat

Wudayhi

Jawb

Ghazal

N UAY Y I M

J U WAY L

B ER WAT H

K H U FF

UN

AY

ZA

H

Serpukhovian

GzhelianKasimovian

Moscovian

Bashkirian

Kungurian

Tournaisian

Artinskian

Sakmarian

Asselian

Visean

Changhsingian

Wuchiapingian

CapitainianWordianRoadian

PA

LE

OZ

OIC

PE

RM

IA

NC

AR

BO

NI

FE

RO

US

CIS

URA

LIA

NGUADALU

PIAN

MIS

SISS

IPPI

AN

PENN

SYLVAN

IAN

LOPIN

GIAN

260

270

280

290

300

310

320

330

340

350

H E R C Y N I A N O R O G E N Y H E R C Y N I A N O R O G E N Y

Gla

ciat

ion

Deg

laci

atio

n

G l a c i a l e v e n t s

G E N E R A L I Z E DL I T H O L O G Y

SYST

EM/

PER

IOD

SOU

RCE

INTE

RVAL

SER

IES/

EPO

CH

S T A G E / A G EERA

GR

OU

P

TIM

E (M

a)MEMBER

R E S E R V O I RFORMATION

G E N E R A L I Z E D S T R A T I G R A P H YSEA LEVEL CHANGE

100 -1000.0200300

P K U

P U U

Figure 3

Fig. 3. Lithostratigraphy and age constraint of the Unayzah Group in central Saudi Arabia.


ground to a fine powder. The samples were fused, using high

purity lithium metaborate (LiBO2) flux with uncertainty not

exceeding ± 0.05 mg. Then they were dissolved, using diluted

nitric acid to 1 M.

Data was acquired using inductively coupled plasma-opti-

cal emission spectrometry (ICP-OES) and inductively coupled

plasma-mass emission spectrometry (ICP-MS) machines for the majority of the elements to be detailed. More than 55 el-ements could be identified, including a large number of trace and rare earth elements (REEs) and all the possible major elements. The REEs were subdivided into light rare earth elements (LREEs), middle rare earth elements (MREEs) and heavy rare earth elements (HREEs).

Data for trace elements and REEs are provided as ppm values, while data for major elements are provided as wt% oxide values. A standard reference material was used to as-sess the data quality during the analysis of the core and cut-tings samples. The repeatability of analysis — or analytical precision — was checked, as was the closeness of the “mea-sured” value to the “estimated” value of each element in a sample — or accuracy.

Principal Component Analysis (PCA)

Principal component analysis (PCA) is a statistical technique used to identify important element associations, given that elements occurring together in the same area of the eigen-vector (EV) plots are likely to have similar mineralogical affinities17-22. The principal component scores for the original vectors indicate the association of certain elements as well as the relationship between those elements and mineralogy. The principal element associations are predicted through an inter-pretation of PCA results. Statistical techniques are generally based on binary and ternary diagrams generated through the PCA and the EVs, which aid in the definition of each chemical boundary. These techniques are employed to attach a higher level of confidence to the boundaries set between each of the generated packages (zones). The resulting well-to-well correla-tion scheme is then compared with another previous scheme. After that, it is integrated with previously published sedimen-tological and biostratigraphical interpretations.

Interpretation of Geochemical Data

The geochemical profiles were separated into sandstone and mudrock datasets to minimize problems caused by lithological types or grain size. The second step was to create and use key elemental ratios, which could demonstrate well-defined re-petitive signatures from well to well or from location to loca-tion, for their final identification. These signatures, identified through the elemental ratios, helped to produce hierarchical correlation schemes, including zones, subzones and divisions.

The mineralogical affinities of elements are generally im-portant for understanding the provenance of a given element and its depositional environment. If it is difficult to conduct such an analysis, then this understanding can be achieved by comparing the geochemical and mineralogical data with X-ray diffraction and heavy mineral datasets, for example, or by using graphical and statistical techniques, such as the discrimi-nate function analysis.

300400500

Equator

Late OrdovicianGlaciation

Hercynian Orogeny

480 Ma

442 Ma

270 Ma

235 Ma

15

0

30

45

60

N

S

345 Ma

LATI

TUD

E

TIME (Ma)

305 Ma

Permo-Carb.Glaciation

ORDOVICIAN DEVONIAN CARBONIFEROUS PERMIAN

Glaciation

LATE CARBONIFEROUS: 310 Ma

Tarim

NorthChina

Apulia Taurides Sanandaj-Sirjan

NW Iran

SibumasuHelmand

PALEO-TETHYSOCEAN

Siberia

30 S

30 N

Equator

GondwanaSupercontinent

LaurussiaSupercontinent

HunSuperterrane

CimmeriaSuperterrane

Other

Mid-ocean rift

SubductionUD

Central Iran

Arabia

India

Antarctica

AfricaSouth

America

NorthAmerica

Baltica

Kazakh

Kara

SouthTibet North Tibet

Adria

Pontides

Hellenic-Moesia

MexicanTerranes

HERCYNIAN

OROGEN

URALIANOROGEN

GondwanaGlaciation

SouthPole

PANGEA(Laurussia and

Gondwana)

PANGEA(Laurussia and

Gondwana)

Australia

Rheic Ocean

EXPLANATION

A

B

Fig. 4. Paleogeographic reconstruction and the Permo-Carboniferous glaciation: (a) Paleozoic Arabian Plate with paleolatitude positions reaching about 60° S near 305 Ma (the Permian-Carboniferous), which coincides with the known glaciation period, and (b) Paleogeographic reconstruction of the Permo-Carboniferous Adapted from Ruban et al. (2007)11 and modified after Torsvik and Cocks (2004)12.


0

5

10

15

20

25

30

35

40

45

0 2 4 6 80

500

1000

1500

2000

2500

3000

3500

4000

0 20 40 60 80

0

500

1000

1500

2000

2500

3000

3500

0 10 20 30 400

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4

R² = 0.4635 R² = 0.0251

R² = 0.2605R² = 0.0165

CaO (%) Na2O (%)

K2O (%)

Th (

ppm

)Sr

(ppm

)

Sr (p

pm)

Zr (

ppm

)

Zr (ppm)

C3-2C3-1C2-2C2-1C1

Key

Fig. 5a. Binary diagrams of selected elements.

0

20

40

60

80

100

0 10 20 300

1

2

3

4

5

6

7

0 10 20 300

5

10

15

20

25

30

35

40

0 10 20 30

0

50

100

150

200

250

300

0 2 4 6 8

R² = 0.9499

R² = 0.8793

R² = 0.6109

R² = 0.9847

R² = 0.1945

0

5

10

15

20

25

30

35

40

45

0 2 4 6 80

5

10

15

20

25

30

35

40

45

0 20 40 60 80

0

10

20

30

40

50

60

70

0 1 2 30.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 20 40 60 80

R² = 0.7007

R² = 0.972R² = 0.9084

R² = 0.4635

0

500

1000

1500

2000

2500

3000

3500

4000

0 50 100

R² = 0.9898

SiO

2 (%

)

Al2O3(%) Al2O3(%)

K2O

(%)

Al2O3(%)

Ga

(ppm

)

K2O(%)

Th (

ppm

)

K2O(%)

Rb (

ppm

)

Th (

ppm

)

Nb (ppm)

Nb

(ppm

)

TiO2 (%) Nb (ppm)

Ta (p

pm)

Zr (p

pm)

Hf (ppm)

C3-2C3-1C2-2C2-1C1

Key

Fig. 5b. Binary diagrams of selected elements (continuity).

