Paleozoic petroleum systems of Saudi Arabia: a basin modeling approach

Transcription

1 GeoArabia, Vol. 1, No. 3, 25 Gulf PetroLink, Bahrain Paleozoic petroleum systems of Saudi Arabia: a basin modeling approach Mahdi Abu-Ali and Ralf Littke ABSTRACT The major Paleozoic petroleum system of Saudi Arabia is qualitatively characterized by a proven Silurian ( Member, Qalibah Formation) source rock, Devonian ( Formation), Permian and Carboniferous (Khuff and formations) reservoirs, a laterally extensive, regional Permian seal (basal Khuff clastics and Khuff evaporites), and four-way closed Hercynian structures. Hydrocarbons found in these systems include non-associated gas in Eastern Arabia and extra light oil in Central Arabia. A basin modeling approach was used to quantify important aspects of the petroleum system. Firstly, seventeen regional wells were selected to establish a reference tool for the three-dimensional (3-D) basin model using multiple one-dimensional (1-D) models. This was accomplished by studying core material from source rocks and other lithologies for thermal maturity and kerogen quality. The major emphasis was on the Silurian section, other Paleozoic intervals and to a lesser extent on the Mesozoic cover from which only few samples were studied. Although vitrinite macerals, solid bitumen, and other vitrinite-like particles were not abundant in most of the investigated samples, enough measured data established valid maturitydepth trends allowing for calibrated models of temperature history. Sensitivity analyses for maturity support the view that thermal boundary conditions and Hercynian uplift and erosion did not greatly influence the Paleozoic petroleum systems. Secondly, a 3-D basin model was constructed using major geologic horizon maps spanning the whole stratigraphic column. This model was used to gain insight into the general maturity distribution, acquire a better control of the model boundary conditions and investigate charge, drainage, migration and filling history of the main Paleozoic reservoirs. The 3-D hydrocarbon migration simulation results qualitatively account for the present gas accumulations in the Permian- Triassic Khuff and Carboniferous-Permian reservoirs in the Ghawar area. This kind of study illustrates the importance of basin modeling when used with other geologic data to describe petroleum systems. It provides a predictive exploratory tool for efficiently modeling hydrocarbon distribution from known fields. Real earth models can only be described in 3-D as pressure variations and fluid movements in the subsurface are impossible to address in 1-D and 2-D domains. INTRODUCTION Petroleum system analysis has become an increasingly important aspect in assessing the hydrocarbon potential of a given basin. Understanding the principal components of a petroleum system (source, reservoir, trap, seal, migration and timing; Demaison and Huizinga, 1991; Magoon and Dow, 1994) is a pre-requisite for a successful exploration program. Basin modeling, though focused on the source rock component of the petroleum system, is a major first step. Basin modeling assesses the potential of the basin in terms of source rock availability, temperature history and organic maturity (e.g. Lopatin, 1971; Ungerer et al., 199; Yalcin et al., 1997). This paper presents a 3-D basin model that focuses on the generation and migration of hydrocarbons for the Paleozoic sequence in the Ghawar anticline and Central Arabia, a region of about 162, sq km (Figure 1). The study is based on the analyses of the quality and thermal maturity of the principal Silurian hot shale source rock using organic petrology and Rock-Eval pyrolysis data. The thermal and maturation histories were calibrated using organic petrography and source Copyright Gulf PetroLink 216. All Rights Reserved. Downloaded by munirelmahdy87@gmail.com IP:

2 Abu-Ali and Littke Oil field Safaniya 28 Gas field Habari Well Gulf Arabian Manifa Sharar Study well Fault Juraybi'at Abu Hadriya Al Haba Khursaniyah Bakr Berri Abu Sa'fah Samin Fadhili 547 Berwath Formation 355 Qatif Tawil Formation 26 Fazran 554 Qu Jaham a ib sa 311 Abqaiq M em Ain Dar be r Shedgum Ghawar Khurais Saq/Qasim formations Qirdi Niban Shama Yabrin Formation 51 Jawb Abu Markhah Khuzama Nisalah Hawtah Member Hercynian Subcrop Nuayyim Hazmiyzh Layla Tinat Waqr Shaden Abu Rakiz Burmah Sharawra Member Jufayn Wudayhi Mulayh Tawil Formation 331 Ghazal Raghib Hilwah Haradh Mazalij Dilam Shiblah Formation Warid Abu Shidad Hawiyah Harmaliyah Sharawra Member Jubah Formation 358 Abu Jifan Uthmaniyah Proterozoic Basement 313 Dukhan Ghinah Umm Jurf Usaylah Berwath Sharawra Jubah Saq/Qasim Tawil Proterozoic Km N Tukhman 5 51 Figure 1: Location map showing wells used in the study area and major oil and gas fields. The 17 wells used in the study area are shown in black circles. The Tayma (TAYM) well falls outside the study area. The Hercynian subcrop map shows the formations that are encountered below the Carboniferous-Permian and Permian part of the Khuff Formation. Copyright Gulf PetroLink 216. All Rights Reserved. Downloaded by munirelmahdy87@gmail.com IP:

3 Cisuralian Unfimian- TataKazanian rian Khuff A Khuf C E Late CAMBRIAN 51 D Basal Khuff Aeolian TST Possible SourceTST Interval Potential Source Interval Marine Major Source Rock Interval P1 (287) Intra-Formational Hiatus (Erosional or non-depositional) Juwayl Stephanian TST Playa Fluviolacustrine Fluvio-glacial Incisions Scale Change Fluvioglacial Westphalian Serpukhovian P4 (254) (Oman) B C Oil and Non-associated Gas Reservoir P3 (255) P2 (261) Sakmarian Major Maximum FloodingSurface (MFS) [after Sharlandet al. (41) (24) with ages in Ma calibrated to GTS, 24] Hercynian Orogeny (Erosional Hiatus) D2 C1 (333) Berwath Hiatal Surface Sequence Boundary (SB) Higher Frequency SB Inferred MFS Famennian (361) LST = Lowstand Systems Tract TST = Transgressive Systems Tract Jubah Frasnian Givetian Eifelian Emsian Pragian Lochkovian Pridolian Ludlow Llandoverian Ludfordian Gorstian Homerian Sheirnwoodian Telychian Aeronian Late Rhuddanian 49 5 Khuff (A-C) B Wenlock E. M. 47 Tr3 (249.75) = Highstand Systems Tract M. L. 46 Artinskian Tournaisian 45 SILURIAN 44 ORDOVICIAN 43 TST Non-associated Gas Resrvoir TST Tr5 A Visean L. 42 (226) Sudair Kungurian 4 41 Oil Reservoir Tr6 (241) Olenekian Induan Changsingian Wuchiapingian Capitanian Wordian Roadian DEVONIAN 39 (22) D3 PALEOZOIC 38-1 Tr Tr7 Anisian Namurian CARBONIFEROUS (211) (234) M. Late 3 2? Carbonate Tr8 Ladinian Asselian Gzhelian Kasimovian Moscovian Bashkirian 3 Minjur Jilh Jilh Pennsylvanian 29 Carnian Late E. 28. Relative Sea-Level Changes Evaporite 27.5 Systems Tracts? Minjur Mississippian PERMIAN 26 Basinward Shale 1. Norian 25 Member Sandstone Onlap Curve Major MFSs (Age) Landward Hettangian Rhaetian E. 24 stratigraphic Extent) Sinemurian TRIASSIC 23 LEGEND Arabian Platform Regional Sea-Level Changes Source Generalized Arabian Reservoir Depositional Profile (Chrono- Intervals Stratigraphy Intervals Formation Guada- Lopingian lupian 22 MESOZOIC 2 Generalized Standard Stages / Ages Late JURASSIC ERAS Systems/ Periods Series/ Epochs Time (Ma) Ashgillian (Hirnantian) Caradocian Llandeilian Llanvirnian Arenigian Tremadocian Merioneth St. Davids Lenan Botomian Atdabanian Tommotian Nemakit - Daldynian Precambrian Murayr (Fiyadh) D2 Hammamiyat (41) Subbat Qasr D1 (48) Long Term Curve Sha iba Tawil S2 (418) il Unconfor m ity Pre-Taw Sharawra Qalibah Sarah Sarah Quwarah Ra an Kahfah Hanadir Sajir Risha Qasim Saq Midddle Sarah Hot Shale S1 (44) Quwarah O4 (454) Kahfah O3 (469) O2 (483) O1 (486) Cm3 (Oman) Burj Siq (496) Cm2 (56) Pre-Siq Unconformity Northern Gulf Salt Basin (Ara) (Jibalah Group) (Volcanics) (Oman) Cm1 (538) Figure 2: Cambrian-Triassic Saudi Arabian Stratigraphic Column showing major reservoir and source intervals (modified after Haq and Al-Qahtani, 25). Ages are in million years and Copyright Gulf PetroLink 216. All Rights Reserved. Downloaded by munirelmahdy87@gmail.com IP: calibrated to GTS Short Term Curve 1

