Energy & Fuels 1994,8, 1425-1442
1425
Organic Geochemistry of the Paleozoic Petroleum System of Saudi Arabia G. A. Cole,* M. A. Abu-Ali, S. M. Aoudeh, W. J. Carrigan, H. H. Chen, E. L. Colling, W. J. Gwathney, A. A. Al-Hajji, H. I. Halpern, P. J. Jones, S. H. Al-Sharidi, and M. H. Tobey The Saudi Arabian Oil Company, Box 62, Dhahran 31311, Saudi Arabia Received May 3, 199@
The Paleozoic petroleum system of the Central and Northern Provinces of Saudi Arabia contains one of the most prolific oil-prone source rocks of Silurian age worldwide, the Qusaiba shale of the Qalibah Formation. This source rock is responsible for charging the large, gentle structures of the Central Province with billions of barrels of light, sweet, crude oil. Organic geochemical methods such as biomarkers, organic petrography, and characterization of the source rock facies have been used to determine the most likely candidate source rock for the Paleozoic oils of Saudi Arabia. By using detailed extract and oil characterizations, the basal “hot” shales of the Qusaiba Member of the Silurian Qalibah Formation have been correlated to the oils reservoired in the Permian sands of the Central Province of Saudi Arabia. The Qusaiba shales consist of light to dark gray, clastic, marine shales and can be divided into two main zones: a low gamma-ray response and a high gamma-ray response, both having their own organic geochemical signatures. The low gamma ray zone has almost no source potential and comprises most of the Qalibah shale sequence. The shales within this sequence are light gray, have poor to moderate organic richness and little to no generative potential, and contain an organic matter assemblage deposited under mostly oxic, marine conditions. The organic matter type consists of amorphous matter, marine algae (phytoplankton), liptodetrinite, inertinite, and occasional chitinozoans and graptolites. The hot gamma-ray zone is located at the base of the Qusaiba Member of the Qalibah Formation and consists of dark gray to black, organic-rich, oil-prone, marine shales. The kerogen assemblage consists of abundant amorphous matter, marine algae, acritarchs, and abundant chitinozoans and graptolites. Carbon isotopes and sterane biomarkers resulted in an excellent correlation between the basal Qusaiba shales and the oils. No other potential source rocks correlated as well, nor did these other potential sources contain the source rock quality, or vertical and lateral regional persistence as the Silurian basal Qusaiba shales.
Introduction Saudi Arabia is without question one of the most prolific oil producing regions of the world where approximately 26% of the world‘s known oil reserves are found. Two-thirds of the oil reserves are found in Jurassic carbonate reservoirs. The remaining hydrocarbons (oil and gas) are found in Cretaceous and Upper Paleozoic reservoirs of the Arabian Basin and the central part of Saudi Arabia (Figure 1). Excellent conditions existed for source rock, reservoir rock, and cap rock development during deposition of the Paleozoic succession of Saudi Arabia. Several previous publications have described the regional tectonic and depositional settings of the important parameters required to form the regionally extensive Silurian to Permian petroleum These publications describe the formation of the potential source rock system, the @Abstractpublished in Advance ACS Abstracts, July 15, 1994. (1)Powers, R. W.; Ramirez, L. F.; Redmond, C. D.; Elberg, E. L. Jr. U.S.G.S. Prof. Paper 560-D,1966,147. (2)al-Laboun, A. A. AAPG Memoir 40, 1986,373-390. (3)Beydoun, Z.R. The Middle East: Regional Geology and Petroleum Resources: Scientific Press: London, 1988,292. (4)Beydoun, Z.R. AAPG Stud. Geol. 33, 1991,No.33,76. (5)Husseini, M. I. AAPG Bull. 1991,75,108-120. (6)Mahmoud. M.D.;Vaslet, D.: Husseini, M. I. AAPG Bull. 1992, 76,1491-1506. (7)McGillivray, J. G,Husseini, M. I. AAPG Bull. 1992,76,14731490.
0887-0624/94/2508-1425$04.50/0
*//4//
Ousaiba Subcrop Ousalba Outcrop
Figure 1. Location m a p showing locations of rock and oil samples discussed in this paper. Line A-B-C-D is the location of the Central Area cross-section illustrated in Figure 24. The maturity contours are determined on the Silurian basal Qusaiba shale unit.
reservoir facies, and the carbonate seals. Other publications have discussed the relationships of source rocks and the various oil a ~ c u m u l a t i o n sFigure . ~ ~ ~ ~2~shows (8)Alsharhan, A. S; Kendall, C. G. ST.C. AAPG Bull. 1986,70,9771002. (9)Abu-Mi, M.A,; Franz, U. A,; Shen, J.; Monnier, F.; Mahmoud, M.D.;Chambers, T. M. Soc. Pet. Eng. 1991,SPE 21376,345-355.
0 1994 American Chemical Society
Cole et al.
1426 Energy & Fuels, Vol. 8, No. 6, 1994
I
PALEOZOIC ROCKS OF SAUDI ARABIA
I
Figure 2. Generalized stratigraphic column for the Paleozoic sequence in Saudi Arabia showing ages, formations, lithology, reservoirs (O), and source rocks (Wj, for the key components of this petroleum system Reprinted with permission from ref 7 . Copyright The American Association of Petroleum Geologists 1992.
a generalized Paleozoic stratigraphic column for the Central and Northern Areas of Saudi Arabia and identifies the primary source rock, reservoirs, and seals that form this petroleum system. Potential source rocks are distributed throughout much of the Phanerozoic in the Arabian Plate r e g i ~ n . ~ - ' ~ Ayres et a l l 2 considered the Callovian to Oxfordian interval of the Tuwaiq Mountain and Hanifa Formations to be the principal sources of the oil contained (10) Christensen, R. M.; Wilson, A. 0.; Kushnir, D. W.; Slentz, L. W. Confidential Chevron Report to Saudi Aramco 1977, 141 pp. (11)Ala, M. A.; Kinghorn, R. R. F.; Rahman, M. J . Pet. Geol. 1980, 3, 61-89.
