Micro-pore structure in an inter-salt shale oil reservoir and the relationship with physical properties in the fourth section of the third member of Qianjiang Formation, Qianjiang Sag, Jianghan Basin

Abstract

The micro-pore structure of an inter-salt shale oil reservoir and its relationship with physical properties in the fourth section of the third member of Qianjiang Formation (Eq34) in the Qianjiang Sag of Jianghan Basin were studied using X-ray diffraction, thin section observation, conventional scanning electron microscopy (SEM), argon ion polishing SEM and three-dimensional pore reconstruction of focused ion beam, microscopy combined with quantitative analysis of mercury intrusion and nitrogen adsorption pore volume. According to the level of mineral content, the reservoir is divided into three categories: dolomitic mudstone, argillaceous dolomite and muddy dolomite. The mineral composition and structure of the reservoir are highly heterogeneous, and the conventional core-plug porosity varies greatly, ranging from 1% to 13%. The porosity of dolomitic mudstone is low, the porosity of argillaceous dolomite and muddy dolomite is high, and the porosity of mudstone with high content of glauberite is low. The reservoir microstructure changes greatly, and the dolomitic mudstone has a directional arrangement structure, mainly with elongated or flat micropores. The pore size is small, with a pore diameter of 20-50 nm, and the largest connecting throat is small, ranging from 22 to 42 nm. The argillaceous dolomite mainly contains residual intergranular pores and pores between clay layers. The pore diameter becomes larger, mainly 20 to 80 nm. The maximum connecting throat varies greatly, between 16 and 158 nm. The muddy dolomite is mainly composed of uniform grain structure, polygonal equidimensional intergranular micropores. The pore size is large, 80-180 nm. The largest connecting throat is large, at 158-196 nm. The pore connection of dolomitic mudstone, argillaceous dolomite and muddy dolomite has the characteristics of "fracture-fracture, pore-fracture, pore-pore connection", respectively. The muddy dolomite has high porosity, wide connecting throats, and favorable pore structure, indicating the best shale oil storage space.

Keywords

• mineral composition
• porosity
• micro structure
• shale oil reservoir
• qianjiang formation
• qianjiang sag
• jianghan basin