Peculiarities of Molecular Composition of Bitumoids in Bazhenov Deposits in the Northern Part of the West Siberian Platform

The aim of the investigation is to study the molecular composition of hydrocarbon compounds (HC) in the pore space of the Bazhenov horizon rocks in the northern part of Western Siberia. The oil and gas source rock samples with mixed and continental organic matter from two wells G5 and G6 were investigated. The biomarker analysis of the bitumen from open, blocked, and closed pores showed significant differences in the composition of HC, which allowed to divide them into two groups. The presence of migrated HC in the open pores of the studied samples was determined by the differences in the biomarker parameter values characterizing the conditions of organic matter sedimentary conditions. The composition of migrated bitumen expresses the oxidative conditions of the parent predominantly continental organic matter sedimentation. Bitumen in the closed pores, on the contrary, was formed from predominantly marine OM in the reducing conditions. The maturity parameters of bitumen confirmed that the migrated HC have a different source. The values of Ts/(Ts+Tm), Ts/C30, 4MDBT/1MDBT, TAI/(I+II) for migrated bitumen from G5 and G6 wells characterize the stages of early-middle oil window respectively, closed pore HC – the stage of immature kerogen to early oil window.