Source rock potential and depositional environments of the Jurassic and Cretaceous coals from the Gyda Peninsula, Western Siberia

Abstract

Whether the coals in Western Siberia generated commercial volumes of liquid hydrocarbons is an important issue that remains controversial because it has not yet been definitively resolved. Organic petrography and geochemical investigations including Rock-Eval pyrolysis, elemental, isotope, and biomarker analyses were carried out to characterize in detail the hydrocarbon generation potential, type of organic matter, and depositional environment of the Cretaceous and Jurassic coals from the Gyda Peninsula in Western Siberia. The results indicate that the Jurassic coals primarily contain mixed gas-oil-prone Type II-III kerogen. In contrast, the Cretaceous coals exhibit a diverse generation potential with varied proportions of gas- and oil-prone kerogen types. Vitrinite reflectance of the studied coals suggests an immature to early mature stage for the Cretaceous coals and the thermally mature stage for the Jurassic coals, which is supported by maturity-related biomarker parameters and T_max values. The carbon and nitrogen isotopic signatures in combination with biomarker data and maceral composition predominantly indicate terrestrial input. The presence of retene and simonellite suggests the contribution of conifers. Based on the obtained results, the Jurassic coals were likely to be deposited in limno-telmatic conditions with a rise in the water-level due to the sea expansion. The Cretaceous coals are probably deposited in bogs developed on the lower delta plain in limno-telmatic to telmatic environments with sudden episodes of flooding. Considering the type and thermal maturity of organic matter, the Jurassic coals show strong potential for hydrocarbon generation.