Mineralogical Evolution of the Cretaceous Strata in the Songliao Basin, Northeastern China: Implications for Thermal History and Paleoenvironmental Evolution

The Songliao Basin in northeastern China is one of the largest and longest-lived Cretaceous sedimentary basins enriched in petroleum and geothermal resources worldwide. Although the modern Songliao Basin has a high geothermal gradient, the geological thermal history of the basin has not been well constrained. The SK-2 drilling program, as the second stage of the International Continental Drilling Project of Cretaceous Songliao Basin, is for recovering extensive Early Cretaceous terrestrial strata and providing valuable materials for decoding the mineralogical evolution and the paleoenvironmental changes. Here, we present whole-rock and clay mineralogical analysis on 72 core samples covering 3346-5705 m of the Shahezi Formation in the SK-2 borehole. The whole-rock minerals mainly include clay minerals, quartz, plagioclase, as well as some calcite, K-feldspar, siderite, and pyrite. The clay mineral assemblages include illite, chlorite, and illite-smectite interlayer minerals. Above 4500 m, clay minerals are dominated by illite and illite-smectite interlayers. Below 4500 m, more plagioclase, K-feldspar, and calcite are present, while illite-smectite interlayers are completely replaced by illite. The whole-rock and clay mineralogical evolution of the Shahezi Formation is primarily controlled by thermal diagenesis, although paleoenvironmental change may act as a minor contribution. Combined with published data from the Upper Cretaceous in SK-1 cores, we infer that Cretaceous greenhouse climatic and environmental changes left fingerprints on whole-rock and clay mineralogical assemblages and that the Songliao Basin reached a maximum burial depth and a peak of thermal evolution at the end of the Cretaceous.