Fig. 5c. Binary diagrams of selected elements (continuity).

0

5

10

15

20

25

30

35

40

45

0 2 4 60

500

1000

1500

2000

2500

3000

3500

4000

0 20 40 60

0

10

20

30

40

50

60

70

80

0 20 40 60 800

10

20

30

40

50

60

70

80

0 20 40 60

R² = 0.716 R² = 0.329

R² = 0.1635 R² = 0.1856

Th (

ppm

)

Ta (ppm)

Th (

ppm

)

Ta (ppm)

U (

ppm

)

Th (ppm)

U (

ppm

)

Nb (ppm)

C3-2C3-1C2-2C2-1C1

Key


RESULTS

Mineralogy and Geochemistry

Principal components and binary diagrams. In the absence of petrographic or mineralogical data, PCA was used to establish element:mineral links. Many authors have shown that the expectations related to element:mineral links are likely to be accurate, based on the comparison of geochemical and miner-alogical data21, 23-25. Alternatively, the use of binary diagrams, established using a multivariate statistical method, can help to either confirm or approximate the estimated element:mineral links. Links can be estimated when a strong correlation exists between two elements (R2), thereby indicating that a similar mineralogical association is possible17, 23, 26, 27. The diagrams are displayed in Figs. 5a, 5b and 5c.

The PCA method was employed to reduce the total num-ber of variables in the elemental concentration dataset17-22 to a smaller number of variables known as “principal com-ponents”28. In general, principal component-1 and principal

component-2 account for around 80% of the total variation in the entire sandstone elemental concentration. The principal component score assigned to each sample is determined from the EVs. The PCA derived EV plots for the entire sandstone elemental concentration are shown in Fig. 6a. The elements, which are plotted in the same field of data, indicate that they are linked to the same mineralogical affinity.

Figure 6b shows that at least six common element groups — associations — can be depicted through the EV analysis:

• Group 1: Includes silicon (Si), whose concentration in the sandstone is related generally to the abundance of quartz. It is indicative of grain size.

• Group 2: Includes calcium (Ca), magnesium (Mg), manganese (Mn) and strontium (Sr), which are probably associated with the carbonate minerals, such as calcite. As shown in Fig. 5a, the relationship and correlation between Sr and Ca is weak, and therefore, it is as sumed that these two elements share different mineral compositions.

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

-0.1 -0.1 0.0 0.1 0.1 0.2 0.2

MgCa

SrMn Mo

Co

NaU

Be

PW

Ta

TiNbTh

K AlSc

GaTl VFe

CsRb

Cr

Ni

S

Cl

Si

Zr Hf

Y HREE

MREE

LREE

Group 2

Group 6

Group 5

Group 1

Cu

Zn

Br

SnBa

Pb

Group 3

Group 4

EV1

EV2

-20

-10

10

20

5 10

-10 -5 15 20 25 30

Heavy mineral influenceand decreasing grain size

Clay mineral, micas, feldspar influence

PC1

PC2

0

0 5 10A B

Group 7

Fig. 6. EV and PCA cross plots: (a) PC1 vs. PC2, and (b) EV1 vs. EV2, for data derived by PCA of all sandstone samples. All oxides have been abbreviated, i.e., Si = SiO2.

Cross Plot

Group Group of Elements Mineral Association

EV1 vs. EV2

Group 1 Si Quartz

Group 2 Ca, Mg, Mn and Sr Carbonate minerals

Group 3 Nb, Ti, Ta, Th, LREE, MREE, HREE and Y Heavy minerals

Group 4 Na Plagioclase

Group 5 Fe, Cs, V, Rb, Ga, Sc, Al and K Clay minerals, micas, feldspars

Group 6 Zr and Hf Zircon

Table 1. EV1 vs. EV2 cross plot and related groups of elements


• Group 3: Includes niobium (Nb), titanium (Ti), tantalum (Ta), thorium (Th), LREE, MREE, HREE and yttrium (Y), which typically reflect the abundance of the heavy minerals in the group17, 29-31. An analysis should be performed to decide which heavy minerals are controlling the elements. According to Green and Pearson (1983)32 and Klemme et al. (2005)33, MREE and HREE are associated with titanite. In general, titaniferous heavy minerals include Nb and/or Ta34. The presence of a very good correlation between Ti vs. Nb (R2 = 0.90) and Nb vs. Ta (R2 = 0.97), Fig. 5a, suggests that Ti-rich heavy minerals are controlling these elements in the Unayzah Group sediments.

• Group 4: Includes sodium (Na), which is associated in general with the abundance of plagioclase21.

• Group 5: Includes iron (Fe), cesium (Cs), vanadium (V),

rubidium (Rb), gallium (Ga), scandium (Sc), aluminum (Al) and potassium (K), which are indicative of clay minerals, mica and feldspars content. The elements’ concentrations that are plotted in the vicinity of Al, Sc and Ga are considered to be controlled by clay miner als. Typically, Ga and Sc are associated with kaolinite, whereas Rb, Cs and K are more common in illite and smectite21.

• Group 6: Includes zirconium (Zr) and hafnium (Hf), which usually reflect the abundance of a zircon heavy mineral30, 35.

Table 1 summarizes the EV1 vs. EV2 cross plot and its re-lated groups of elements.

The Al vs. Si binary diagram, Fig. 5b, also shows a very weak association between the two elements (R² = 0.19), ex-plained by the fact that Si and Al are mainly concentrated in

WELL - B

Fm

Unay

zah A

UB

100 1500-0 210-100 2400-0 10-0 150API 0 100 Chem

ozon

es

C3-2

C3-1

C2-2

0 4-0 80 0 150 0 80-

C1

& C

3

C2

C1

& C

3

C2 C1 & C2 C3 C3-2

C3-

1

C2-

1b

C2-

1a -

C2-

1c

C3-

1b -

C3-

1d

C3-

1a -

C3-

1c

C2-2 C2-1

Basa

l Khu

ff Clas

tics

Unay

zah

A

WELL - 1

Fm

Basa

l Khu

ff C

last

ics

Una

yzah

AU

nayz

ah B

Una

yzah

C

100 1500-100 2400- 0 210-2 10-0 75 0 120- Chem

ozon

es

C3-2

C3-1d

C3-1c

C3-1bC3-1a

C2-2

C2-1c

C2-1b

C2-1a

C1

0 4-0 100- 20 220 0 70-

C1

& C

3

C2

C1

& C

3

C2

C1

& C

2

C3 C3-

2

C3-

1

C2-

1b

C2-

1a -

C2-

1c

C3-

1b -

C3-

1d

C3-

1a -

C3-

1c

C2-2 C2-1

MA: moving average

Basa

l Khu

ff Clas

tics

Unay

zah

AUn

ayza

h B

Unay

zah

C

MA: moving average

WELL - A

Fm

Bas

al K

huff

Cla

stic

sU

nayz

ah A

Una

yzah

B

Unayzah C

100 1500-100 2400- 0 210-0 10-

DEP

TH

GR

0 150API 0 100 Chem

ozon

es

0 6-0 80 0 180 0 70-

C1

& C

3

C2

C1

& C

3

C2

C1

& C

2

C3 C3-2 C3-

1

C2-

1b

C2-

1a -

C2-

1c

C3-

1b -

C3-

1d

C3-

1a -

C3-

1c

C2-2 C2-1

BK

CU

nayz

ah A

Una

yzah

B

C3-2

C3-1d

C3-1c

C3-1b

C2-2

C2-1c

C2-1b

C2-1a

Lith

olog

ical

subd

ivis

ion

Lith

olog

ical

subd

ivis

ion

Lith

olog

ical

subd

ivis

ion

UB: Unayzah B

Unay

zah

B

WELL B

WELL 1 WELL A

N

0 20km

SCALE

Scal

e

50ft

Si/Al Zr/Nb Zr/Th Nb/U (Rb+Cs)/La

Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)


Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)GR

API

DEP

TH


Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)GR

DEP

TH

Fig. 8. Chemostratigraphic correlation proposed for Province 1. Geochemical data acquired for sandstone core samples were used to construct this scheme.

