THE VARIATIONS OF MICROSCOPIC PORE STRUCTURE AND COMPONENT AND EFFECTS ON THE HYDROCARBONS CONTENT IN THE HYDROCARBONS GENERATION OF SHALE AND MUDSTONE

The study collected and selected the mudstone and shale samples of Chang 7, Ordos Basin and then divided a sample into five samples, the five samples were matured to different maturity stages through hydrous pyrolysis, the samples of different maturities were measured and analyzed by nano-CT, N₂ adsorption, and X ray diffraction, and the evolutions of micro pore-structure and component were analyzed in the spectrum of maturity to over-maturity quantitatively and qualitatively. The component, structure, and physical parameters of shale and mudstone samples of different maturities, which were geologically evolved and from Shahejie Formation, Dongpu Sag, were measured, and the study determined the main factors influencing oil bearing content and built the comprehensive evolution model of mudstone and shale oil bearing content of the Shahejie Formation, Dongpu Sag.

The study suggests: (1) the distribution of organic matter (OM) can control the distribution and connection of OM pores, the position of high OM content is favorable for the OM pores distribution. The conversion of OM to hydrocarbons prefers to form smaller diameter OM pores and fewer OM pores when the TOC is lower; (2) the macropore and mesopore control the OM pore volume corporately. The specific surface area (SSA) of OM pore increases and decreases with increasing maturity in the stage of maturity to over-maturity, the pore type controlling the SSA of OM pore shifts from mesopore to micropore according to increasing maturity; (3) the conversion of OM to hydrocarbons improves the connection of OM pores; (4) in the shale and mudstone samples of Shahejie Formation, Dongpu Sag, the maturity is in the range of 0.78%~1.72% with depth increasing. The TOC ranges from 0.17% to 0.72% and shows no distinctive varying pattern and strong heterogeneity. The clay mineral content fluctuates in the range of 36%~62%, increasing and then decreasing with the maturity. The mesopore size distribution mainly falls in the ranges of 2~5 nm and 9~50 nm, and the pore number of falling in the range of 9~50 nm increases distinctively when maturity increases; (5) the correlations between the porosity and organic carbon content, main factors controlling oil bearing content, and the oil bearing content are positive, but the relationship between the clay mineral content and the oil bearing content is negative, the high porosity and OM content are favorable factors for retentate hydrocarbons. The maturity range of 0.8%~1.2% is the most favorable maturity interval for the retentate oil content.