(12) Ayers, M. G.; Bilal, M.; Jones, R. W.; Slentz, L. W.; Tartir, M.; Wilson, A. 0. AAPG Bull. 1982, 66, 1-9. (13) Stoneley, R. Classic Petroleum Provinces: Geological Society Special Publication No. 50, 1990, 293-298. (14) Droste, H. Sedimentary Geol. 1990, 69,281-296. (15) Cole, G. A,; Carrigan, W. J.; Colling, E. L.; Halpern, H. I.; AlKhadhrawi, M. R.; Jones, 1993, P. J. CSPG Memoir 17, in press.
within the southern part of the Eastern Province. In the Central Area the primary Paleozoic source rock is the high gamma response marine shale zone at the base of the Silurian Qusaiba Member of the Qalibah Format i ~ n . ~Additional , ~ , ~ source rocks may be present within the Paleozoic succession such as the Ordovician Hanadir, Devonian Jauf, and the Permian Unayzah and Khuff, but these potential source rocks are not as thick and/or regionally extensive as the Silurian basal Qusaiba. This paper has several primary objectives which are to (1)describe the most likely Paleozoic source rocks that generated, expelled, and charged the Permian sand (Unayzah Formation) and carbonate (Khuff Formation) reservoirs; (2) determine which source rock best correlates with the reservoired oils in the central part of Saudi Arabia; and (3) model the potential source rock or rocks to determine the timing of generation and
Energy & Fuels, Vol. 8, No. 6, 1994 1427
Paleozoic Petroleum System of Saudi Arabia
,?- ,
c o n t o u ~a m in meters
Figure 5. Isopach map of the basal “hot” shales of the Qusaiba Member, Silurian Qalibah formation (modified from McGillivray and Husseini‘).
IGlaciogenic Diamictites
A Cold Water Fauna
.Antarctic
Fauna
Figure 3. Position of Saudi Arabia during the Early Paleozoic as part of Gondwandand (after McGillivray and Husseini‘ and Husseini;21reprinted by permission. Copyright The American Association of Petroleum Geologists 1992).
30‘s
Margnrl Msrne Sheif fine Claalim
\
Excellent Clastic Source Rack Development Barinwards (slowand disla positions1
Marine Ciaslics
Figure 4. Paleogeographic map of eastern Saudi Arabia for early Silurian time (modified from Ziegler et ~ l . , Mahmoud ~l et ~ 1 . ~ 1 .
expulsion. Figure 1 shows the locations of the samples used in this study.
Geologic Setting Throughout the Paleozoic, the northeast margin of the Afro-Arabian Plate evolved as an extensive, stable, slowly subsiding continental shelf along the margin of Gondwana. The location and paleogeographic setting of the Central and Northern Areas during late Ordovician through Silurian time are shown on Figure 3 and in more detail in Figure 4. The Middle to Late Ordovician sedimentation consisted of littoral to prodelta marine clastic sequence~.~ Two coarsening upward sequences are recognized within the Qasim Formation. The lowermost shale unit, the Hanadir Member, represents the initial transgression following Saq Formation deposition and contains some thin beds which attain moderate organic richness and potential. The overlying Kahfah Member of the Qasim Formation comprises mostly coarse clastics deposited during a regressive cycle. Following Kahfah deposition, another short transgressive episode occurred and re-
sulted in the deposition of the shales of the Ra’an Member of the Qasim Formation, which was capped off by the Quwarah Member sands during late Ordovician time. During the late Ordovician, the polar glaciers expanded across Gondwana and covered most of Saudi Arabia.16J7 The advance and retreat of the glaciers resulted in the deposition of the Sarah and Zarqa Formations. These two formations represent glacial and periglacial sequences and consist of tillites and proglacia1 sands (Zarqa Formation) and finer sands in the Sarah F o r m a t i ~ n . ~ J By ~ J ~Early Silurian time, the retreat and melting of the glaciers resulted in an abrupt sea-level rise and the deposition of the upward-coarsening Qalibah Formation in Saudi Arabia.20,21The lowermost unit, the Qusaiba Member, consists of two distinct units that are regionally present. The lowermost unit is a relatively thin (6-75 m thick as shown in Figure 5) radioactive “hot”shale which was deposited during the maximum flooding event of this transgression. These shales are dark gray t o black, organic-rich, and oil-prone where immature. Overlying the “hot” shale unit is a thick sequence of nonradioactive, light to medium gray shale that contains poor to moderate organic richness and mixed oil and gas potential. Overlying the Qusaiba Member is the siltier shale sequence belonging to the Sharawra Member. The Sharawra Member is disconformably overlain by the Devonian Tawil Formation sandstone which resulted from a sea-level drop during the Late Silurian.22 Following deposition of the Tawil braided stream and littoral marine sediments, another minor sea-level rise occurred and resulted in the sedimentation of the marine Devonian Jauf Formation. The Jauf consists of interbedded mixed clastics with some small carbonate units. The lowermost Sha’iba Member contains some thin organic-rich, mixed oil and gas-prone beds. The final depositional episode before the Hercynian event (16)McClure, H. A. Paleogeogr. Paleoclimat. Paleoecol. 1978,25, 315-326. (17)McClure, H.A.;Hussey, E.; Kaill, I. Geologisches Jahrbuch, Reihe B 1988,68,3-31. (18)Vaslet, D.,Kellogg, K. S.; Berthiaux, A.; Le Strat, P.; Vincent, P. L. Saudi Arabian Deputy Ministry for Mineral Resources Geoscience Map GM-116A, 1987,scale 1:250000. (19)Vaslet, D.Episodes 1990,13, 147-161. (20)Vaslet, D. Saudi Arabian Directorate General for Mineral Resources Technical Record BRGM-TR-07-1 1987,24. (21)Husseini, M.I. J . Pet. Geol. 1990,13, 267-288. (22)Vail, P.R.;Mitchum, R. M. Jr.; Thompson 111,S. AAPG Memoir 1977,26,83-97.