4 Abu-Ali and Littke rock geochemistry data from 132 samples representing 17 key wells. Of the 132 samples, 126 were extracted from cores and only 6 from cuttings; this ensures that the samples represent true depth. This aspect of the study established a regional database on maturity distribution and source rock characteristics in the Paleozoic section of Saudi Arabia. This data was used to construct 1-D burial history models for the individual wells. Based on the burial history models of the 17 wells, a 3-D model was developed using a coarse grid for maturity and general migration predictions. The 3-D model only covers 16 wells as the 17th well (TAYM) is located outside the study area. Silurian source rocks from the 16 wells represent either early-mature or postmature early Silurian intervals. TAYM well was chosen because it provides geochemical data on organically immature source rock interval (Abu-Ali et al., 1999). The next step was to build a 3-D basin model using regional geological maps that span the postordovician stratigraphic column. The objective of the 3-D petroleum system analysis is to predict hydrocarbon properties, simulate general hydrocarbon migration trends and filling history, determine drainage areas and assess future hydrocarbon exploration areas. PETROLEUM SYSTEM The Paleozoic petroleum geology of Saudi Arabia has been discussed in several publications (Abu-Ali et al., 1991, 1999 and 21; McGillivray and Al-Husseini, 1992; Mahmoud et al., 1992; Cole et al., 1994; Wender et al., 1998; Jones and Stump, 1999; Konert et al., 21). Figure 2 displays the stratigraphic column of the Paleozoic strata in Saudi Arabia. Source Rocks The only proven source rock for the Paleozoic hydrocarbons in Eastern and Central Saudi Arabia is the early Silurian basal hot shale of the Member of the Qalibah Formation (Abu-Ali et al., 1991, 1999, 21; Mahmoud et al., 1992; McGillivray and Al-Husseini, 1992; Cole at al., 1994; Jones and Stump, 1999). The basal contains type II organic matter with a hot shale thickness ranging from 1 25 ft (3 7 m, Mahmoud et al., 1992; Wender et al., 1998; Abu-Ali et al., 1999, 21). Carbon isotopes strongly correlate hydrocarbons found in the Devonian, Carboniferous and Permian reservoirs to the source rock extracts. Biomarker data also clearly distinguishes the Silurian shales as the main source rock for the Paleozoic oils and condensates (Abu-Ali et al., 1991; Moldowan et al., 1994; Cole et al., 1994). The Silurian source rock is also well established in North Africa (Yahi et al., 21; Lüning et al., 2) and reflects a regional transgression that followed the deglaciation of Gondwana Land. There are also, to a lesser extent, isotopic correlations between Paleozoic hydrocarbons and other potential source rocks; these include but are not restricted to the Ordovician Ra an and Hanadir shales, Devonian shales, Permian shales, Permian basal Khuff shales (Abu-Ali et al., 1991; Cole et al., 1994). These rocks are of limited thickness and lateral extent and have not been thoroughly sampled and analyzed. Deeper infra-cambrian units have not been penetrated in Saudi Arabia, but may be source rocks as proven in Oman (Grantham et al., 1987). Proterozoic-Cambrian basins have been identified on seismic in the western Rub Al-Khali Basin. Reservoir Rocks Sandstone reservoirs are proven in the Late Ordovician glaciogenic Sarah Formation in Northern Saudi Arabia (e.g., Kahf field) and Jordan (e.g., Risha field). This reservoir occurs immediately below the hot shale. The marine Devonian Formation is a good reservoir which tested gas and condensate along the eastern flanks of the Hawiyah and Shedgum fields (Figure 1). The Late Carboniferous-Permian Formation constitutes the main Paleozoic hydrocarbonbearing reservoir (McGillivray and Al-Husseini, 1992; Senalp and Al-Duaiji, 1995; Evans et al., 1997; Wender et al., 1998). Above the, the Permian- Triassic Khuff carbonates constitute the main reservoir for non-associated gas in Arabia (e.g., Ghawar Superfield, Qatar s North Field). Copyright Gulf PetroLink 216. All Rights Reserved. Downloaded by munirelmahdy87@gmail.com IP:

