Lake-bottom hydrothermal activities and their influence on high-quality source rock development: A case from Chang 7 source rocks in Ordos Basin

By optical microscopy and SEM-energy spectrometry, some phenomena that relate to lake-bottom hydrothermal activities, such as the presence of siliceous rocks, ankerite laminar deposits, evidence for marcasite brassil anhydrite symbiosis systems and authigenetic albite filling in early diagenetic fractures, have been discovered in the Chang 7 high-quality source beds of the Yanchang Formation, Ordos Basin. Various geochemical data of the Chang 7 high-quality source rocks testify further to the existence of lake-bottom hydrothermal activities, and they include abnormally abundant S2- content (on average 7.37%), Mo, Cu, U and Mn positive anomalies, high ratios of U to Th, the positive carbon isotope composition of crystalline ankerite (δ13C from 2.88 to 3.03‰), depleted oxygen isotope composition (δ18O PDB from −16.41 to −16.17‰), the enrichment of microelements like Cu and Mn, and isotopically enriched in 34S sulfur isotope compositions of strawberry-shaped pyrite (δ34S CDT from 2.37 to 5.90‰). In the early sedimentary period of the Chang 7, the activation of floor ruptures by violent regional structural activities was a dynamic factor that produced lake-bottom hydrothermal activities. The hydrothermal activities played an important role in accelerating the extensive development of the Chang 7 high-quality source rocks.