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

-0.2 -0.1 0.0 0.1 0.2

Ca

Mg

Mn

SrS

P

Pb

BaZn

Cu

Mo

U Na Co Ni

CsFe

Cr

TlBeBrSn

K ScV

RbGa

AlThNbTa

Ti

Cl

LREE

MREE

HREE

Y

HfZr

Si

Group 2

Group 7Heavy Minerals

Group 6 Group 5Group 1

W

Group 3

Group 4

-10

-5

0

5

10

15

20

25

-40 -30 -20 -10

0

10

Carbonate mineral influence

Heavy mineral influence

Clay mineral,micas, feldspar influence

Zircon influence

EV3

EV2

PC2

PC3

0

0.0

Fig. 7. Construction of the correlation scheme through five provinces using six wells. The correlation scheme is based on three chemozones: C1, C2 and C3, in ascending order.


quartz and clay minerals, respectively. While a good correla-

tion is observed in the K vs. Al diagram (R² = 0.61), it was

not interpreted as strong enough to share the same phase. The

Al vs. Ga (R² = 0.94) and K vs. Rb (R² = 0.87) diagrams sug-

gest that these elements are most likely to be concentrated in

clay minerals. In addition, Fig. 5a shows a strong association

between Nb vs. Ta and Ti vs. Nb (R² = 0.97 and R² = 0.90,

respectively). These elements are generally concentrated in

heavy minerals like rutile, anatase and sphene, and in opaque

heavy minerals such as titanomagnetite, magnetite and illmen-

tite32-34. Figure 5c shows a very strong relationship between

Zr and Hf (R² = 0.98), both of which also have strong miner-

alogical affinities with zircon.

Thorium (Th) shows a strong relationship with Ta (R² =

0.70), but a weak relationship to Zr (R² = 0.13), Fig. 5c. It is

possible that Th is linked only with the Ta-bearing heavy min-

erals rather than a variety of heavy minerals. Uranium (U) also

displays a very weak correlation in the Zr vs. U (R² = 0.02) and

U vs. Nb (R² = 0.16) diagrams. Additionally, U displays a poor

correlation coefficient with Th (R² = 0.19), Fig. 5a.

Figure 5a shows no significant trend developed between

Sr vs. Ca (R² = 0.26), which explains why Sr is not to be

linked to the carbonate minerals, although it is associated

with Group 2 in Fig. 6b. In addition, the weak relationship

developed between Sr vs. Na (R² = 0.01) suggests that Sr is

not concentrated in the plagioclase either. The absence of a

stronger correlation suggests that Sr is partly concentrated in

other mineral phases.

Key elemental ratios used. Variations in Zr/Nb, Zr/Th,

(Zr*Hf)/(Nb*Ta), (Zr*Hf)/Nb and Zr/(Nb*Ta) were used in

this study. In general, these ratios are related to changes in

WELL 2

Fm

Basa

l Khu

ff C

lastic

sUn

ayza

h A

100 1500-100 2400- 0 210-2 10-0 150 0 120-

C3-1ii

C2-2

C2-1

C1

0 4-0 100- 20 220 0 4-

Fm

BK

CUn

ayza

h A

Unay

zah

B?

100 1500-100 2400- 0 210-2 10-0 100 0 120-

Che

moz

ones

C3-2

C3-Aii

C3-Ai

C2-2

C2-1

C1

0 4-0 100- 20 220 0 4-

C3-Aii

BK

CU

nayz

ah A

Una

yzah

BU

nayz

ah C

Una

yzah

AU

nayz

ah B

Una

yzah

C

Top Unayzah A

C3-1 is interpreted differently (C3-A).the (Rb+Cs)/La is working reversely.Nb/U doesn’t show similar signature than the other provinces.

Che

moz

ones

C2-1 (Zr*Hf)/(Nb*Ta) is interpreted differently in this province .Nb/U does not show same signature than other provinces.

Fm

Unay

zah

A

100 1500-100 2400- 0 210-2 10-0 100 0 120- Che

moz

ones

0 4-0 100- 20 220 0 4-

WELL E

Fm

BKC

Unay

zah

A

100 1500-0 1000- 0 210-2 10-0 100- 0 120- Che

moz

ones

C1

0 4-0 250- 20 220 0 4-

C3-B

C3-Ai

C2-2

C2-1

C1

C3-Aii

C2-1 (Zr*Hf)/(Nb*Ta) is interpreted differently in this province .Nb/U does not show same signature than other provinces.

C2-1

C2-2

C3-1

C3-2

Unay

zah

AUn

ayza

h B

Unay

zah

CBK

C

(Rb+Cs)/La is interpreted differently in this provinceC2-1 (Zr*Hf)/(Nb*Ta) does not show same signature than other provinces .Nb/U does not show same signature than other provincesUB: Unayzah B

Unay

zah

A

UB

Unay

zah

CBK

CLi

thol

ogic

alsu

bdiv

isio

n

Lith

olog

ical

subd

ivis

ion

Lith

olog

ical

subd

ivis

ion

Lith

olog

ical

subd

ivis

ion

Well E

Well CWell 2

N

0 20km

SCALE

WELL C

Well D

Top Unayzah A

WELL D

Scal

e

50ft


Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)GR

DEP

TH


Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)GR

DEP

TH


Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)GR

DEP

TH


Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)GR

DEP

TH



Elements Ratios Comments

Zr/Nb Zr/NbZircon vs. Nb-bearing heavy minerals, i.e., rutile, anatase, ilmenite,

sphene and magnetite

Zr, Th Zr/Th Zircon vs. Th-bearing heavy minerals

Nb, U Nb/U Nb-bearing heavy minerals vs. U-bearing heavy minerals

Rb, Cs, La (Rb+Cs)/La Rb is mainly linked to clay minerals, micas and K-feldspars

