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

End_Page 15---------------

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 the 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. Other 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 the 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 owing 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, which prevented any mixing with Mesozoic hydrocarbons.

The Precambrian source rocks have been proved 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 because of multiples, seismic transparency and near-surface problems; (2) the prediction of porosity in the tight, deeply buried reservoirs; and (3) the hostile subsurface environments.

End_of_Record - Last_Page 17---------------