Sedimentology and Diagenetic History with Reference to Reservoir Quality, Triassic Lower Jilh (Kra Al Maru), Kuwait

Abstract

The name Kra Al Maru has been assigned to the additional unit at the lowermost part of the Middle Triassic Jilh Formation in Mutriba and Kra Al-Maru area in western Kuwait. Stratigraphically the interval corresponds to the Jilh C Member of Jilh Formation, and it is divisible into a lower (KM-B) and an upper (KM-A) unit. Microfacies comprises anhydrite, dolomudstone, dolowackestone, argillaceous dolostone and dolomitic shales with minor dolopackstone, dolograinstone, lime mudstone, lime wackestone. Carbonaceous matter and terrigenous material is present at places. Anhydrite is present as early nodules and crystals, as well as late cement and vug fillings. Facies associations of both units are bioturbated, highly variable with common organic matters. The distinguishing feature of the lower unit (KM-B) is having less anhydrite than the upper unit (KM-A). The lower unit was deposited in intertidal to subtidal and lagoonal environments, as a shallowing upward sequence that grade upward to algal laminated wackestone and anhydrite. The presence of few sub-aerial exposure surfaces indicate dissolution that might have developed at the end of cycle and are indicative of slightly humid conditions. The upper unit was deposited in an intertidal to supratidal, Sabkha environments under arid climate. Diagenetic events include compaction, dolomitization, and replacement by anhydrite, fracturing and stylolization. Primary porosities were reduced by compaction, overdolomitisation and late stage cementation. Both cemented and uncemented fractures are observed in the core and microfractures are seen in core plugs and have led to increased fracture porosity and permeability. The Lower unit is ranked and pursued as new prospective units within the Jilh Formation.