Al, Ca, Mg, K, Na Al/(Ca+Mg+K+Na) Weathering indicator

Zr, Hf, Nb, Ta (Zr*Hf)/(Nb*Ta)Zircon vs. Nb and Ta-bearing heavy minerals, i.e., magnetite, illmenite,

rutile, anatase and/or sphene

Zr, Hf, Nb (Zr*Hf)/NbZircon vs. Nb-bearing heavy minerals, i.e., rutile, anatase, ilmenite,

sphene and magnetite

Zr, Nb, Ta Zr/(Nb*Ta)Zircon vs. Nb and Ta-bearing heavy minerals, i.e., magnetite, illmenite,

rutile, anatase and/or sphene

Table 2. Elements and their mineralogical affinities

C1 is very similar to C2-2

WELL G

-Un

ayza

h A

100 1500-100 2400- 0 210-2 10-0 150 0 120- Che

moz

ones

C3-1

C2-2

C2-1c

C2-1b

C2-1a

C1

0 30-0 100- 20 220 0 4-

WELL F

Fm

-Un

ayza

h A

100 1500-100 2400- 0 210-2 10-0 150 0 120- Che

moz

ones

C2-1

C1

0 30-0 35- 20 220 0 4-

Nb/U does not show similar signature than in Province II

Lith

olog

ical

subd

ivis

ion

Lith

olog

ical

subd

ivis

ion

C3-1

C2-2

Nb/U is acting differently in this province

Unay

zah

AUn

ayza

h B

Unay

zah

C

Unay

zah

BUn

ayza

h C

Unay

zah

A

Well 6

Well F

Well G

N

0 20km

SCALE

C2-2 is used reverserly

WELL 6

Fm

-?U

nayz

ah B

100 1500-100 3400- 0 1000-2 10-0 100- 0 120- Che

moz

ones

C3-2

C3-1c

C3-1b

C3-1a

C2-2

C2-1c

C2-1b

C2-1a

C1

0 4-0 100- 20 220 0 4-

Lith

olog

ical

subd

ivis

ion

Unay

zah

AUn

ayza

h B

Unay

zah

ABK

C

Scal

e

50ft


Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)GR

DE

PTH


Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)GR

DE

PTH


Al/(Ca+Mg+K+Na)

(Zr*Hf)/(Nb*Ta)

(Zr*Hf)/Nb

Zr/(Nb*Ta)GR

DE

PTH


WELL 3

Fm

Unay

zah

A-

Zr/(Nb*Ta)100 1500-

(Zr*Hf)/(Nb*Ta)100 2400-

(Zr*Hf)/Nb0 210-

Nb/U2 10-

Dep

th

GR0 150

Zr/Nb0 120- C

hem

ozon

es

C2-2

C1

Al/(Ca+Mg+K+Na)0 40-

Si/Al0 250-

Zr/Th20 220

(Rb+Cs)/La0 4-

WELL H

Fm

Unay

zah

A

Zr/(Nb*Ta)100 1500-

(Zr*Hf)/(Nb*Ta)100 3600-

(Zr*Hf)/Nb0 350-

Nb/U2 10-

Dep

th

GR0 75

Zr/Nb0 120- C

hem

ozon

es

C2-2

C2-1c


Si/Al0 100-

Zr/Th20 220

(Rb+Cs)/La0 1-

C1

& C

3

C2

C1

& C

3

C2 C1 & C2 C3

C2-

1a -

C2-

1c

C2-1 C2-2

C2-

1b

C2-1b

Unay

zah

AUn

ayza

h B

Lith

olog

ical

subd

ivis

ion

C1

& C

3C2

C1

& C

3

C2 C1 & C2

C2-

1a -

C2-

1c

C2-1 C2-2

C2-

1bC3

C2-1c

Lith

olog

ical

subd

ivis

ion

Unay

zah

AUn

ayza

h B

Unay

zah

C

C2-1b

Well H

Well 3

N

0 20km

SCALE

Scal

e

50ft



provenance and source area. The elements are concentrated in

a variety of heavy mineral suites, such as zircon, rutile, anatase,

ilmenite, sphene and magnetite, which are ultra-stable heavy

minerals36 and therefore unaffected by alteration. The elements

Th, Nb and Ta generally share multiple mineralogical affini-

ties — with both clay minerals and heavy minerals — though

the PCA multivariate statistical method as well as EV analysis

showed their closer association with heavy mineral suites.

In addition, Nb is concentrated in a variety of heavy

minerals, including rutile, anatase, titanite, titanomagnetite

and ilmenite37, while Zr and Hf are typically associated with

zircon35. Close inspection of the PCA analysis shows that Nb

is associated with Ti, Ta and Th (Group 3), which typically

reflects the abundance of heavy minerals17, 29, 30.

Although Th is generally associated with monazite24, it can

also be concentrated in zircon, apatite and opaque heavy min-

erals with variable concentrations25.

The (Zr*Hf)/(Nb*Ta), (Zr*Hf)/Nb and Zr/(Nb*Ta) ratios

also served chemostratigraphic purposes in this study. While

Zr and Hf are considered to constitute most of the zircon

composition, Nb and Ta are associated with Nb-bearing and

Ta-bearing heavy minerals, respectively.

The results of the PCA generally revealed a close associ-

ation between the abundance of Nb and Ta and an element

association composed of Ti and Th. Consequently, changes in

depositional conditions and provenance trends show signifi-

cant variations in the key ratios previously listed: Zr/Nb, Zr/

Th, (Zr*Hf)/(Nb*Ta), (Zr*Hf)/Nb and Zr/(Nb*Ta). Taking

into consideration the proximity of each defined province,

previously seen in Fig. 1, to the related source area, the signa-

ture of these key elemental ratios will differ from one location

to another, Fig. 7.

The (Rb+Cs)/La and Al/(Ca+Mg+K+Na) ratios were also

used in this study. Rb is mainly linked to clay minerals, micas

and K-feldspars, while the ratio Al/(Ca+Mg+K+Na) is typi-

cally used as a weathering indicator17, 21 where values of this

ratio generally increase with the intensity of subaerial weath-

ering. Table 2 summarizes the mineralogical affinities of the

key elements used.

Chemostratigraphy Analysis

In the present study, a threefold hierarchical order scheme

was developed comprising zones, subzones and divisions. This

scheme was based primarily on changes and variations in the

key elemental ratios.

Figures 8 to 12 show the proposed correlation scheme in

the studied wells. The Si/Al ratio is plotted with the key ele-

mental ratios to model the detrital proportion of quartz vs.

clay minerals. Al is associated with clay minerals, including

kaolinite, which reflect increasing grain size. The purpose of

plotting the Si/Al ratio is to constrain the variation in grain

Well 5Well I

Well 4

N

0 20km

SCALE

WELL 5

Member

Wud

ayhi

Jaw

b

Fm

Una

yzah

AU

nayz

ah B

Zr/(Nb*Ta)100 1500-

(Zr*Hf)/(Nb*Ta)100 4500-

(Zr*Hf)/Nb0 600-

Nb/U2 6-

Dept

h

GR0 75

Zr/Nb50 120- C

hem

ozon

es

C1

Al/(Ca+Mg+K+Na)0 8-

Si/Al0 100-

Zr/Th20 100

(Rb+Cs)/La0 4-

C1

& C

3

C2 C1 & C2C3 C3-

2

C3-

1

C2-

1a -

C2-

1c

C3-

1B

C3-

1A

C2-2 C2-1C2C1 &C3 C2-

1b

C2-1

C2-2

C3-1A

C3-1B

Nb/U works reverselyUB: Unayzah B

Unay

zah

AUB

Unay

zah

C

WELL I

Member

Tina

tW

uday

hiJa

wb

Fm

Una

yzah

AU

nayz

ah B

Zr/(Nb*Ta)100 1500-

(Zr*Hf)/(Nb*Ta)100 2400-

(Zr*Hf)/Nb0 210-

Nb/U2 10-

Dept

h

GR0 250

Zr/Nb0 120- C

hem

ozon

es

C3-1B

C2-2

C2-1

C1


Si/Al0 100-

Zr/Th20 220

(Rb+Cs)/La0 8-

Lith

olog

ical

subd

ivis

ion

C1

& C

3

C2

C1

& C

2

C3 C3-

2

C3-

1

C2-

1a -

C2-

1c

C3-

1B

C3-

1A

C2-2C2-1C2C1 &C3 C2-

1b

Zr/(Nb*Ta) works reverselyUndiff.: undifferentiatedUB: Unayzah BBKC: Basal Khuff Clastics(Rb+Cs)/La does not show the same signature elesewhere

Undiff

Unay

zah

AUB

Unay

zah

CBK

C?

