Geochemical characteristics of oil shale in the Triassic Chang7 subsection, southern Ordos basin, China, and palaeo-environment reconstruction

The lacustrine facies oil shale layer present in the Chang7 subsection of the Yanchang Formation in the Ordos basin represents good hydrocarbon source rocks. The palaeo-environment of the oil shale deposition is one of the most important controlling factors of the types and abundance of organic matter. In this study, organic geochemistry, element geochemistry and paleontology were investigated to depict its sedimentary environment. The study results show that the oil shale produced from the southern Ordos basin is high in organic matter at immature to low mature stage. Analysis of the maceral composition has shown that the kerogen of organic matter is of sapropelic-humic type. The shallow lake facies is the major facies belt for the development of oil shale in this area. Through the analysis of sporopollen and several geochemical parameters, e.g., Sr/Cu, climate index and Sr/Ba, it is indicated that the oil shale was deposited under warm-humid climatic conditions and a low salinity water column. The reducing water bottom, indicated by Ni/V and V/(V+Ni) ratios, provided ideal conditions for the preservation of organic matter in the oil shale.