Differential reservoir formation mechanisms of shale oil reservoirs in the Qingshankou Formation, Cretaceous system, Songliao Basin

To investigate the lithofacies assemblages, reservoir types, oil content, and mobility of shale oil reservoirs in the Cretaceous Qingshankou Formation of the Songliao Basin, a comprehensive set of analyses was conducted, including thin-section mosaic imaging, argon-ion-polished field-emission scanning electron microscopy (FE-SEM), whole-rock X-ray diffraction (XRD), total organic carbon (TOC) determination, petrophysical analysis, and laser confocal microscopy (CLSM). These methods were used to systematically characterize the reservoir development features and differential formation mechanisms of shale reservoirs in the Qingshankou Formation. The results indicate that: (1) the shale reservoirs are characterized by high feldspar content, high clay mineral content, and moderate carbonate mineral content. Feldspar mainly occurs in banded patterns, whereas carbonate minerals are mainly present as micrite, nodular aggregates, and banded structures; (2) shale laminations are poorly developed and show limited diversity, being mainly composed of felsic and carbonate laminations, whereas bioclastic laminations are rare. The mineral assemblages of shale laminations are dominated by two types: “felsic minerals + clay minerals” and “carbonate minerals + clay minerals”; (3) the shale succession mainly consists of two lithofacies, namely massive felsic mudstone and laminated felsic shale. Reservoir space is dominated by intergranular pores associated with clay minerals, whereas lamination-related fractures and high-angle fractures are only weakly developed. Locally, a small number of dissolution pores, intergranular pores, and organic pores can be observed; (4) shale oil mainly occurs in a dispersed state, with light/heavy ratios ranging from 0.45 to 0.57; and (5) sedimentary environment controls lithofacies assemblages, diagenesis controls pore evolution, tectonic activity controls fracture development, and hydrocarbon-generation and thermal-evolution history controls hydrocarbon occurrence and distribution.