The multi-scale pore system and its implication for shale oil occurrence in alkaline lacustrine mixed sedimentary shale reservoirs: A case study from the Fengcheng Formation, Mahu Sag, Junggar Basin

The pore structure of shale oil reservoir significantly affects the occurrence and mobility of hydrocarbons. The potential of a new type of alkaline lake shale oil has been demonstrated， but there are few reports on the pore system of alkaline lake shale， which restricts the efficient exploration and development of shale oil. This study investigates the Fengcheng Formation shale in the Mahu Sag of the Junggar Basin， employing methods such as low-temperature nitrogecn adsorption（LTNA）， mercury intrusion capillary pressure （MICP）， and nuclear magnetic resonance（NMR） to quantitatively characterize the multi-scale pore structure and fractal characteristics of shale， while evaluating the applicability of these methods. Based on a comprehensive analysis of material composition， different pore types， and fractal dimensions， the controlling factors for the development of different pore types and their seepage capacity are discussed. And a new perspective is provided for the study of the occurrence of shale oil from the standpoint of pore structure and fractals. The results indicate that inorganic mineral pores are the main development in alkaline lake shale， with the pore morphology being characterized by slit-like and ink-bottle shapes. The multi-scale pore size distribution（PSD） shows that II-micropores（10-100nm） and mesopores （100-1000nm） are the main contributors to the pore system. The development of II-micropores is associated with feldspar and calcareous minerals， the development of I-micropores（<10nm） and mesopores is related to quartz content， while large pores are mainly found in interlayer fissures of clay minerals. The development of I-micropores increases the roughness of pore surface and enhances the adsorption capacity of the pores， while the development of II-micropores associated with calcareous minerals hinders pore seepage capacity. Mesopores and macropores（>1000nm） exhibit good flowability. The high content of siliceous minerals plays a positive role in the pore system of alkaline lake shale. The shale with higher fractal dimension D_min exhibits greater adsorption capacity， which hinders the accumulation of free-state shale oil. Different types of pore space play different roles in the occurrence of shale oil， with free-state shale oil primarily occurring in micro-fractures and inorganic mineral pores， and the pore size is exceeding 10 nm.