SEDIMENTARY DIAGENETIC INTERPRETATION AND RESERVOIR CHARACTERISTICS OF THE MIDDLE JURASSIC (ARAEJ FORMATION) IN THE SOUTHERN ARABIAN GULF

Oil and gas condensates discovered so far in the Middle Jurassic Araej Formation occur in several structures in western and central offshore Abu Dhabi and Qatar. The source for the oil is thought to have migrat ed updip primarily from the Hanifa/Diyab Formation (Upper Jurassic), w hereas the source of the gas condensate is believed to be from Middle Triassic-Lower Jurassic formations. The Araej Formation is described f rom well data and thin sections from wells drilled in the south and so uth-west Arabian Gulf. It is a carbonate sequence that has a thickness ranging from 180 to 300 m (590 to 980 ft) and is divided into three m embers: Lower Araej, Uwainat and Upper Araej. The Lower Araej Member i s made up of slightly argillaceous lime mudstones and wackestones with subordinate interbeds of peloidal/bioclastic packstones and grainston es. The Uwainat Member consists of peloidal bioclastic packstones and grainstones with subordinate wackestones and lime mudstones. The Upper Araej Member consists predominantly of variably cemented grainstones and packstones with minor wackestones. The sediments of the Araej Form ation were deposited as a shallowing upwards sequence in quiet shelf w aters grading to a shallow to very shallow marine shelf. Lithological variation within each member suggests higher order shallowing upwards sequences produced by sea-level fluctuations. Petrographic analysis of the Araej suggests a diagenetic path which includes neomorphism and r ecrystallization of allochems and marine cements to low Mg-calcite, di ssolution and partial dolomitization of muddy lithologies. Pressure so lution seams and stylolitization improved the permeability and these c ompactional features are often filled by bitumen. Some seams are fille d by coarse mosaic calcite crystals or coarse rhombic dolomite, which is believed to be a late diagenetic cement. Porosity is highly variabl e, but never exceeds 16%. Intercrystalline, interparticle, mouldic and vuggy porosities are the dominant pore types.