CoZe et al.
1428 Energy & Fuels, Vol. 8, No. 6, 1994
Table 1. Source Rock Interpretation Guide" total organic carbon (wt %) SZ pyrolysis yield (mg of HC/g of rock)
poor
moderate
good
very good
excellent
4.0 220.0
40.0 I
S2 Yield (mg HClg rock) h
150
E
125
* 100
0
c al a
U
75
E 50 25 0
Hydrogen Index Figure 7. Total organic carbon (% TOC), Sz pyrolytic yield (in mg hydrocarbondg rock), and hydrogen index (in mg of
hydrocarbondg of TOC) distributions for the Paleozoic sequences of Saudi Arabia. matter assemblages as based on visual kerogen assessment30,34,35 of these potential source rocks were as follows: 1. Qusaiba Member, Qalibah Formation: These marine, clastic shales, consisted dominantly of amorphous organic matter which was 250% fluorescent, but also contained large amounts of chitinozoans greater than graptolites. The remainder of the organic matter was (34) Burgess, J. D. Carbonaceous Materials as Indicators of Metamorphism; GSA Special Paper 153, 1974, pp 19-30. (35) Batten, D. J. Organic Maturation Studies and Fossil Fuel Exploration; Academic Press: London, 1981;pp 201-224.
composed of acritarchs and liptinites. There was some minor inertinitic material, but this was difficult to discern from the graptolite debris in the strew mounts. 2. Ordovician Hanadir Member, Qasim Formation: The organic-rich shales consisted of abundant, partially fluorescent amorphous matter with subordinate amounts of algal spores, acritarchs, and inertinites. 3. Devonian Jauf Formation: The organic-rich shales were highly variable in their organic matter constituents. The more oil-prone samples consisted of abundant, fluorescent amorphous matter with subordinate liptinite and humic matter. Minor bituminites and
Paleozoic Petroleum System of Saudi Arabia 8
Energy & Fuels, Vol. 8, No. 6,1994 1431 l2
Khuff/Unayzah
b
11 n
+
Upper Qusaiba Shales
j
10
A
0
9
0
L a
HI = 200
f u
1
7
F ;
.
W
9 a
5
4
t u 3 v ) 2
1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 0.0 0.5
3.5
Total Organic Carbon (wt. Yo) Figure 8. Cross plot between Sz pyrolytic yield and % TOC for the KhufWnayzah Formations. Most samples contain poor t o moderate source quality (0.6%VRe) samples plot along a gas-prone trend with a slope-derived HI of 137, and a third set are inert and fall along the x-axis.
Tawil, and Silurian Sharawra, (3) the Silurian Upper Qusaiba, (4)the Silurian basal Qusaiba “hot” shales, and ( 5 ) the Silurian-OrdovicianS a r M a r q a , Ordovician Qasim, and Ordovician Saq. Figure 7 illustrates histograms for the organic richness (% TOC), the potential productivity (Sa yields from Rock-Eva1 pyrolysis), and the hydrocarbon potential (hydrogen index for oil or gas proneness). The corresponding data are listed in Table 2. Total Organic Carbon (TOC;Organic Richness). As observed in the TOC histograms, there are some organicrich beds throughout most of the Paleozoic sequence. However, if samples with l . O % W e ) and have an average TOC of 2.7%. Based on the total organic carbon distributions (Figure 71, the most organic rich sequence identified within the Paleozoic succession of Saudi Arabia is the basal Qusaiba shale unit which also coincides with a high gamma ray log response. This organic rich marine clastic shale unit is regionally extensive (Figure 5) and thick (exceeds 75 m) and is interpreted as representing the maximum flooding event during marine transgression. These shales maintain good to excellent TOC whether in an immature setting, or a mature setting. Potential Productivity (Sz Pyrolytic Yield). As clearly observed from S2 yield histograms (Figure 7), there are some productive beds throughout most of the Paleozoic sequence that have enough S2 yield to be considered potential source rocks. Because hydrocarbon yield or potential productivity is affected by maturity, it is often difficult to compare various data sets. For discussion purposes, those samples with 5.0 mg of HC/g of rock SZ yields. In contrast, the basal Qusaiba “hot” shales contained excellent potential as based on an average SZ yield of 16.5 mg of HC/g of rock. Of the 355 samples analyzed, 65% contained >5.0 mg of HC/g of rock SS yield. In order t o better evaluate the potential of the basal Qusaiba “hot” shales, the pyrolytic data were divided into two groups according to maturity. The first group represents the samples where maturity is ‘0.6% VRe, and the second group are the samples with ~ 0 . 6 % VRe. The samples with 10.6%VRe have a unimodal distribution with a peak population in the 20-40 mg of HC/g of rock Sa yield range, indicating that these shales have excellent potential. The average SZyield for this data set is 22.0 mg of HC/g of rock. The samples with >0.6% VRe represent mostly overmature samples ( ~ 1 . 0 % VRe) and have an average S2 yield of 2.9 mg of HC/g of rock. Based on the pyrolytic yield distributions (Figure 71, the only unit identified within the Paleozoic sequence of Saudi Arabia that contains excellent potential for generating and expelling significant amounts of hydrocarbons is the basal Qusaiba shale unit. Even where
