GEOLOGY AND HYDROCARBON OCCURRENCES OF THE CLASTIC PERMO-CARBONIFEROUS IN THE CENTRAL AND EASTERN ARABIAN BASIN

The siliciclastic lower part of the Permo-Carboniferous in the Arabian Basin represents a cyclic transgressive and regressive unit, consisti ng of sandstones, shales and thin beds of argillaceous limestone. This unit crops out in small exposures in central and northwest Saudi Arab ia, but is widespread in the subsurface of central and eastern Arabia. It is known as the Unayzah Formation in Saudi Arabia and in the weste rn and southern Arabian Gulf region, and as the Haushi Group in Oman. The Permo-Carboniferous clastics in the Arabian Basin proved to be pro spective for hydrocarbons. Oil and gas were encountered in Saudi Arabi a, Qatar, the United Arab Emirates (U.A.E.) and Oman. The Unayzah Form ation in the subsurface of the U.A.E. ranges in thickness from 140 to 206 m. The relatively thin upper section consists of pyritic siltstone and terrigenous mudstone with minor sandstone, whereas the lower sect ion is dominated by a thick sequence of very fine to coarse-grained, s ubangular to subrounded, moderately to poorly sorted quartzitic sandst ones with minor interbeds of siltstone. Minor quantities of clay miner als, plagioclase, dolomite and pyrite occur in the sandstones. The for mation is interpreted to be of fluvial origin. Six lithostratigraphic units were identified in the U.A.E. Three units have moderate to good reservoir potential, while others act as seals over these reservoirs. Porosity in the reservoir units ranges from less than 1 to 27%, and pe rmeability from less than 1 to 75 md. The sediments in the U.A.E. ate highly affected by diagenesis. The main diagenetic events include sili ca cementation (which occurs as quartz overgrowths), precipitation of illite and minor kaolinite, carbonate cementation (calcite and dolomit e) filling pores around the quartz grains, and locally a partial disso lution and leaching of feldspar grains.