WELL 4

Member

Basa

lKh

uff

Clas

tics

Tina

tW

uday

hi

Fm

Bas

al K

huff

Cla

stic

sU

nayz

ah A

Zr/(Nb*Ta)100 800-

(Zr*Hf)/(Nb*Ta)100 2400-

(Zr*Hf)/Nb0 210-

Nb/U2 10-

Dept

h

GR0 150

Zr/Nb0 100- C

hem

ozon

es

C3-2

C3-1

C2-2

C1


Si/Al0 100-

Zr/Th20 200

(Rb+Cs)/La0 5-

C1

& C

3

C2

C1

& C

2

C3 C3-

2

C3-

1

C2-

1a -

C2-

1c

C3-

1B

C3-

1A

C2-2C2-1C2

C1

&C3

C2-

1b

Lith

olog

ical

subd

ivis

ion

Lith

olog

ical

subd

ivis

ion

Unay

zah

AUB

Unay

zah

CBK

C?

UB: Unayzah BBKC: Basal Khuff Clastics

C2-1

Scal

e

50ft



size. The correlation panels shown in Table 3 were produced province-by-province as previously shown in Fig. 1.

Chemostratigraphic zones. Three chemostratigraphic zones were identified in this study using only the sandstone dataset. These zones were labeled C1, C2 and C3 in stratigraphic order.

Zones C1 and C3 generally produce lower Zr/Nb and Zr/Th values than does C2. Through comparison, it is possible to distinguish Zone C3 from Zone C1, particularly basal C1, due to the slightly higher ratio values overall.

The differentiation of the zones is shown in the correlation panels of Figs. 8 to 11, with the principal characteristics of each zone summarized in Fig. 13.

Chemostratigraphic subzones. Two chemostratigraphic sub-

zones each were recognized in Zones C2 and C3. Subzones

C2-1 and C2-2 occur at the base and top of Zone C2, re-

spectively. Subzone C2-2 has higher Zr/(Nb*Ta) ratios than

occurs in the underlying subzone C2-1.

Subzones C3-1 and C3-2 were labeled in ascending strati-

graphic order. C3-1 has the highest (Rb+Cs)/La values. In

Province II, the C3 subzones are labeled C3-A and C3-B in

stratigraphic order, where C3-A has lower (Rb+Cs)/La values.

The principal geochemical characteristics of each subzone are

summarized in Fig. 13.

Chemostratigraphic divisions. Chemostratigraphic divisions

were noted in subzones C2-1 and C3-1. Of the C2-1 divi-

sions, C2-1a occurs at the base and produces higher (Zr*Hf)/

(Nb*Ta) ratios than in the overlying divisions of C2-1b and

C2-1c. Subzone C3-1 is divided into four divisions, labeled

C3-1a to C3-1d in ascending order. C3-1a and C3-1c pro-

duce higher values of (Zr*Hf)/Nb than in C3-1b and C3-1d.

The principal geochemical parameters used to recognize the

chemostratigraphic divisions are summarized in Fig. 13. In

Province II, the divisions C3-1a to C3-1d are labeled C3-Ai,

C3-Aii and C3-Aiii, which are recognized in C3-1 by vari-

ations in the (Zr*Hf)/Nb ratio as well. In Province V, divi-

sions C3-1A and C3-1B are defined by the variations of the

(Zr*Hf)/Nb ratio.

DISCUSSION

During the last two decades, many authors have defined

and adopted different Unayzah subdivisions1-5, 13-15, 31, 38. This

includes the definition of a silty unnamed member5 as a

stratigraphic position occurring between the Unayzah B and

Unayzah A members.

Regional

Province I

Province II

Province III

Province IV

Province V

Zone

Zr/N

b

Zr/T

h

Nb/

U

Subz

one

(Rb+

Cs)/

La

Zr/(

Nb*

Ta)

Subd

ivis

ion

(Zr*

Hf)/

Nb

(Zr*

Hf)/

(Nb*

Ta)

Subz

one

(Rb+

Cs)/

La

Zr/(

Nb*

Ta)

Subd

ivis

ion

(Zr*

Hf)/

Nb

Subz

one

(Rb+

Cs)/

La

Zr/(

Nb*

Ta)

Subd

ivis

ion

(Zr*

Hf)/

Nb

(Zr*

Hf)/

(Nb*

Ta)

Subz

one

Zr/(

Nb*

Ta)

Subz

one

(Rb+

Cs)/

La

Zr/(

Nb*

Ta)

Subd

ivis

ion

(Zr*

Hf)/

Nb

C3 <55 < 100 >8

C3-2 <40 C3-B >2 C3-2 <2 C3-2 <2

C3-1 >40

C3-1d <130

C3-A <2 C3-Aiii <55

C3-1 >2

C3-1d <50

C3-1 >2 C3-1c >130 C3-1c >50

C3-1b <130 C3-Aii >55 C3-1b <50 C3-1b <200 C3-1a >130 C3-Ai <55 C3-1a >50 C3-1a >200

C2 >55 >100 <8

C2-2 <1000 C2-2 <800 C2-2 RGHB

<800 C2-2 <750 C2-2 <800

DILM >450

C2-1 >1000

C2-1c >1300

C2-1 >800 C2-1 RGHB >800

C2-1c >2100

C2-1 >750 C2-1 >800 C2-1b <1300 C2-1b <2100

C2-1a >1300 DILM <450 C2-1a >2100

C1 <55 <100 <8

Fig. 13. Chemical characteristics of the zones, subzones and divisions.

Province Well Name

Province I

Well 1

Well A

Well B

Province II

Well 2

Well C

Well D

Well E

Province III

Well 6

Well F

Well G

Province IVWell 3

Well H

Province V

Well 5

Well 4

Well I

Table 3. Different provinces and wells used in this study


Using palynology, the ages of the various members consti-tuting the terrestrial material-rich Unayzah Group have been constrained. They range from Carboniferous to early Permian.

The majority of authors have had difficulty tracing any valuable correlations along central Saudi Arabia, with most Unayzah samples shown as barren. Of course, this has pre-sented a problem for hydrocarbon exploration, pointing to the general inability to create significant correlation schemes at large basin scales. Since sedimentology along with stratigraphy are unable to correlate barren sandstone sections, chemostra-tigraphy may prove to be a powerful tool when applied to dif-ferent stratigraphic intervals in different locations. Because the geochemical signature of a sediment is generally determined by its composition and mineralogical content, the intensity/strength of these mineralogical factors will alter and modify the initial geochemistry signature of a given sediment37, 39. Heavy mineral assemblages, for example, generate a strong signature of the source rock from where they come. Analysis of the element ratios — Zr/Nb, Zr/Th (Zr*Hf)/(Nb*Ta) and Zr/(Nb*Ta) — used to model the heavy mineral assemblages, in fact, supports the differentiation of the three chemozones: C1 to C3.

