Lithofacies and Pore Structure of Fine-grained Sedimentary Rocks of Qing-1 Member of Cretaceous in the Southern Songliao Basin

Fine-grained sedimentary rocks often contain hydrocarbon and mineral resources. Compared with coarse-grained sedimentary rocks, fine-grained sedimentary rocks are less studied. To elucidate the lithofacies and pore structure of lacustrine fine-grained rocks, the 340.6 m continuous core of Cretaceous Qing-1 Member from five wells in the southern central depression of the Songliao Basin was analyzed using X-ray diffraction, Rock-Eval pyrolysis, low-temperature nitrogen adsorption, high-pressure mercury injection, argon ion polishing-field emission scanning electron microscopy, and laser scanning confocal microscopy. Based on mineral compositions, organic matter abundance and sedimentary structure, lacustrine fine-grained rocks in the study area were divided into ten lithofacies, with their spatial distributions mainly influenced by tectonic cycle, climate cycle and provenance. Furthermore, pore structure characteristics of different lithofacies are summarized. (1) The siliceous mudstone lithofacies with low TOC content and the laminated/layered clay-bearing siliceous mudstone lithofacies with medium TOC content have the highest proportion of first-class pores (diameter > 100 nm), making it the most favourable lithofacies for the accumulation of shale oil and shale gas. (2) The massive clay-bearing siliceous mudstone lithofacies with low TOC content has the highest proportion of second-class pores (diameter ranges from 10 to 100 nm), making it a favourable lithofacies for the enrichment of shale gas. (3) The massive clay-bearing siliceous mudstone lithofacies with high TOC content has the highest proportion of third-class pores (diameter < 10 nm), making it intermediate in gas storage and flow. Laser confocal oil analysis shows that the heavy component of oil is mainly distributed in the clay lamina, while the light part with higher mobility is mainly concentrated in the silty lamina.