Rock Fabric of Lacustrine Shale and Its Influence on Residual Oil Distribution in the Upper Cretaceous Qingshankou Formation, Songliao Basin

Rock fabric and its influence on residual oil distribution are key issues to the highly efficient development of shale oil. This study targeted the rock fabric and residual oil distribution, and samples were selected from the first and second members of the Upper Cretaceous Qingshankou Formation in the Songliao Basin. Multiple methods were used to analyze rock fabric, including material composition, scanning electron microscopy (SEM), micro-computed tomography (micro-CT), and low-temperature nitrogen adsorption (LT-NA). The residual oil distribution was investigated by methods of oil extraction and LT-NA. The organic matter of most samples belongs to type I, and clay is the main mineral component, which can be as high as 65.4%. There are a lot of inorganic pores at the nanoscale, while the organic pores mainly range from 10 nm to 2 mu m, and at the microscale, the pore connectivity is poor in both formations. The apparent and intrinsic average specific surface areas (SSAs) are 5.35 and 10.23 m2/g, respectively, indicating that the average SSA of post-oil extraction is nearly 2 times that of pre-oil extraction. The residual oil has a wide distribution among different pores, ranging from 1 to 200 nm. In most cases, the residual oil mainly exists in pores between 1 and 5 nm, indicating small pores holding abundant oil. The pore space ratio for residual oil has a negative relationship with clay content, total organic matter (TOC), pyrolysis hydrocarbon (S2), and intrinsic average pore diameter, indicating that higher clay content is detrimental to liquid hydrocarbon generation. Higher TOC and S2 mean less generated liquid hydrocarbon, and a higher intrinsic average pore diameter means fewer nano organic pores for liquid hydrocarbon. This study is conducive to understanding the rock fabric of lacustrine shale and its influence on residual oil distribution.