Controlling Factors and Quantitative Characterization of Pore Development in Marine-Continental Transitional Shale

The pore characteristics are studied in the overmatured marine-continental transitional (MCT) shale and simulated shale under different thermal maturity conditions, based on transitional and simulated shale samples in the eastern margin of Ordos Basin. The work uses high-pressure mercury intrusion (MICP), field emission scanning electron microscopy (FESEM), helium-mercury method, X-ray diffraction of whole-rock minerals, and hydrocarbon-generating thermal simulation to quantitatively analyze pore characteristics and main controlling factors of pore development. The results show that the shallow bay and lake facies (SBLF) shale has great exploration potential, while the delta facies (DF) shale has poor exploration potential. The SBLF shale is mainly characterized by pie shale, high quartz and carbonate, low clay, high porosity, and pore volume. The DF shale mainly develops dot shale with low quartz and carbonate content, high clay content, low porosity, and pore volume. Kaolinite has the strongest inhibition on MTC shale pore development. The pore volume of MTC shale decreases first and then increases with maturity. The pie shale is more conducive to the increase of pore volume than the dot shale. The effect of doubled TOC on porosity is greater than that of maturity in the dot shale. The effect of doubled TOC on porosity is less than that of maturity in the pie shale. Organic matter (OM) has the greatest impact on pore development, controlled by the OM content, sedimentary facies, and maturity. OM content, sedimentary facies, and maturity can be used to jointly characterize the MTC shale pore development, providing guidance for multiparameter quantitative characterization of pore development and determining the enrichment area of shale gas.