5 Another reservoir is the Silurian mid- Member sandstones which tested non-associated gas in Tukhman (Figure 1). The sandstones of the Late Silurian- Devonian Tawil Formation have poor reservoir quality due to the development of diagenetic cements. Seal Rocks Amongst all petroleum system components, the nature of the sealing rocks is the least studied and understood. The seal of the Sarah reservoir is the shale. The reservoir is sealed by interbedded shales and dolomites (D3B palynological marker) at the top of the Devonian Formation. A recent capillary pressure study of the Devonian D3B seal showed that a ft interval of this unit can hold a gas column of approximately 2, ft before it is breached. Faulting with more than 5 ft throw will breach the D3B seal. Along the flanks of Ghawar, the reservoir is truncated and sealed by the Khuff Formation. The shales of the basal Khuff clastics are the regional seal for the Carboniferous-Permian reservoirs. The seal efficiency of the basal Khuff clastics may determine whether hydrocarbons leaked vertically due to thinning and/or fracturing, or laterally via fault seal sand juxtaposition. The interbedded Khuff evaporites are the main seals for the Khuff A, B and C reservoirs (Figure 2). The overlying Sudair Formation shales provide a regional seal for the Khuff Formation. Structural Evolution and Traps The regional structural growth history of the Arabian Plate is illustrated in Figures 3 7. Figure 3 shows the regional compressional growth during the Carboniferous Hercynian Orogeny as mapped by isopach from the base Member to base Khuff Formation. Blank areas indicate Ordovician or older subcrops and highlight regions where structural growth was greatest. For example, over parts of the Ghawar structure the Hercynian Unconformity eroded down into the Ordovician and Cambrian, while the Carboniferous and early Permian section is absent. The blue color represents low areas during the Hercynian Orogeny. Significant removal of section occurred over Ghawar field and the areas to the west and southeast (red-yellow color depicting thin areas). Most traps in the Paleozoic section are structural; a few traps are faulted, stratigraphic and combination stratigraphic-structural. Most of the structural closures were initiated in Hercynian times and continued to grow with time. The stratigraphic traps are reservoir bounded by the Hercynian Unconformity and/or reservoir with strong lateral thickness and porosity variations. Figure 4 shows the isopach map from base Khuff Formation to top Jilh Formation. It indicates minor extensional growth during the Permian to Triassic time (Figure 5), and a shift of the basin center towards the north. Most of the growth took place in the south. The Triassic to Cretaceous (Aptian) time is represented by the isopach between the tops of the Jilh and Shu aiba formations (Figure 5). During this phase a further shift of the basin center to the north occurred, while more section was removed in the south. The Late Cretaceous period is marked by the obduction of the Neo-Tethys Ocean onto Oman and along the Zagros region. Deeper burial occurred in Eastern and Southern Arabia, while most of the structural growth was in the west (Figure 6). Tectonic compression caused significant structural growth over the majority of the existing structures. In the Tertiary regional tilting to the east and north is evident by comparing the Aruma structure (Figure 7) with the previous figures representing Triassic and Hercynian growths. Figure 8 shows an east-west cross section across Arabia. The major structures of Ghawar (Saudi Arabia) and Dukhan (Qatar) are bounded by major faults. The section displays the basin in the middle, where hydrocarbons have been generated and migrating toward the Ghawar and Dukhan structures to the east and west flanks of the basin. Copyright Gulf PetroLink 216. All Rights Reserved. Downloaded by munirelmahdy87@gmail.com IP:

6 Abu-Ali and Littke BASE QUSAIBA TO BASE KHUFF 35 48o 5o 28o o (feet) -1 24o 1, -15 2, 3, -2 4, 5, -25 6, 7, -3 8, 9, 22o , Figure 3: Carboniferous growth is represented by base Member to base Khuff Formation isopach. Red indicates areas of thinning and blue areas of thickening. Blank areas are where the base Member has been eroded. Copyright Gulf PetroLink 216. All Rights Reserved. Downloaded by munirelmahdy87@gmail.com IP:

7 BASE KHUFF TO TOP JILH 35 48o 5o 28o o (feet) -1 1, 24o 2, -15 3, 4, -2 5, 6, -25 7, 8, -3 9, 1, 22o , Figure 4: Permo-Triassic growth is represented by base Khuff to top Jilh isopach. Red indicates areas of thinning and blue areas of thickening. Copyright Gulf PetroLink 216. All Rights Reserved. Downloaded by munirelmahdy87@gmail.com IP: