Distribution pattern of tricyclic terpanes and its influencing factors in the Permian source rocks from Mahu Depression in the Junggar Basin

Abstract

Biomarkers extracted from the crude oil and source rock samples obtained from the northwestern margin of Junggar Basin are characterized by high abundance of tricyclic terpanes (TTs).The distribution pattern of TTs has been used to distinguish oil families and to conduct oil-source correlations.Comprehensive geochemical analyses of source rock samples reveal that different distribution patterns of TTs correspond to different sets of source rocks, i.e.the Fengcheng Formation(P1f) features in “Going Up” of C20, C21 and C23 TTs, with higher amounts of C24 and C25 TTs for mudstones in comparison with carbonate rocks; the Lower Wuerhe Formation(P2w)and the Jiamuhe Formation(P1j)are characterized by “Going Down” of C20, C21 and C23 TTs.The influences of sedimentary environment, kerogen type, maturity and hydrocarbon expulsion on distribution of TTs were studied through microscopic observation of thin sections and hydrous pyrolysis simulation.We concluded that the TTs distribution patterns of P1f and P2w are mainly controlled by sedimentary environment and kerogen type.The organic matters of the P1f source rocks were mainly derived from autochthonous algae or bacteria and were deposited in an evaporative reducing salt lake, while allochthonous organic matters were relatively abundant in the P2w and were deposited in a semi-reducing fresh lacustrine environment.The P1j source rocks are currently over-mature and have experienced intense carbonization.Burial thermal evolution has limited impacts on the distribution patterns of TTs during mature and high-mature stage, but can result in the increase of low carbon number terpanes during over-mature stage.Hydrocarbon expulsion has limited influences on the TTs distribution pattern.