mature, the basal Qusaiba shale unit has better residual potential than any other Paleozoic unit based on average pyrolytic yields. Hydrocarbon Type (Hydrogen Index). As clearly observed from TOC and SZ yield histograms, there are some organic-richbeds throughout most of the Paleozoic sequence that have enough SZ yield to be considered potential source rocks. To determine the hydrocarbon proneness of the sediments, the hydrogen index (HI = [(SZyield/% TOC) x 1001)was determined. Basically, an HI of 150 indicates inert kerogen, 50-200 indicates gas-prone kerogen, 200-400 indicates mixed oil- and gas-prone kerogen, and 400 indicates oil-prone kerogen. Because hydrocarbon yield decreases with maturity, so does HI. For discussion purposes here, those samples with 1400 HI will be considered as nonoil source rocks (even though 200-400 HIS will expel gas and oil). Based on this criterion, the only significant oil-prone source rock interval encountered within the Paleozoic sequence in Saudi Arabia is the basal Qusaiba “hot” shales, even though other formations such as the KhuWUnayzah, the Jauf, and the Qasim contain some thin intervals that have >400 HI. Overall, however,
Paleozoic Petroleum System of Saudi Arabia
E
‘
Energy & Fuels, Vol. 8, No. 6, 1994 1435
Jauf Source Rocks
9 7
V
r
4
7
7
4 7
. r
3
7
‘ a ! 7
‘
4 7
9 ‘ ?J a!
9
(u
A
7
pristane/p hyta ne ratio Figure 17. Frequency histogram of the pristanelphytane ratios calculated for the Central Area oils and condensates and the various potential source rock units. 2.00T 1.75: 1.50:
co
0
+ 0
Qusaiba Source Rocks Jauf Source Rocks Hanadirsource Rocks Unayzah Source Rocks Paleozoic Oils
0.00 0 25
0.50 0 75
*./
1.00 1.25 1 50
1.75 2.00
pristanelmC17 Figure 18. PristanehC1.i versus phytaneln-Clg for Paleozoic oils and the potential source rocks. Note that the oils and the basal Qusaiba extracts plot along a single trend indicating that the oils are likely derived from this source rock unit.
these formations contain poor oil-proneness. Their average HI values are listed in Table 2. A total of 974 samples from the Silurian Qusaiba Member, Qalibah Formation were analyzed for their respective hydrocarbon proneness or type. Six hundred twenty-three (623) samples from the “cool” upper Qusaiba have an average HI of 126 mg HC/g TOC and 87% of the samples had HIS 1200. Seventy-nine (79) of 623 samples had HIS ’200, but only 5 samples had HIS ’400.
The basal Qusaiba “hot” shales, however, contained excellent mixed oil/gas-prone potential as based on an average HI of 339 mg of HC/g of TOC for all 351
samples. However, in order to better evaluate the hydrocarbon proneness of the basal Qusaiba “hot” shales, the data set was divided into two groups according to maturity. The first group represents the samples where maturity is ~0.6%VRe, and the second group are the samples with >0.6% VRe. The samples with 10.6% VRe have a unimodal distribution with a peak frequency in the 400-600 mg of HC/g of TOC range, indicating that these shales have excellent oil-prone potential (average HI of 434). Because of the greater maturity for the second sample set and the associated effects, these values are not be considered representative of the true potential of these organic rich marine shales. Because these mature shales contain good to excellent organic richness, and can be regionally correlated based on their high gamma ray log responses, their original potential was probably similar to the immature basal Qusaiba “hot” shales. From Figure 7, it is clearly shown that the basal Qusaiba shales contain the best source rock quality within the Paleozoic sequence. The immature basal Qusaiba shales have excellent organic richness and potential, and are regionally extensive and thick (Figure 51, and the mature shales have good residual organic richness and gas potential. SZYield versus %TOC Cross Plots. Another method of classifying the potential of the Paleozoic potential source rock units is through an S2 versus TOC diagram (modified from Langford and Blanc-Valler~n~~ ). Figures 8-13 illustrate these diagrams for the Paleozoic formations discussed above. Figure 8 shows the S2 uersus TOC diagram for the KhufWUnayzah sample set. These data fall primarily along the 100 HI line and have 12.0% TOC and c5.0 (36) Langford, F. F.; Blanc-Valleron, M. M. AAPG Bull. 1990, 74, 799-804.
Cole et al.
1436 Energy & Fuels, Vol. 8, No. 6, 1994 lusaiba Incipiently Mature Extract n/z 191 Hopanes and Tricyclics
aut Formation, Sha’iba Member Extract i / z 191 (hopanes + tricyclics)
0)
a
.
f
T r icyclics
Extended Hopanes
cn
ianadir Incipiently Mature Extract n/z 191 Hopanes and Tricyclics
Jnayzah Formation Extract n/z 191 (hopanes and tricyclics)
C30 Hopane
2
.
nature Basal Qusaiba Extract about I .O% W e ) n/z 191 (hopanes + tricyclics) C29 Norhopane
C30 Hopane
1
U ,
\-I!
