Lithofacies composition and geological significance of black rock series: a case study from the lower Cambrian Qiongzhusi Formation in Huize area, eastern Yunnan province

Lithofacies is an important basis for the characterization of source rocks, shale oil and tight oil reservoirs and the evaluation of oil and gas reservoirs. In this study, a well-developed Qiongzhusi Formation section in Huize area was selected to study the lithofacies features and their evolution and the following conclusions are obtained. In the geological section, the lower part of the Qiongzhusi Formation black rock series is mainly composed of felsic fine-grained sedimentary rock (I) and dolomite or limestone (II). The middle part of the section is clayey felsic fine-grained sedimentary rock (I2), and the upper part is felsic clay rock (III2). Lithologic changes in Qiongzhusi Formation were controlled by terrigenous debris supply, hydrodynamic conditions and water depth. The felsic fine-grained sedimentary rocks formed when the water body was relatively deep in the early stage and the felsic clay rocks deposited when the terrigenous clastic supply was weakened in the middle to late stage have a high content of original organic matter, which is a favorable site for the evaluation of shale gas reservoirs.