Geology and hydrocarbon occurrences of the elastic Permo-Carboniferous in the central and eastern Arabian Basin

Abstract

The siliciclastic lower part of the Permo-Carboniferous in the Arabian Basin represents a cyclic transgressive and regressive unit, consisting of sandstones, shales and thin beds of argillaceous limestone. This unit crops out in small exposures in central and northwest Saudi Arabia, but is widespread in the subsurface of central and eastern Arabia. It is known as the Unayzah Formation in Saudi Arabia and in the western and southern Arabian Gulf region, and as the Haushi Group in Oman. The Permo-Carboniferous clastics in the Arabian Basin proved to be prospective for hydrocarbons. Oil and gas were encountered in Saudi Arabia, Qatar, the united Arab Emirates (U.A.E.) and Oman. The Unayzah Formation 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 section is dominated by a thick sequence of very fine to coarse-grained, subangular to subrounded, moderately to poorly sorted quartzitic sandstones with minor interbeds of siltstone. Minor quantities of clay minerals, plagioclase, dolomite and pyrite occur in the sandstones. The formation is interpreted to be of fluvial origin. Six lithostratigraphic units were identified in the U.A.E. Thee units have moderate to good reservoir potential, while others act as seals over these reservoirs. Porosity in the reservoir units ranges from less than I to 27%, and permeability from less than 1 to 75 md. The sediments in the U.A.E. are highly affected by diagenesis. The main diagenetic events include silica cementation (which occurs as quartz overgrowths), precipitation of illite and minor kaolinite, carbonate cementation (calcite and dolomite) filling pores around the quartz grains, and locally a partial dissolution and leaching of feldspar grains.