I
0
?a Bx
Tricyclics
Clusaiba-derivedOil irom the Central Province n / z 191 (hopanes + tricyclics)
%
5z 0
C30 Hopane
Extended HopanesMature Pattern
Figure 19. mlz 191 (hopane and tricyclic) biomarker fragmentogramsfor representative extracts from the Unayzah Formation, Jauf, and Qusaiba, as well as a representative oil from the Paleozoic.
mg of HC/g of rock Sa yield, indicating that the samples are mainly gas-prone. Occasional samples follow the 300 HI trend, but overall, the KhuffXJnayzah is classified as a nonsource contributor for the Paleozoic petroleum system. Figure 9 shows the S2 uersus TOC diagram for the Devonian Jubah, Jauf, and Tawil Formations and the Silurian Sharawra Member of the Qalibah Formation. The majority of these data fall along the inert and/or overmature (HI = 50) and gas-prone (HI = 200) trends and contain nonsource rock quality: 12.0% TOC and 1 5 . 0 mg of HC/g of rock S2 yield. However, about half of the Jauf Formation data follow an oil-prone trend with an HI >400. Most of these oil-prone shales have poor source richness (12.0% TOC), but several samples have good organic richness and potential. The Jauf Formation, therefore, may be a contributor of hydrocarbons in the Paleozoic petroleum system, but its lateral continuity is poorly understood. The Devonian Jubah and Tawil, and the Silurian Sharawra are likely nonsource units within the Paleozoic system. Figure 10 shows the S2 versus TOC diagram for the Qusaiba low gamma ray sample set. These data fall between the 50 and 300 HI lines, indicating gasproneness. On the basis of the poor TOC and pyrolytic
yields, overall, the upper Qusaiba is not a potential source rock for the oils. However, given the thickness of this unit, it may contribute dry gas to the petroleum system at advanced maturity (Le., >1.5%VRe). Figure 11 shows the Sa uersus TOC diagram for the basal Qusaiba “hot” shale sample set. The samples were divided by maturity and their respective TOC and Sa yields were plotted. The less mature samples (10.6% VRe) plot along a single trend that has a slope-derived HI of 498 indicating excellent oil-prone potential (r2= 0.94) suggesting that the kerogen assemblage within the basal Qusaiba belongs to a single uniform type of kerogen with very little variability. The second data set, representing the more mature samples, plots along a gas-prone trend with a slope-derived HI of 137 (r2= 0.88). There is more variability in this data set because of the larger maturity range covered by the data (about 0.6% to ’2.0% VRe). Figure 12 shows an expanded plot of Figure 11. When this plot is compared to Figure 11, two observations can be made. First, the oil-prone, less mature trend shows some minor variability in the poorer quality samples. Second, the mature trend now illustrates the effects of maturity more clearly. There are now two apparent trends in the mature samples. The first trend lies above
Paleozoic Petroleum System of Saudi Arabia Jnayzah Formation Extract n/z 217 (steranes)
Energy & Fuels, Vol. 8, No. 6, 1994 1437 Qusaiba Incipiently Mature Extract m/z 21 7 Steranes
luuuZoR ~ 2 7 1
c29
C27
I?
E
C28
C29
Diasteranes
\
5
8 c2s
Low Molecular Weight Steranes
Jauf Formation Sha'iba Member Extract n/z 217 (steranes)
Hanadir Incipiently Mature Extract
m/z 21 7 Steranes
0
C27 % Diasteranes
t
Low Molecular Weight Steranes
1900 Paleozoic samples from Saudi Arabia for organic richness and pyrolytic yields, only one regional source
Cole et al.
1438 Energy & Fuels, Vol. 8,No. 6, 1994
I 0
lo00
Site D - Deep Basin p€l € I Ord. ISil]Dev.l M I P I P I Tr.Jur. I
Cret.
I Tert.
Fm. Tertiary
-
cretaceous
JurassicTriassic
Khuffl Unayzah
L
CahniferousDevonian Sharawra
f I -
1 600
I 1.3 - 1.8% VRe 1 >1.8% VRe I 500
Qusaiba Basal Qusaiba Source Qasrm Saq - Siq
1 80°C
I 400
I 300
I
I
I
200
100
0
Age (million years) Figure 22. Burial historyhhermal model showing the isotherms (in “ C ) and maturity windows through time for the siv2D pseudowell.
rock was identified. This unit was the basal Qusaiba “hot” shales of Silurian Age. Other units with some localized source rock potential are the Permian KhuW Unayzah Formations, the Devonian Jauf Formation, and the Hanadir Member of the Qasim Formation. All other Paleozoic units are considered nonsource rocks. The basal Qusaiba “hot” shale unit is the only Paleozoic unit identified that contains both excellent organic richness and potential. Because the source rock part of the Qusaiba coincides with the gamma ray “hot” zone, it is possible to trace this source rock horizon throughout the Middle East. In central Saudi Arabia this “hot”shale is responsible for charging the Unayzah reservoir^.^*^*^ Based on this conclusion, it was possible to map the regional extent of the “hot”shale zone of the Qusaiba from well logs as shown in Figure 5. The basal Qusaiba “hot” shales are regionally extensive across Saudi Arabia. The unit ranges in thickness from 6 m to over 75 m, and, as indicated by the source rock data presented here, it maintains its excellent organic richness and potential regionally. Throughout the Middle East and northern Africa, these same “hot” shales have been recognized at the base of the Silurian section. Over 100 m of Silurian “hot” shales have been reported at Kuh-e Gakum in Iran.37 These thermally spent (>2.0% VRe) shales contain up to 4.3% TOC and may represent source rock potential at the easternmost part of the basin. South of Saudi Arabia, “hot” shales near the base of the (37)Bordenave, M.L.; Burwood, R. Adu. Org. Geochem., 1989: 1990, 16, NOS. 1-3, 369-387.
