Paleozoic Hydrocarbon Habitat in the Arabian Plate*
Abdulkader M. Afifi1
Search and Discovery Article #10075 (2005)
Posted January 26, 2005
*Adapted from AAPG Distinguished Lecture, 2004. Online adaptation of the presentation of ãGhawar: The Anatomy of the Worldâs Largest Oil Field,ä the other AAPG Distinguished Lecture presented by the author in 2004, is also posted on Search and Discovery.
1Saudi Aramco, Dhahran, Saudi Arabia ([email protected])
The Paleozoic section became prospective during the seventies following discovery of oil in Oman and delineation of the enormous gas reserves in the Khuff Formation. Exploration has since targeted the Paleozoic section throughout the Middle East and has resulted in major oil and gas discoveries in Oman, Qatar, and Saudi Arabia. Our improved knowledge of the Paleozoic geology is a direct outcome of these activities.
The Paleozoic section in Arabia was deposited along the continental margin of Gondwana in predominantly clastic environments that ranged from continental near the Arabian Shield to outer shelf in Iran and Syria. Carbonate deposition became dominant during the Permian opening of the Neo-Tethys. Six megasequences are recognized, separated by regional unconformities. The paleo-environments ranged from arid to glacial, reflecting the drift and rotation of the region during the Paleozoic from equatorial to high southern latitudes and back.
Rifting during the Late Precambrian initially formed salt basins in Oman and the Arabian Gulf region. Subsequently, the Cambro-Ordovician clastic sequences were deposited over a leveled continental platform. However, during Late Ordovician, this margin probably separated into two terranes along the Zagros suture zone. The Hercynian orogeny during the Carboniferous caused widespread intraplate deformation, including broad upwarps in Egypt, Central Arabia, northern Syria, and Oman, which underwent extensive erosion. Another manifestation of the Hercynian deformation are transpressional basement horsts in Eastern Arabia that host the major oil and gas fields. The Hercynian deformation occurred in at least two pulses and probably resulted from collision along the northern margin of Gondwana. The Hercynian deformation was followed during the Permian by rifting and opening of the Neo-Tethys along the Zagros fault zone and deposition of Khuff carbonates and evaporates along the new passive margin.
Two petroleum systems are recognized, sourced by rocks of Precambrian and Silurian age. The Silurian petroleum system is sourced by the Qusaiba hot shales which extend over large areas in the Arabian plate. The hydrocarbons are trapped in Ordovician, Silurian, Devonian, and Permo-Carboniferous sandstone reservoirs except in high relief structures, where the upward propagation of basement faults has breached seals, allowing charge into the Permian carbonate reservoirs. The Paleozoic hydrocarbon system is oil-prone along the basin margin in Central Arabia but is predominantly gas-prone elsewhere due to the deep burial of the source rocks. It is estimated by USGS to have generated mean recoverable conventional resources of 37 billion barrels of oil and 808 trillion cubic feet of gas. In Arabia, the Paleozoic hydrocarbons are ultimately sealed by the thick Triassic shales that prevented any mixing with Mesozoic hydrocarbons.
The Precambrian source rocks have been proven to be effective in the interior salt basins of Oman, where they have charged Precambrian, Paleozoic, and Mesozoic reservoirs along faults and salt diapirs. The Hercynian uplift of Oman largely removed the Silurian source rocks and effectively saved the Precambrian source rocks from excess burial.
The main challenges to exploration and development are: (1) the difficulties in seismic imaging of the Paleozoic section due to multiples, seismic transparency, and near-surface problems; (2) the prediction of porosity in the tight, deeply buried reservoirs; and (3) the hostile subsurface environments.
The Arabian plate contains a wide range of tectonic elements, from Precambrian shield and Precambrian (Infracambrian) basin to Cenozoic fold-thrust belt, locally with ophiolite (Figure 1). During the Paleozoic, the plate was affected by drift and rotation that resulted in changes from equatorial to high southern latitudes and back to equatorial (at the end of the Triassic), with two periods of glaciation (Figure 2). Structure of the foredeep basin becomes increasingly complex toward the foldbelt(s) (Figure 3).
