Hydrocarbon-generating potential of the source rocks of the Sinian Doushantuo Formation on the western side of the Xuefeng Mountain

Abstract：Based on systematic observation, sampling and organic geochemical studies of Sinian Doushantuo Formation source rocks on the western side of the Xuefeng Mountain, this paper summarizes source rocks distribution, organic matter types, formation of source rocks and sedimentary environment, maturity of organic matter and hydrocarbon-generation history. The platform-basin deposition model of Sinian Doushantuo Formation has a close relationship with black shale distribution and organic carbon content. The source rocks are mainly distributed in northern Guizhou, eastern Guizhou, and southeastern Upper Yangtze plate. Black shale in northern Guizhou and eastern Guizhou is from 20m to 70 m in thickness, and high-value areas of organic carbon (2.0%) are mainly distributed in northern Guizhou, eastern Guizhou, and southeastern Upper Yangtze plate such as Shimen - Taoyuan - Anhua area. Organic macerals suggest that organic matters of the black shale are of Type I. The mass fraction of group elements is characterized by saturated hydrocarbon> non-hydrocarbon > aromatic hydrocarbon. The ratio of saturated hydrocarbon and aromatic hydrocarbon in basins and slopes is generally higher than that in the restricted platform, which suggests that the black shale series of Doushantuo Formation is sapropelitic source rock and that the deposition of parent materials was somewhat related to the depositional environment throughout the depositional period. Kerogen δ13Corg values in different depositional environments of Doushantuo Formation imply that there existed remarkable differences in the abundances of organic matters and sources of marine plankton, maritime bacteria and marine algae. For the specific sedimentary organic matter (marine plankton, maritime bacteria and marine algae) , the deeper the water, the lower the kerogen δ13Corg value, and the higher the organic matter content, indicating that the organic carbon content of source rocks was decided by the organic matter productivity, the formation of source rocks and the sedimentary environment. The reflectance of equivalent vitrinite (Rc) varies from 1.78% to 3.05% in the study area, and the Rc of Doushantuo Formation can be calculated by methylphenanthrene index. The Rc is 1.96% in Songlin, and from 2.06% to 2.10% in Meiziwan of Meitan, suggesting that most of the samples have reached the overmature stage, and a minor part of samples have reached the higher mature wet gas stage. The modeling results of hydrocarbon-generating history show that the source rocks of Doushantuo Formation at Zhangjiajie entered into the oil-generation stage in late early Silurian period (434 Ma) , entered into the oil-generation peak stage in early Middle Triassic period (244 Ma), and entered into the condensed oil and wet oil stage in late Middle Triassic, when the Ro (%) reached 1.3. The results show that the source rocks of Doushantuo Formation were deposited in the late rift basins characterized by higher paleogeothermal gradient, lower threshold depth of oil generation and earlier oil generation, which was in favor of the accumulation of Sinian Dengying Formation oil and gas in the early time.