Effects of smectite-illitization and hydrocarbon generation on the pore structure: a case study from the continental shales in China

Pore structure of shale significantly affects the occurrence and reserves of natural gas and oil. At present, the quantitative characterization of pore structure related to clay minerals in high-clay continental shale remains inadequate. The mechanism of clay mineral transformation and pore preservation of continental shale is still unclear. To address this gap, an organic-rich, high-clay shale from the first member of the Qingshankou Formation (K 2 qn 1 ) in the Gulong Sag, Songliao Basin, China, was analyzed, based on the measurement of argon ion polishing-field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), total organic carbon (TOC), vitrinite reflectance (Ro), pore structure parameters, and fractal dimension obtained from the isothermal N 2 adsorption experiment, integrated with well logging data. Quantitative characterization of the modification on shale pore structure by smectite illitization and hydrocarbon generation from organic matter was conducted. The primary factors influencing pore structure, and pore preservation mechanism were then elucidated. Research result shows that the mesopores (2–50 nm) formed during smectite illitization and micropores (< 2 nm) produced by hydrocarbon generation from the organic matters significantly contributed to the pore volume and specific pore surface area of the shale. The microcrystalline quartz, formed in the smectite illitization, caused the loss of some pores, but the rigid frame composed of the microcrystalline quartz, as well as the overpressure generated partly by hydrocarbon generation from organic matter and partly by smectite-to-illite transformation, preserved some pores. This work highlights that smectite illitization and hydrocarbon generation from organic matter are crucial mechanisms for pore formation and preservation in continental shale with a high content of clay minerals. • Qingshankou shale pore volume positively correlates with clay mineral content. • Smectite illitization boosts pore volume, enhancing roughness and complexity. • Smectite illitization exerts dual impacts on shale pore system. • Smectite illitization and hydrocarbon generation impact the pores evolution.