Regional unconformities, with two related to the Carboniferous Hercynian orogeny, divide Paleozoic strata into megasequences (Figures 5, 6, and 7. Deposition before the orogeny was dominated by clastics, whereas Permian deposits are characterized by the Khuff carbonates and evaporites. Much earlier, Late Precambrian rifting resulted in salt basins in Oman and the Arabian Gulf region (Figure 8). Extensive erosion resulting from the Hercynian orogeny is reflected by the subcrop patterns below the pre-Late Carboniferous unconformity (Figure 9).
Features of the Paleozoic stratigraphic section and interpretations of environments of deposition are illustrated in Figures 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and 21. Some of the more striking features may be summarized as follows:
Widespread Middle Cambrian carbonates (Figure 10)
Widespread Middle Ordovician marine shales (Hanadir maximum flooding event) (Figure 13)
Widespread Lower Silurian organic marine shale (Figure 16)
Upper Carboniferous (post-Hercynian) glacial deposits (Figure 19)
Lower Permian carbonates in the southern part of the Arabian plate (Figure 20)
Widespread Upper Permian evaporites (Figure 21)
Regional cross-sections (Figures 22, 23, and 24), in addition to tectonic, structural, and subcrop maps (Figures 1, 3, and 9), illustrate the general structural patterns and features of the Arabian plate. Basement faults in the foredeep basin are well shown in Figure 22, and inversion of Precambrian and Paleozoic subsident areas in Oman and Syria are illustrated in Figures 23 and 24, respectively. The Hercynian orogeny resulted in intraplate broad upwarps in Oman and Central Arabia, and downwarp in northern Syria (Figure 25) and associated widespread erosion. On the Central Arabia upwarp, Silurian strata were completely eroded (Figure 26). Transpressional basement horsts in Eastern Arabia are the underlying structure for the traps of the giant oil and gas fields (e.g., Ghawar÷Figures 27, 28, 29, and 30).
Two hydrocarbon systems, based on source rock, have been identified: Silurian and Precambrian (Figure 31). The source for the former is the Qusaiba hot shales, which were deposited over large areas of the Arabian plate (Figures 16). The hydrocarbons are trapped in Ordovician, Silurian, Devonian, and Permo-Carboniferous sandstone reservoirs and Permian carbonate reservoirs (Figures 32, 33, and 34). Except for Central Arabia, the system is gas-prone, due deep burial of the source rocks. The Precambrian source rocks have charged Precambrian, Paleozoic, and Mesozoic reservoirs in the interior salt basins of Oman (Figures 35 and 36).
á Silurian has high potential: USGS estimates it generated 808 TCF and 37 billion barrels of conventional resources.
Acknowledgment and appreciation is expressed to: AAPG, Saudi Aramco (Tom Connally, Dan Evans, John Filatoff, Said Al-Hajri, M. Dia Mahmoud, Jim McGillivray, Abdulla Al-Naim, Paul Nicholson), Shell (Geert Konert), Petroleum Development Oman (Henk Droste), ExxonMobil, and Gulf Petrolink (Moujahed Husseini).
Dr. Abdulkader M. Afifi was educated in Saudi Arabia (B.S., University of Petroleum and Minerals, 1977) and in the United States (M.S. Colorado School of Mines. 1981; Ph.D., University of Michigan, 1990).
His experience includes:
1980-86 = U.S. Geological Survey Mission, Saudi Arabia; Geological Mapping, Geochemical and Stable Isotopic Studies of the Mahd Adh Dhahab Gold District.
1991-Present - Saudi Aramco, Dhahran, Saudi Arabia. Several technical and supervisory positions in the Exploration Organization including Chief Explorationist and Chief Geologist. Currently, Senior Geological Consultant, Upstream Ventures Department.