Hybrid Sedimentary Conditions of Organic-Rich Shales in Faulted Lacustrine Basin during Volcanic Eruption Episode: A Case Study of Shahezi Formation (K1sh Fm.), Lishu Faulted Depression, South Songliao Basin

JLYY-1 well was drilled to explore the continental shale gas resources by the China Geological Survey of the Shahezi Formation in the south of Songliao basin, where 51 m hybrid sedimentary shale layer contributed to a high-yield shale gas breakthrough. Through coring observation and indoor comprehensive analysis, it is found that this set of 51 m shale has the following characteristics. First, hybrid sedimentary(carbonate minerals and siliceous minerals account for a high proportion, followed by volcanic materials). Second, shale is intercalated with multiple sets of thin-layer tuff or tuffaceous mud/shale. Thirdly, a large number of bubbles gushing along the foliation surface were observed in the field core immersion experiment. Based on the element logging data of well JLYY-1, combined with the test results of scanning electron microscope imaging, optical microscopic imaging, and Roqscan mineral composition imaging, it is summarized as the formation mode of organic-rich hybrid sedimentary shale in the faulted continental basin during volcanic activity periods, and the formation conditions are as follows. (1) The input of multiple organic sources improves the abundance of organic matter in shale contributing to the co-preservation of type I, II and type III kerogens. (2) In the later stage of volcanic activity, the migration of nutrient elements leads to algae bloom in the intervening shales. (3) Carbonate diagenesis contributes to hydrocarbon generation in the hybrid sedimentary shales which are sedimented in a closed saltwater environment under dry and hot climates. (4) The oxidation stratification of the water body forms four types of laminae of shale in this section: siliceous, clayey, calcareous, and pyrite laminae. © 2022, Editorial Department of Earth Science. All right reserved.