Hydrocarbon Generation Potential and Paleo-Depositional Environments of the Laffan Formation in the Binak Oilfield: Results from Rock-Eval Pyrolysis and Organic Petrographic Studies

In this study, the hydrocarbon potential and depositional environments of the Coniacian Laffan formation were investigated in the Binak oilfield, SW Iran. With an average thickness of 80 m, the Laffan formation consists mainly of gray shales and thin argillaceous limestones in the study area. In order to investigate the hydrocarbon potential, 22 cutting samples from 5 wells of the Binak oilfield were analyzed by Rock-Eval 6 pyrolysis and organic petrographic techniques. The hydrogen index (HI) versus Tmax diagrams indicated mixed-type II/III kerogen with a maturity corresponding to the early stages of the oil window (Tmax ≈ 435 °C). In addition, plots of S1+ S2 versus TOC were consistent with a weak to excellent hydrocarbon potential for the Laffan formation. On the other hand, organic petrographic techniques indicated that the primary organic constituents of the Laffan formation are inertinite and bituminite with subordinate amounts of amorphous organic matter (AOM). In other words, the contained organic matter was mainly composed of inertinite and lacked significant hydrocarbon potential. An abundance of inertinite and the conspicuous absence of vitrinite macerals in the studied samples suggested that the Laffan formation was deposited under sub-oxic marine conditions. Furthermore, the presence of bituminite in the studied samples greatly influenced the Rock-Eval pyrolysis readings, so geochemical evaluation of the Laffan formation using only Rock-Eval pyrolysis data may lead to erroneous interpretations. Therefore, a combination of Rock-Eval and organic petrographic methods is necessary for reliable geochemical evaluation of the Laffan formation. The results of this study can be useful for a better understanding of the Cretaceous hydrocarbon system in the study area.