Organic geochemical characteristics and mineralogy of the Asmari basal shale unit in the central Dezful Embayment

Asmari Formation, comprises mixed carbonate-siliciclastic lithologies in the central Dezful Embayment, with thick shale intervals at the base. In this study mineralogy, organic petrography, thermal maturity and generation potential of the basal shale sediments of the Asmari Formation is investigated in Ahwaz, Marun and Kupal oilfields. Clay mineralogy including kaolinite and mixed layer illite-smectite are the most abundant minerals. Secondary important minerals are calcite and quartz. Microscopic examinations indicate that the organic matter in this unit have mostly originated from continental settings and comprises vitrinite, inertinite, pollen, woody phytoclasts with minor marine liptinite macerals and amorphous materials. TOC contents range from 0.2 to 1.25 wt.% (avg. 0.75 wt%), indicating poor to fair oil generation potential. HI values ranging between 141 to 244 (mg HC/g TOC), is consistent with type III and II/III kerogen. The dominance of terrestrial organic matter (kerogen type III), with relatively high oxygen index indicates that the organic matter was most likely deposited under oxic/suboxic conditions when the relative sea level was low. Vitrinite reflectance (0.45-0.7 %Ro) and Tmax values (417-444 ℃, avg. 432 ℃) suggest that the Asmari basal shale is thermally immature to early mature in the study area.