Chemostratigraphic Correlation

Interpretation of the geochemical results has led to the estab-lishment of hierarchical chemical stratigraphic schemes for the sandstone datasets, Figs. 8 to 11. As mentioned in the pre-ceding section, three chemostratigraphic zones — C1, C2 and C3 in ascending order — have been defined for the Unayzah Group, which includes the basal part of the BKC sediments.

Zone C1 is generally related to the Ghazal member (Unayzah C), the lithostratigraphic subdivision at the base of the Juwayl formation. It is separated from the underlying strata — generally Silurian-Ordovician sediments in the stud-ied area — by the PUU, also called the pre-Haradh uncon-formity4. The sandstones of Zone C1 can be differentiated from those of the overlying C2 and C3 zones by their low Zr/Nb and Zr/Th ratios, as well as by their low Nb/U values. In Provinces III and V — Wells 5 and 6 — differences in the Zr/Th ratio cannot differentiate C1 from C2, and C1 is similar to C3. Nonetheless, the C1, C2 and C3 chemozones are still distinguishable based on the Zr/N ratio. It may be that the source of the sediments in Provinces III and V was different, and therefore, the signature of the ratio Zr/Th has been char-acterized differently in this area, Figs. 10 and 12.

Zone C2 is roughly equivalent to the Jawb member (Unayzah B), comprising subzone C2-1, and the Wudayhi member (lower Unayzah A), comprising subzone C2-2. The sandstones of Zone C2 have Zr/Nb and Zr/Th values that are significantly higher than those of Zones C1 and C3. Within Zone C2, the subzone C2-2 sediments have Zr/Nb and Zr/Th values marginally higher than the sediments of subzone C2-1 and markedly higher than sediments in the overlying C3-1 subzone. Provenance framework data39 show these

sandstones plotting in the Kaolinite-Chlorite field. The geo-chemical change in the Zr/Nb and Zr/Th ratios, as well as in the Zr*Hf/Nb*Ta, Zr*Hf/Nb and Zr/Nb*Ta parameters, reflects high zircon/rutile ratios. The geochemical and min-eralogical changes shown in this zone are in agreement with data presented by Knox et al. (2010)31. These authors used heavy mineral analysis to argue that in central Saudi Arabia, including the study area, most of the Jawb member (Unayzah B) sandstones were not derived from a reworking of the un-derlying Ghazal member (Unayzah C) sands, but rather repre-sent material transported from elsewhere, and therefore with a different provenance.

Zone C3 is roughly equivalent to the Tinat member (Unayzah A), comprising subzone C3-1, and the BKC mem-ber, comprising C3-2. Zone C3 cannot be recognized in all wells, and in the wells where it is recognized, it is lithologi-cally variable, extending across the PKU, which separates sub-zone C3-1 from subzone C3-2.

The sandstones of Zone C3 have low Zr/Nb and Zr/Th val-ues that are similar to those encountered in Zone C1 — infer-ring that both zones may have a similar source/provenance.

Interestingly, Zone C3 is also characterized by generally higher Al/(Ca+Mg+K+Na) values as well as by the highest (Rb+Cs)/La values — typically found in the C3-1 subzone. This could be interpreted as evidence of intensified weath-ering and a possible reworking of the sandstone. High Al/(Ca+Mg+K+Na) values could also be interpreted as reflect-ing the development of paleosols within the Tinat member (Unayzah A), a development recognized by Melvin et al. (2010)13.

In Well 5, shown in Figs. 7 and 12, subzone C3-2 could not be recognized due to a major unconformity between the Unayzah Group and the Khuff formation that eroded the BKC. In comparison, Wells 4 and 6 have a recorded BKC signature.

Predictive Lithostratigraphic Subdivision

Table 4 shows a summary of the chemostratigraphic zones, subzones and divisions — and the proposed lithostratigraphic

Table 4. Chemostratigraphic zones, subzones and divisions, and their correlation with simplified lithostratigraphic subdivisions for Provinces I, III, II, IV and V

Province Well Name

Province I Well 1 Well A Well B

Province II

Well 2 Well C Well D Well E

Province III Well 6 Well F Well G

Province IV

Well 3 Well H

Province V Well 5 Well 4 Well I

Table 3. Different provinces and wells used in this study

Zones Subzones Divisions Lithostratigraphy

C3

C3-2 BKC

C3-1

C3-1d C3-Aiii C3-1B

Tinat (Upper Unayzah A)

C3-1c C3-Aii C3-1b

C3-Ai C3-1A C3-1a

C2

C2-2 Wudayhi

(Lower Unayzah A)

C2-1

C2-1c Jawb

(Unayzah B) C2-1b C2-1a

C1 Ghazal (Unayzah C)

Table 4. Chemostratigraphic zones, subzones and divisions, and their correlation with simplified lithostratigraphic subdivisions for Provinces I, III, II, IV and V

Province I/III Province II Province IV/V


subdivision correlated to them.Based on the data analysis conducted in the current study,

it is clear that the three chemostratigraphic zones can be rec-ognized on a subregional scale, with Zone C1 being roughly equivalent to the Ghazal member (Unayzah C) and Zone C2 representing the Jawb and Wudayhi members (Unayzah B and lowermost Unayzah A). The Tinat member (Upper Unayzah A) is associated with chemostratigraphic subzone C3-1, while subzone C3-2 is linked to the BKC member of the Khuff for-mation. This is to be expected, as these boundaries are related to more subtle changes in source and/or provenance that may only persist at a field scale rather than at a subregional scale.

CONCLUSIONS

In this study, a series of chemozones have been defined in the Unayzah Group, which can be generally associated with lower rank lithostratigraphic subdivisions, i.e., members. The zones show considerable potential for a high resolution correlation in the subsurface, especially when integrated with sedimentolog-ical and palynological data. The elements and elemental ratios used to define the chemostratigraphic correlation schemes and the chemostratigraphic boundaries in this study include Zr/Nb, Zr/Th (Zr*Hf)/(Nb*Ta), Zr/(Nb*Ta), Al/(Ca+Mg+K+Na) and (Rb+Cs)/La. They reflect changes in sediment provenance and weathering with their strong variation components. These variations led to the differentiation of the three different chemo-zones, labeled C1 to C3 in ascending order.

Zones C1 and C3 are characterized by the lowest values of Zr/Nb and Zr/Th ratios, while Zone C2 is characterized by a high Nb/U ratio. Subzone C2-1 is recognized at the base of Zone C2 and produces higher values of Zr/(Hf*Ta) than in the overlying subzone C2-2. Subzone C3-1 produces gener-ally higher (Rb+Cs)/La values than in the overlying subzone C3-2, while Province II subzone C3-A is characterized by lower (Rb+Cs)/La values than the overlying C3-B subzone. Divisions C2-1a and C2-1c are characterized by higher values of (Zr*Hf)/(Nb*Ta) than in division C2-1b. It is also noted that the divisions C3-1a, C3-1b, C3-1c and C3-1d are defined by the variations in (Zr*Hf)/Nb within them.

