AAPG Middle East Region Geoscience Technology Workshop

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Petrography of Diagenetic Concretions of Miqrat Formation in Huqf Region Outcrops, Central Oman


The middle Cambrian Miqrat Formation of Mahatta Humaid Group outcrops in the Huqf region of central Oman, and is a complex, tight gas-producing reservoir in the subsurface. A large number of single or aggregates of concretions are randomly distributed within sandstone beds of the formation. These concretions are sometimes aligned along the bedding or cross-bedding surfaces and commonly following tectonic fractures. Concretions are spheroidal in shape and range in size from 0.1 cm up to 5 cm diameter, and are dominantly calcite and rarely barite in composition. Field and lab studies of nearly 50 concretion samples revealed that concretions vary in size, distribution and mineral composition. The small size (0.1-1 cm) concretions are common in the supratidal to intertidal facies of the lower Miqrat Formation, whereas the large size (1-5 cm) are abundant in the subtidal facies of the middle Miqrat Formation. The petrographic and geochemical analysis revealed the presence of three types of concretions; type 1, type 2 and type 3. Type 1 concretions are attributed to the sandstones of lower and middle Miqrat Formation that are pervasively cemented by poikilotopic calcite filling relatively large size intergranular pores lacking quartz overgrowth in loosely packed framework grains. Type 2 concretions are restricted to the subtidal sandstones of the middle Miqrat Formation and are cemented entirely by poikilotopic barite in a relatively loosely to moderately packed framework grains. Associated diagenetic alterations with type 1 and type 2 concretions include rare kaolinite, illite, quartz overgrowths, feldspar overgrowths and dissolution of feldspar grains. Petrographically, cementation of loosely to moderately packed framework grains by calcite and barite may suggest their precipitation during eodiagenetic regime at near-surface and subsequent progressive burial at shallow depths. If this is the case, then the required Ca ions for cementation by calcite may come from sea-waters and the Ba and SO4 for cementation by barite may come from the dissolved feldspars and sea-waters respectively. Meso- and telogenetic origin of both calcite and barite cement cannot be excluded and thus isotopes analysis including O, C, S, and Sr are required to shed light on their origin and timing of precipitation. Type 3 concretions are pervasively cemented by poikilotopic calcite along tectonic fractures. Their association with tectonic fractures suggests precipitation under telogenetic regime subsequent to uplift. Understanding calcite and barite concretions distribution and timing of precipitation within Miqrat Formation will help in better prediction of reservoir heterogeneity and compartmentalization, which are key components in hydrocarbon exploration, production, and enhanced recovery plans. Keywords: Barite, Calcite, Concretions, Miqrat Formation, Mahatta Humaid Group, Huqf, Oman