Silurian are reported from Oman.38 E l - B i ~ h l a 4and ~ Ali and Silwadi40 suggest that the Silurian shales are the source of the Khuff gas in the southern Gulf area. , ~ , ~ that ~ the North of Saudi Arabia, B e y d o ~ n ~states Silurian “hot” shales are present and are believed to be the best potential source rock for the Risha gas in Jordan, and some Silurian bituminous marine shales have been reported in Turkey.42 Gontcharov et al.43 suggests that the highly radioactive Silurian shales in Algeria are the source of some Algerian oils. Oil-Oil and Oil-Source Correlation. As discussed in the source rock section of this paper, the most likely source rock units for the Paleozoic reservoired oils of Saudi Arabia are the “hot”shales located at the base of the Silurian Qusaiba Member of the Qalibah Formation. To determine the oil-source and oil-oil relationships, selected source rock extracts and reservoired oils from the Khuff and Unayzah reservoirs were characterized using carbon isotopes (d13C),gas chromatography (GC), and gas chromatography-mass spectrometry (38)Grantham, P. J.; Lijmbach, G. W. M.;Posthuma, J.; Hughes Clarke, M.W.; Willink, R. J. J. Pet. Geol. 1987,11, 61-80. (39)El-Bishlawy, S. H. SOC.Pet. Eng., 4th Middle East Oil Show, 1985, SPE 13678,9-24. (40)Ali, A. R.;Silwadi, S. J. Soc.Pet. Eng. 6th Middle East Oil Show, 1989, SPE 18009,819-832. (41)Beydoun, Z.R.OAPECIADNOC Seminar on Deep Formations in the Arab Countries: Hydrocarbon Potential and Exploration Techniques, Abu Dhabi, October 1989,Proc. Tech. Pap. 1989, 31-84. (42)Ala, M.A.; Moss, B. J. J. Pet. Geol. 1979,1, 3-27. (43)Gontcharov, I.; Hamel, M.; Boudjema, A. Poster Sessions from the 16th International Meeting on Organic Geochemistry; Stavanger, Norway, 1993;261-264.
Energy & Fuels, Vol. 8, No. 6, 1994 1439
Paleozoic Petroleum System of Saudi Arabia
I
Site B - Basin Margin Fm. Tertiary cretaceous
JurassicTriassic
KhuffNnayzah Carboniferous Sharawm Qusaiba Basal Qusaiba Source Qasim Saq - Siq
I
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Age (million years) Figure 23. Burial historyhhermal model showing the isotherms (in “C) and maturity windows through time for the site B pseudowell.
(GCMS). These results were then used to compare the oils to each other and to the prospective source units. Carbon isotopes (d13C)were determined for the source rock bitumen and isolated kerogens for the basal Qusaiba “hot” shale zone, the Jauf, KhufWnayzah, and Hanadir shales (Figures 14 and 15). The 613C values of the kerogens and extracts from the immature basal Qusaiba source rock ranged from -28.8 to -30.8%0 with most of the values lying between -29.0 and -30.5%~~ Permian KhufVUnayzah extracts ranged from -22.8 to -29.3,but most were heavier than -27.0%~~Devonian Jauf extracts had isotopic compositions around -27.0%0, whereas the Ordovician Hanadir was between -27.7 and -28.7%0. These values were then compared to the d13C compositions of the whole oils. A total of 30 oils from the Khuff and Unayzah reservoirs had carbon isotopic compositions ranging from -27.0%0(heaviest)to -30.5%0 (lightest). Because most of the basal Qusaiba “hot shale kerogens and extracts had carbon isotopic compositions between -28.8 and -30.8%0, and most of the oils ranged from -28.8 to -30.5%0, correlation between the basal Qusaiba source rocks and most Paleozoic reservoired oils was excellent. Only two Paleozoic oils, and the Paleozoic condensates were outside the basal Qusaiba range. The two oils had carbon isotopic values around -27.1%0 which was attributed to a possible mix with a localized source (i.e., Khumnayzah or Jauf), and the condensates had heavier isotopic compositions due to maturity effects. The heavier isotopic composition of the condensates are consistent with isotopic values resulting from continuing
kerogen conversion during m a t u r a t i ~ n .Since ~ ~ ~ the ~ condensates would have lighter isotopic compositions prior to advanced maturation, the condensates also have a good match to the basal Qusaiba shales. To better define the relationships between the oils and potential source rock extracts, saturate fraction CIS+gas chromatography was used to characterize them. Regionally, the Paleozoic oils have the following chromatographic characteristics typified by Figure 16: 1. Pristane/phytane ratios are quite variable for the different source rocks and oils. The KhufVUnayzah rock extracts have pristane/phytane ratios ranging from 0.86 to 1.93 and the Jauf extracts from 0.80 to 0.93;the Qusaiba and Hanadir extracts range from 1.2 to ’2.1. The Paleozoic oils have pristane/phytane ratios from 1.2 to 2.3,except for two oils from the Ghinah area (see McGillivray and Husseini’) which have ratios below 1.0. These two oils also have a different saturate alkane envelope than other Paleozoic oils which suggests a different origin, or mix of oil types. No significant differences were observed between the Silurian basal Qusaiba immature extracts and the immature Ordovician Hanadir extracts. This was attributed to similar source rock kerogens. Figure 17 shows a histogram for the pristane/phytane ratios of the oils and potential source rocks. 2. Figure 18 shows a cross plot between the pristane/ n-C17 and phytane/n& ratios. Most Qusaiba source (44) Clayton, C. J. Org. Geochem. 1991,17,887-900. (45) Clayton, C. J. Marine Pet. Geol. 1991,8,232-240
.
