Geochemical characteristics and sedimentary model of shales in Lower member of Zhongjiangou Formation in Wudun Sag, Dunhuang Basin

Analysis on the sedimentary environment of shale is of great significance for the evaluation of shale gas reservoir and the optimization of sweet spot. In order to deeply explore the depositional environment of organic-rich shale and the mechanism of organic matter accumulation in the Lower member of Zhongjiangou Formation of Wudun Sag in Dunhuang Basin, the TOC content, maceral composition, carbon isotope, major elements, trace elements and rare earth element were tested based on DY1 well. The results indicate that the lithologies of the Lower member of Zhongjiangou Formation in Wudun Sag are gray-black carbonaceous shale and silty shale with thin coal seam interlayer. The shale has a high TOC content, ranging from 0.53% to 25.25%, averaged 8.18%, with maturity R_ran ranging from 0.74% to 1.21%. According to the Mo content, P/Ti ratio and the maceral composition of organic matter, the sedimentation water has low primarily productivity and the rich organic matter is mainly contributed by the terrestrial higher plants, which indicates that the paleoproductivity of shales in the environment of shallow water delta and semi-deep lake is not the major controlling factor of organic matter accumulation. V/(V+Ni), Ce_anom, Th/U and the U_EF-Mo_EF covariation reveals that the shale in the Lower member of Zhongjiangou Formation was developed in an anaerobic environment. The indicators such as Sr/Cu, Rb/Sr and climate index C reflect that the paleoclimate was warm to semi-arid. The characteristics of Sr/Ba, Ba/Ga. Ca/(Fe+Ca) and Al₂O₃/MgO suggest that the palaeo-water was fresh to brackish. Besides, Zr/Al, Rb/K and MnO indicate that the sedimentation water was in shallow to semi-deep depth. According to the relationship between the parameters of sedimentary environment and the accumulation of organic matter in DY1 well, sedimentary models of organic-rich shale in the Lower member of Zhongjianggou Formation were established, with two cycles and four sedimentary models experienced form bottom to top, the sedimentation water evolved from shallow delta to semi-deep lake deposition, and then to shallow delta to semi-deep lake deposition again. The organic-rich shale was formed in the semi-deep lake environment with low primary productivity and terrestrial higher plant input and anaerobic conditions, and the anaerobic condition is the key controlling factor for the accumulation of organic matter in the shale in the Lower member of Zhongjiangou Formation. Generally, this study provides theoretical support for Jurassic shale gas accumulation mechanism, resources potential evaluation and favorable area selection in Dunhuang Basin.