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The first sediments to onlap the metamorphosed Precambrian Arabian shield were Infracambrian (Proterozoic) to Middle Cambrian carbonates, clastics, and evaporites. The oldest Arabian reservoir rocks occur in the Precambrian to lower Paleozoic Ara Salt of the Huqf Group, which forms the Birba field of Oman. The Middle Cambrian sequence was followed by Late Cambrian through Early Permian marine sandstones and continental to littoral siltstones and variegated shales. The first commercial oil discovered in the Arabian Gulf region occurs in fluvial sands of the Ordovician to Permian Haima and Haushi Groups of the Marmul field in south Oman. These strata are also productive in other fields and are sealed by unconformable contact with the Al Khlata Formation or beneath shale of he Albian Nahr Umr Formation. The deeply buried kerogen sediments of the Huqf Group to the southeast are believed to be the source rocks for these fields of south Oman.
The Late Permian to Triassic deposits of the Arabian Peninsula are mainly widespread carbonates and evaporites that were deposited during a period of relative tectonic stability. Their deposition on an epeiric shelf was punctuated by a series of transgressions and regressions. Significant gas reserves have been proven in deep wells in the Arabian Gulf. These wells penetrate large deep structures in the Permian Khuff shelf carbonates. These carbonates have developed secondary porosity and lie beneath interbedded shale and dolomites of the Sudair or Suwei Formation. The source of gas in the Khuff is unknown but could lie in more deeply buried formations. The large deep structures of the Khuff are considered to be among the most attractive for gas potential in the region today.
The stable-shelf depositional environment established during the Permian and Triassic continued through the Mesozoic. Jurassic rock units of the Arabian Gulf formed a sedimentary megacycle of epeiric shelf carbonates. These include relatively deep-water mudstones and wackestones to shallow-water grainstones and packstones with the greatest facies variations occurring in the post-Dhruma to Arab "D" rocks. Some of the best and most extensive reservoirs of the area occur in the primary and secondary porosity of the Upper Jurassic Arab Formation, and in the Middle Jurassic Araej and Dhruma Formations of Saudi Arabia, Qatar, and Abu Dhabi. The sources for hydrocarbons in these Jurassic sediments are thought to be in the Dhruma and lower Hanifa Formation (Diyab equivalent) of the gulf area n general. The Jubailah is a source in Qatar and some of the offshore gulf. The most common seals are the argillaceous carbonate at the top of the Dhruma and the Hith anhydrite, the latter also marking the termination of Jurassic deposition across the Arabian platform. The Upper Jurassic "cycles" of marine limestones and anhydrites of the Arab and Hith Formations accumulated in response to changes of sea level caused either by regional epeirogenic movements or by eustasy.
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