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Paleozoic Hydrocarbon Habitat in the Arabian Plate*
By
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])
Abstract
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.
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Paleozoic Tectonics
Paleozoic TectonicsThe 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).
Paleozoic Stratigraphy
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
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
Widespread Upper Ordovician periglacial deposits (Figures 14 and 15) 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)
Hercynian DeformationFigure Captions (22-30)
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
Summary
á
Silurian has high potential: USGS estimates it generated
808 TCF and 37 billion barrels of
Acknowledgments
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
About the AuthorDr. 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. |