Cole et al.
1440 Energy & Fuels, Vol. 8, No. 6, 1994
Immature (1.8% VRe)
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Figure 24. Cross section showing the maturity windows from the edge of the Arabian Plate in the west to the deepest part of the basin at site D.
rock extracts and most Paleozoic oils plot along the same trend. The two Ghinah oils and the Jauf and Unayzah extracts deviate from the Qusaiba trend. Using these chromatographic values and observations, the Paleozoic oils were likely derived from source rocks deposited under dysoxic to anoxic, marine, clastic condition^.^^,^^ When the chromatographic data are combined with the carbon isotopic data, the most likely source rock for the Paleozoic oils are the “hot” shales of the basal Qusaiba Member of the Silurian Qalibah Formation. GCMS mlz 191 (tricyclics and hopanes) and mlz 217 (steranes) traces were used to conduct a more detailed characterization of the oils and extracts. Representative mlz 191 fragmentograms are shown in Figure 19 for Unayzah, Jauf, and two Qusaiba extracts, respectively, and for a representative Paleozoic oil. mlz 217 fragmentograms are shown in Figure 20 for the same samples. Most reservoired oils are similar to each other in both fragmentogram fingerprints and in selected biomarker ratios. The tricyclic and hopane fingerprints are characteristic of oils derived from source rocks deposited in a clastic marine e n v i r ~ n m e n t . ~The ~,~~ steranes are indicative of a marine algal, clastic source rtxk where c29 steranes dominate (c29> c27 >> C28). The C29 sterane abundance is believed to be caused by a large contribution of brown or certain green algae in the source kerogen.46 There are abundant amounts of diasteranes which indicate a high amount of clastic (46)Moldowan, J. M.; Seifert, W. R;Gallegos, E. J. AAPG Bull. 1985,69,1255-1268. (47)Waples, D. W.; Machihara, T. Biomarkers For Geologists-A Practical Guide to the Application of Steranes and Triterpanes in Petroleum Geology;AAPG Methods in Exploration No. 9,1991,91pp.
material (carbonate = low diasteranes, calcareous shale = higher diasteranes). Because of the advanced maturity of the Paleozoic oils, biomarkers were present in very low concentrations. Therefore, direct comparisons to the source rocks could not be made. However, there appear to be more similarities between the basal Qusaiba extracts and the Paleozoic oils than to the other potential source rock extracts. To better illustrate these similarities, Figure 21 shows the distribution of the c27 to C29 aaa20R steranes (after Huang and Meins~hein~~). Most Paleozoic oils plot in the same region of the ternary plot as the basal Qusaiba extracts suggesting that this is the common and most likely source unit for the Paleozoic oil family.
Burial HistoryFChermal Modeling Burial history/thermal models were constructed for 50+ wells (or pseudowells from seismic) in the Central and Northern Area of Saudi Arabia using the BasinMod software package available from Platte River Associates. This modeling software has been successfully applied in other modeling s t u d i e ~ . ~ In~order , ~ ~ to perform a successful burial historylthermal model, specific data are required for each model: 1. The stratigraphic section penetrated by the well is used to establish the geological burial history. The (48)Huang, W. Y.;Meinschein, W. G. Geochim. Cosmochim. Acta 1979,43,739-745. (49)Waples, D.W.; Kamata, H.; Suiza, M. AAPG Bull. 1992,76, 31-46. (50)Waples, D.W.; Suiza, M.; Kamata, H. AAPG Bull. 1992,76, 47-66. (WZiegler, A. M.; Hansen, K. S.; Johnson, M. E.; Kelly, M. A.; Scotese, C.R.;van der Voo, R. Tectonophysics 1977,40,13-51.
