Controls of clay mineral transformation and organic matter on pore networks of the Paleogene lacustrine shale oil system in the Yitong Basin, NE China

Abstract

Shale oil exploration has achieved a breakthrough in the first member of the Paleogene Shuangyang Formation in the Luxiang fault depression of Yitong Basin. However, the characteristics and evolution of medium–high maturity siliceous shale in this lacustrine basin remain to be fully understood. The coupled evolution of organic matter (OM), quartz, different types of clay, and pore space in the Shuangyang shale is investigated considering differences in pore structure compared to marine and continental shales with low-medium maturity. The Shuangyang shale (depth greater than 3400 m), mainly argillaceous-rich siliceous shale, significantly influences pore volume and specific surface area through the effect of mesopores (2–50 nm). Ink-bottle-shaped pores are dominated by OM pores and dissolved interparticle pores. Parallel plate-shaped pores are predominantly OM shrinkage fractures and intracrystalline pores. Shale pore volume and specific surface area decrease with increasing kaolinite content. The presence of chlorite in foliated form and the ordered distribution of illite/smectite mixed layers (I/S) enhances the specific surface area. Pore overpressure from smectite dehydration during the transformation of I/S to illite helps preserve the primary pores. Pore volume increases as I/S decreases and illite increases. In the middle-high maturity stage (Ro = 1.24–1.34 %), OM content emerges as the primary factor influencing pore formation. The coupling effect of siliceous mineral skeleton (quartz) and overpressure is crucial for pore preservation. This study expands our understanding of shale pore systems in argillaceous-rich siliceous shale within the medium–high maturity range and reveals significant discrepancies in shale pore evolution concerning OM and mineral components.