Sedimentary sequence evolution and organic matter accumulation characteristics of the Chang 8–Chang 7 members in the Upper Triassic Yanchang Formation, southwest Ordos Basin, central China

Abstract

Lacustrine organic–rich shales are widely developed in the Chang 7 member of the Yanchang Formation in the Ordos Basin. These shales have been proven to be the source rocks for crude oil and shale oil. The study of sedimentary evolution and the characteristics of organic matter (OM) accumulation is an important factor for the exploration and development of oil and gas resources. Based on the lithological assemblages, lithofacies, mineral contents, log curves and geochemical data, the sequence stratigraphic framework of the Chang 8–Chang 7 members of the Yanchang Formation in the southern margin of the Ordos Basin were established. A complete third–order sequence was identified, and it included four systems tracts: a lowstand systems tract (LST), a transgressive systems tract (TST), a highstand systems tract (HST) and a regressive systems tract (RST). By using organic petrology, organic geochemistry and elemental geochemistry, the abundance and source of OM, paleoclimate, paleowater conditions, paleoproductivity, and deposition rates of the different systems tracts of the Chang 8–Chang 7 are studied in detail. Various factors controlled the accumulation of OM in the different systems tracts. The abundance of OM in the LST is low, the kerogen type is mainly type III (including vitrinite); the abundance of OM increased gradually in the TST, the kerogen types are III and II2 (including vitrinite and sporophyte); the OM is mainly concentrated in the HST, and the kerogen type is mainly type II1 (including alginite), biomarker compounds show that OM has a mixed source of land plant and planktonic material; the abundance of OM in the RST is relatively high, the kerogen type is mainly type II2 and III (including vitrinite and alginite). The main controlling factor for OM accumulation was paleoclimate, which affected the primary productivity of lake and the chemical properties of the water column (redox and paleosalinity), leading to differences in OM accumulation under different climatic conditions. Hydrothermal activities contributed to anoxia and a salinity increase in lake bottom waters, with volcanic ash providing large amounts of essential nutrients for algal blooms. Based on the V/Cr, Sr/Ba, P/Ti and (La/Yb)N ratios with TOC values, the abundance of OM is positively correlated with paleoproductivity and water column reducibility, and negatively correlated with paleosalinity. The abundance of OM first increases and then decreases with increasing sedimentation rate. In general, the accumulation of OM was promoted under warm and humid climates, anoxic fresh to brackish water and adequate paleoproductivity.