Energy & Fuels, Vol. 8, No. 6, 1994 1441
Paleozoic Petroleum System of Saudi Arabia
Table 3. Maturity Ranges Used To Define the Generation and Expulsion Windows Used in This Study maturity vitrinite reflectance range hydrocarbon generation and expulsion no generation or expulsion immature I0.5% VRe initial to significantgeneration; initial expulsion early mature 0.5-0.70% VRe main phase oil expulsion with completion by 1.0% VRe expulsion mature 0.70-1.0% VRe light products/gas generation begins oil preservation 1.0-1.3% VRe gas generation and expulsion of drier gases gas mature > 1.3% VRe
amount of burial and erosion for the observed unconformities in the well were corrected using regional seismic and geological reconstructions. The stratigraphic section below where the well penetrated was estimated using seismic lines. The lithologies modeled were also determined from well logs. 2. The thermal history was based on the tectonic and basin reconstruction of the Central and Northern Areas described in Beydoun4 and McGillivray and Husseini7 and corrected using observed bottom-holetemperatures. 3. Further calibration of the thermal histories was done regionally by comparison to measured maturity indicators (corrected % VRe from zooclasts, TAI, Tmm) and kinetically derived models using organic geochemical input. To establish the thermal maturity windows used in the burial historykhermal models, it was assumed that the kerogens from which the oils were derived belonged to a single group of organic materials. If this is true, then the empirical and kinetic relationships for the basal Qusaiba source unit can be used to determine at what temperature and maturity the oils will be generated and expelled on a regional basis. Kinetic models based on BasinMod data and in-house analytical data indicate that the basal Qusaiba kerogens convert to mostly liquid hydrocarbons by about 1.0% VRe. Initial generation begins at about 0.5-0.6% VR, expulsion starts a t about 0.7%VRe (assuming that 30% of generation has to occur before expulsion begins), and then generation and main phase oil expulsion ends by 1.0%VRe. Based on the above maturity and burial history data, the most plausible thermal history assumes a decreasing heat flow from Ordovician time to present. However, we did assume a slightly higher heat flow preceding and during the Hercynian event as evidenced by reactivation of faulting. If we apply the above parameters to the deep basin burial history (site D; Figures 1 and 22) and to shallower burial (site B; Figures 1and 23), the basal Qusaiba source rock attains generation and expulsion maturities as a result of the increased burial during the late Jurassic through early Tertiary. From the modeling results, the maturity a t the base Qusaiba (source rock interval) was used to illustrate the regional present-day maturity trends across the Central Area (Figure 24). The maturation windows used (see Figure 6) and their respective equivalent % vitrinite reflectance (VRe) ranges were modified from the l i t e r a t ~ r e ~ Oand - ~ ~are summarized in Table 3. The present-day level of thermal maturity (Figure 24) generally increases from the exposed western rim of the basin in central Saudi Arabia (early mature; 2.0m e ) . Based on the present-day maturity trend and the maturity of the oils analyzed from the Central Area fields, most of the reservoirs in the Central Area of Saudi Arabia require long distance migration for filling.
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R Wet Gas Mature (131.8% VRe) Dry Gas Mature (>1.8% VRe)
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Figure 25. Petroleum system parameters for the central area, Saudi Arabia, as based on the site B and D models.
The present-day maturity in the immediate vicinity of the oil fields is not high enough to match the oil maturity, whereas the deeper basin achieves post-oil expulsion maturity over much of the central and northern areas of Saudi Arabia (Figures 5 and 24). This suggests that the oils have migrated from the downdip, more mature areas of the basin. Given the large fetch areas around site D, the thickness of the source rock (the basal Qusaiba “hot” shales attain thicknesses of ’75 m), the advanced maturity of the source rock, and the good regional seal (the Khuff carbonates and evaporites), these factors combine to create an efficient Paleozoic petroleum system. Summary
This paper has attempted to describe the active Paleozoic petroleum system of the central and northern areas of Saudi Arabia. The primary components of the petroleum system are the source rocks, carrier beds (migration pathways), reservoir rocks, timing of trap development, and seal (Figure 25). The best candidate source rock identified within the Paleozoic sequence of Saudi Arabia is the basal Qusaiba “hot”shale unit. This source rock was deposited under dysoxic to anoxic, clastic, marine conditions at the beginning of Silurian time as a result of a major marine
1442 Energy & Fuels, Vol. 8,No. 6, 1994
transgression following Gondwana glaciation. The source rock attains thicknesses exceeding 75 m in some areas. Source rock quality is excellent considering that it has an organic richness exceeding 4% on average and excellent oil-prone potential. Secondary source rock units were identified within the Sha'iba Member of the Devonian Jauf Formation and within the KhuflGZTnayzah transition zone and the Unayzah Formation. These secondary source rocks, however, do not maintain regional thickness or quality. However, it is believed that these secondary sources may be responsible for some of the variations observed in the oils in the Central Area and may even have generated and accumulated as small pools in some fields. The basal Qusaiba extracts have an excellent match to all but two of the Paleozoic oils as based on carbon isotopes, gas chromatography, and biomarkers. Thermal maturity modeling shows that the least mature source rocks lie along the western margin of the basin and maturity increases progressively toward the east. Most of the basal Qusaiba source rocks in the center of the basin had attained post-oil expulsion ('1.0% W e ) by about 100-150 Ma. As burial increased during late Cretaceous and Tertiary time, the center of the basin became gas mature, whereas the margin of the basin around the Arabian Plate was attaining early maturity (0.5-0.7%VRe) in the Central Area. However, the basal Qusaiba in the Northern Area is overmature ('1.8% VRe) along the Iraq border and becomes less mature as it approaches the Arabian Plate.
Cole et al.
In summary, the petroleum prospectivity and potential of the Paleozoic sequence in Saudi Arabia was the result of the coincidence of several factors. The development and the preservation of organic-rich material at the base of the Qusaiba resulted from a major marine transgression. As time progressed, the overlying strata (Devonian age) were significantly eroded during the Hercynian event. Following this erosive period, good quality reservoir rocks were deposited above the source rocks which allowed for efficient expulsion and migration. Thick, regionally extensive seals (Khuff carbonates and evaporites) trapped large volumes of oil in large, low-relief closures. Because of the size of the Northern and Central Areas of Saudi Arabia, and the relatively few well penetrations (except in the Hawtah area of McGillivray and Husseini7), there remains excellent exploration potential in the remaining, unexplored parts of Saudi Arabia, particularly the Rub 'a1 Khali and the Nafud Desert.
Acknowledgment. A significant part of this paper can be attributed to those seldom seen analytical technicians of the Geochemistry Unit at Saudi Aramco. Without their analytical support, papers such as these cannot be written. We thank the anonymous reviewers of the manuscript. Lastly, we appreciate and thank the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Arabian Oil Co. (Saudi Aramco) for permission to publish this paper.
