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.
SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2016
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.
FALL 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY
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.
SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2016
0
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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
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R² = 0.9499
R² = 0.8793
R² = 0.6109
R² = 0.9847
R² = 0.1945
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R² = 0.7007
R² = 0.972R² = 0.9084
R² = 0.4635
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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).
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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
FALL 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY
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
SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2016
• 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
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& C
2
C3 C3-
2
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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
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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)
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)GR
API
DEP
TH
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)GR
DEP
TH
Fig. 8. Chemostratigraphic correlation proposed for Province 1. Geochemical data acquired for sandstone core samples were used to construct this scheme.
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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.
FALL 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY
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
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)GR
DEP
TH
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)GR
DEP
TH
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)GR
DEP
TH
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)GR
DEP
TH
Fig. 9. Chemostratigraphic correlation proposed for Province 2. Geochemical data acquired for sandstone core samples were used to construct this scheme.
SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2016
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
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)GR
DE
PTH
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)GR
DE
PTH
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)GR
DE
PTH
Fig. 10. Chemostratigraphic correlation proposed for Province 3. Geochemical data acquired for sandstone core samples were used to construct this scheme.
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
Al/(Ca+Mg+K+Na)0 35-
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
Fig. 11. Chemostratigraphic correlation proposed for Province 4. Geochemical data acquired for sandstone core samples were used to construct this scheme.
FALL 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY
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
Al/(Ca+Mg+K+Na)0 25-
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
Al/(Ca+Mg+K+Na)0 18-
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
Fig. 12. Chemostratigraphic correlation proposed for Province 5. Geochemical data acquired for sandstone core samples were used to construct this scheme.
SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2016
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
FALL 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY
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
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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.
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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.