Zone C1 is recognized in the Ghazal member; subzone C2-1 is recognized in the Jawb member; subdivision C2-2 is recognized in the Wudayhi member; subdivision C3-1 is recognized in the Tinat member; and finally, subzone C3-2 defines the BKC, which always occurs above the PKU.

ACKNOWLEDGMENTS

The author would like to thank the management of Saudi Aramco for their support and permission to publish this article. The author also would like to thank Marco Vecoli, Mohammed Al-Qattan, Neil Craigie, Michael Pittman, Conrad Allen and Nigel Hooker for their contributions.

REFERENCES

1. Ferguson, G.S. and Chambers, T.M.: “Subsurface Stratigraphy, Depositional History and Reservoir Development of the Early-to-Late Permian Unayzah Formation in Central Saudi Arabia,” SPE paper 21394, presented at the Middle East Oil Show, Bahrain, November 16-19, 1991.

2. Al-Husseini, M.I.: “Pre-Unayzah Unconformity, Saudi Arabia,” in M.I. Al-Husseini (ed.), Carboniferous, Permian

and Early Triassic Arabian Stratigraphy, GeoArabia Special Publication 3, Gulf PetroLink, Bahrain, 2004, pp. 15-59.

3. McGillivray, J.G. and Husseini, M.I.: “The Paleozoic Petroleum Geology of Central Arabia,” American

Association of Petroleum Geologists Bulletin, Vol. 76, No. 10, October 1992, pp. 1473-1490.

4. Senalp, M. and Al-Duaiji, A.A.: “Sequence Stratigraphy of the ‘Unayzah Reservoir in Central Saudi Arabia,” Saudi

Aramco Journal of Technology, Summer 2001, pp. 20-43.

5. Melvin, J. and Sprague, R.A.: “Advances in Arabian Stratigraphy: Origin and Stratigraphic Architecture of Glaciogenic Sediments in Permian-Carboniferous Lower Unayzah Sandstones, Eastern Central Saudi Arabia,” GeoArabia, Vol. 11, No. 4, January 2006, pp. 105-152.

6. Sharland, P.R., Archer, R., Casey, D.M., Davies, R.B., Hall, S.H., Heward, A., et al.: “Arabian Plate Sequence Stratigraphy,” GeoArabia Special Publication 2, Gulf PetroLink, Bahrain, January 2001, 371 p.

7. Owens, B., Filatoff, J.D., Clayton, G. and Al-Hajri, S.: “Evidence of Mid-Carboniferous Miospore Assemblages from Central Saudi Arabia,” in S. Al-Hajri and B. Owens (eds.), Stratigraphic Palynology of the Palaeozoic of Saudi

Arabia, GeoArabia Special Publication 1, Gulf PetroLink, Bahrain, 2000, pp. 154-167.

8. Stephenson, M.H. and Filatoff, J.D.: “Correlation of Carboniferous-Permian Palynological Assemblages from Oman and Saudi Arabia,” in S. Al-Hajri and B. Owens (eds.), Stratigraphic Palynology of the Palaeozoic of Saudi

Arabia, GeoArabia Special Publication 1, Gulf PetroLink, Bahrain, 2000, pp. 168-191.

9. Stephenson, M.H., Osterloff, P.L. and Filatoff, J.D.: “Palynological Biozonation of the Permian of Oman and Saudi Arabia: Progress and Challenges,” GeoArabia, Vol. 8, No. 3, January 2003, pp. 467-496.

10. Price, R., Norton, A.K., Melvin, J., Heine, C.J., Filatoff, J.D., Sprague, R.A., et al.: “Saudi Aramco Permian-Carboniferous (Unayzah) Stratigraphic Nomenclature of Saudi Arabia,” paper 118714, presented at the 8th Middle East Geosciences Conference and Exhibition, Bahrain, March 3-5, 2008.


11. Ruban, D.A., Al-Husseini, M.I. and Iwasaki, Y.: “Review of Middle East Paleozoic Plate Tectonics,” GeoArabia, Vol. 12, No. 3, January 2007, pp. 35-56.

12. Torsvik, T.H. and Cocks, L.R.M.: “Earth Geography from 400 to 250 Ma: A Palaeomagnetic, Faunal and Facies Review,” Journal of the Geological Society, Vol. 161, No. 4, July 2004, pp. 555-572.

13. Melvin, J., Sprague, R.A. and Heine, C.J.: “From Bergs to Ergs: The Late Paleozoic Gondwanan Glaciation and Its Aftermath in Saudi Arabia,” Special Paper of the

Geological Society of America, Vol. 468, August 2010, pp. 37-43.

14. Senalp, M. and Al-Duaiji, A.: “Stratigraphy and Sedimentation of the Unayzah Reservoir, Central Saudi Arabia,” American Association of Petroleum Geologists

Bulletin, Vol. 79, No. 8, August 1995, pp. 837-847.

15. Aktas, G.G., Al-Khalifa, M. and Al-Qassab, H.: “Sequence Stratigraphy and Reservoir Architecture of Unayzah Continental Deposits, Hawtah Trend, Saudi Arabia,” GeoArabia, Vol. 5, No. 1, 2000, p. 11.

16. Wender, L.E., Bryant, J.W., Dickens, M.F., Neville, A.S. and Al-Moqbel, A.M.: “Paleozoic (pre-Khuff) Hydrocarbon Geology of the Ghawar Area, Eastern Saudi Arabia,” GeoArabia, Vol. 3, No. 2, January 1998, pp. 273-302.

17. Pearce, T.J., Wray, D.S., Ratcliffe, K.T., Wright, D.K., Moscariello, A. and Maritime, C.: “Chemostratigraphy of the Upper Carboniferous Schooner Formation, Southern North Sea,” in J.D. Collinson, D.J. Evans, D.W. Holliday and N.S. Jones (eds.), Carboniferous Hydrocarbon

Geology: The Southern North Sea and Surrounding

Onshore Areas, Occasional Publica tions series 7, Yorkshire Geological Society, Leicester, U.K., 2005, pp. 147-164.

18. Svendsen, J., Friis, H., Stollhofen, H. and Hartley, N.: “Facies Discrimination in a Mixed Fluvio-Eolian Setting Using Elemental Whole-Rock Geochemistry — Applications for Reservoir Characterization,” Journal of

Sedimentary Research, Vol. 77, No. 1, January 2007, pp. 23-33.

19. Ellwood, B.B., Tomkin, J.H., Ratcliffe, K.T., Wright, M. and Kafafy, A.M.: “Magnetic Sus ceptibility and Geochemistry for the Cenomanian/Turonian Boundary GSSP with Correlation to Time Equivalent Core,” paper 122-3, presented at the GSA Denver Annual Meeting, Denver, Colorado, October 28-31, 2007.

20. Pe-Piper, G., Triantafyllidis, S. and Piper, D.J.W.: “Geochemical Identification of Clastic Sediment Provenance from Known Sources of Similar Geology: The Cretaceous Scotian Basin, Canada,” Journal of

Sedimentary Research, Vol. 78, No. 9, September 2008, pp. 595-607.

