Geochemistry and organic petrology of organic-rich shales of the Upper Jurassic-Lower Cretaceous Bazhenov Horizon in the Frolov Region, West Siberian Basin: Implications for the reconstruction of the organic matter origin and paleoredox conditions

The objective of this study is the reconstruction of paleodepositional conditions that have prevailed in the Bazhenov Sea during the sedimentation of the organic matter-rich black shales of the Upper Jurassic-Lower Cretaceous Bazhenov Horizon from the western West Siberian Basin, with the aim to gain insights into the primary forces and mechanisms favouring enhanced organic matter preservation and accumulation. Elemental analysis, rock pyrolysis and petrographic observations were carried out on a total of 298 core samples from a 60.6 m thick well section, covering black shales of the Tutleim Formation and parts of the under- and overlying Abalak and Frolov Formations from the south-western West Siberian Basin (Frolov region). The results reveal systematic depth variation of sedimentary and diagenetic conditions, e.g. detrital influx and paleoredox conditions. The sediments were deposited in a marine environment with a significant contribution of authigenic minerals and subordinate detrital influence. The organic matter composition suggests an aquatic and bacterial origin with minor input of land plant-derived material. The enrichment of redox-sensitive inorganic and organic compounds results from reducing and periodically sulfidic conditions in the bottom and uppermost pore water columns during deposition. Sections that show evidence of anoxia or euxinic conditions simultaneously represent the lowest terrigenous influence, suggesting that anoxia is closely linked to sea-level fluctuations in the course of eustatic transgression-regression cycles. A distinctive correlation pattern of organic matter quantities and redox proxies suggests that organic matter accumulation and preservation are mainly controlled by paleoredox conditions and, therefore, regression-transgression cycles. The most prolific source rock sections are defined as the central Tutleim units, T4 and T5, marking the transition from marine transgression to regression at the Upper Jurassic-Lower Cretaceous boundary.