21. Ratcliffe, K.T., Wright, A.M., Montgomery, P., Palfrey, A., Vonk, A., Vermeulen, J., et al.: “Application of

Chemostratigraphy to the Mungaroo Formation, the Gorgon Field, Offshore Northwest Australia,” Australian

Petroleum Production and Exploration Association

Journal, 50th Anniversary Issue, 2010, pp. 371-388.

22. Sano, J.L., Ratcliffe, K.T. and Spain, D.R.: “Chemostratigraphy of the Haynesville Shale,” in U. Hammes and J. Gale (eds.), Geology of the Haynesville

Gas Shale in East Texas and West Louisiana, USA, AAPG Memoir series 105, American Association of Petroleum Geologists, Tulsa, Oklahoma, 2013, pp. 137-154.

23. Pearce, T.J., Besley, B.M., Wray, D.S. and Wright, D.K.: “Chemostratigraphy: A Method to Improve Interwell Correlation in Barren Sequences — A Case Study Using Onshore Duckmantian/ Stephanian Sequences (West Midlands, U.K.),” Sedimentary Geology, Vol. 124, No. 1, March 1999, pp. 197-220.

24. Mange, M.A. and Morton, A.C.: “Geochemistry of Heavy Minerals,” in M.A. Mange and D.T. Wright (eds), Heavy Minerals in Use, Developments in Sedimentology series 58, Elsevier Science, Amsterdam, The Netherlands, 2007, pp. 345-391.

25. Craigie, N.W.: “Applications of Chemostratigraphy in Cretaceous Sediments Encountered in the North Central Rub’ al-Khali Basin, Saudi Arabia,” Journal of African

Earth Sciences, Vol. 104, April 2015, pp. 27-42.

26. Ratcliffe, K.T., Hughes, A.D., Lawton, D.E., Wray, D.S., Bessa, F., Pearce, T.J., et al.: “A Regional Chemostratigraphically Defined Correlation Frame work for the Late Triassic TAG-I Formation in Blocks 402 and 405a, Algeria,” Petroleum Geoscience, Vol. 12, No. 1, February 2006, pp. 3-12.

27. Ratcliffe, K.T., Morton, A., Ritcey, D. and Evenchick, C.E.: “Whole-Rock Geochemistry and Heavy Mineral Analysis as Petroleum Exploration Tools in the Bowser and Sustut Basins, British Columbia, Canada,” Bulletin of

Canadian Petroleum Geology, Vol. 55, No. 4, December 2007, pp. 320-336.

28. Shaw, P.J.A.: Multivariate Statistics for the

Environmental Sciences, Hodder Arnold, London, U.K., 2003, 233 p.

29. Morton, A.C. and Hallsworth, C.: “Identifying Provenance-Specific Features of Detrital Heavy Mineral Assemblages in Sandstones,” Sedimentary Geology, Vol. 90, Nos. 3-4, May 1994, pp. 241-256.

30. Armstrong, H.A., Turner, B.R., Makhlouf, I.M., Weedon, G.P., Williams, M., Al Smadi, A., et al.: “Origin Sequence Stratigraphy and Depositional Environment of an Upper Ordovician (Hirnantian) Deglacial Black Shale, Jordan,” Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 220, Nos. 3-4, May 2005, pp. 273-289.

31. Knox, R.W., Cocker, J.D. and Filatoff, J.D.: “Heavy


Mineral Stratigraphy of the Unayzah Formation and Basal Khuff Clastics (Carboniferous to Permian) of Central Saudi Arabia,” GeoArabia-Middle East Petroleum Geosciences, Vol. 15, No. 3, January 2010, pp. 17-80.

32. Green, T.H. and Pearson, N.J.: “Effect of Pressure on Rare Earth Element Partition Coefficients in Common Magmas,” Nature, Vol. 305, September 1983, pp. 414-416.

33. Klemme, S., Prowatke, S., Hametner, K. and Günther, D.: “Partitioning of Trace Elements between Rutile and Silicate Melts: Implications for Subduction Zones,” Geochimica et Cosmochimica Acta, Vol. 69, No. 9, May 2005, pp. 2361-2371.

34. Schock, H.H.: “Distribution of Rare-Earth and Other Trace Elements in Magnetites,” Chemical Geology, Vol. 26, Nos. 1-2, July 1979, pp. 119-133.

35. Fujimaki, H.: “Partition Coefficients of Hf, Zr, and REE between Zircon, Apatite, and Liquid,” Contributions to Mineralogy and Petrology, Vol. 94, No. 1, September 1986, pp. 42-45.

36. Pettijohn, F.J., Potter, P.E. and Siever, R.: Sand and Sandstone, 2nd edition, Springer-Verlag, New York, 1987, 553 p.

37. McLennan, S.M.: “Rare Earth Elements in Sedimentary Rocks: Influence of Provenance and Sedimentary Processes,” Reviews in Mineralogy and Geochemistry, Vol. 21, No. 1, January 1989, pp. 169-200.

38. Al-Qassab, H.M., Fitzmaurice, J., Al-Ali, Z.A., Al-Khalifa, M.A., Aktas, G.A. and Glover, P.W.: “Cross-Discipline Integration in Reservoir Modeling: The Impact on Fluid Flow Simulation and Reservoir Management,” SPE paper 62902, presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, October 1-4, 2000.

39. Fralick, P.W. and Kronberg, B.I.: “Geochemical Discrimination of Clastic Sedimentary Rock Sources,” Sedimentary Geology, Vol. 113, Nos. 1-2, October 1997, pp. 111-124.

40. Soua, M.: “Chemostratigraphy and Provenance of the Carboniferous-Permian Unayzah Group and Basal Khuff Clastics Deposits Encountered in Central Arabia,” paper presented at the 12th Middle East Geosciences Conference and Exhibition, Bahrain, March 7-10, 2016.

BIOGRAPHY

Dr. Mohamed Soua joined Saudi Aramco in 2014 as the Chemostratigraphy Lab Coordinator working in the Exploration Technical Services Department, where he is involved in many projects. Prior to this, Mohamed worked at Entreprise

Tunisienne d’Activites Petroliere (ETAP), the Tunisian national oil company, from 2007 to 2012 as Chief Geologist, and then in 2012, he became an Associate Professor in the Faculty of Sciences of Tunis (Tunis-El Manar University), teaching petroleum geology, paleogeography, paleontology and geophysics.

Mohamed is also an editor of the Arabian Journal of Earth Sciences, the Journal of Geosciences and Geomatics and the Journal of Open Transactions on Geosciences.

He has published more than 60 papers, book chapters and technical reports in several geological fields, including chemostratigraphy, biostratigraphy (foraminifera and radiolarians), cyclostratigraphy (orbital forcing and time-series analysis) and sequence stratigraphy.

Mohamed’s professional activities are focused on the oceanic anoxic events of the Earth’s Phanerozoic history and the integration of sedimentological as well as biostra-tigraphic data to enhance the knowledge of the mechanism leading to these events.

He is a member of the Society of Petroleum Engineers (SPE) and the Tunisian Association of International Geological Studies (ATEIG).

Mohamed received his B.S., M.S. and Ph.D. degrees from Tunis-El Manar University, Tunis, Tunisia, with the cooperation of Lille 1 University, Lille, France.

Documents

Chemostratigraphic Approach: A Tool to Unravel the ... · A Tool to Unravel the Stratigraphy of the ... Well 1 Well 3 N 0 km 20 SCALE ... correlation in the subsurface. INTRODUCTION