Influence of Mineral Compositions on Shale Pore Development of Longmaxi Formation in the Dingshan Area, Southeastern Sichuan Basin, China

摘要

Influences of the characteristics of organic matters and mineral compositions on the development of shale microscopic pores were discussed through performing low-temperature nitrogen adsorption, whole-rock "X" diffraction, and field emission scanning electron microscope on black organic-rich shale samples of Longmaxi Formation in the Dingshan area, southeastern Sichuan Basin, in combination with the characteristics of shale organic matters. The shale in the Dingshan area has complex mineral composition, which is mainly quartz and clay minerals. Both contents range from 23% to 72% and 36% to 70%, respectively. The vertical variation is obvious, and there are few feldspar and carbonate rocks; the pyrite content is more than 2%. The bottom shale has high brittleness and gradually decreases in the vertical direction. The brittleness index is between 0.481 and 0.627 and is concentrated above 0.5. It shows strong compressibility and is easy to form a complex network system in hydraulic fracturing. Quartz and feldspar mainly provide secondary dissolved pores, intercrystalline mineral pores, and nanoedge gaps in contact with organic matter. They have no obvious correlation with the specific surface area of shale but have a weak correlation with pore volume. They mainly control the development of macropores. Organic matter develops many hydrocarbon-generating pores, which strongly correlate with the specific surface area and a weak correlation with pore volume. It mainly controls the development of micromesopores. Clay minerals mainly provide a large number of interlaminar pores and interlayer fractures in the clay. The intergranular pores of clay and clay have a weak correlation with pore volume and specific surfaces. They contribute to the development of shale micropores, mesopores, and macropores. Pyrite mainly provides intercrystalline pores and mold pores. By restricting the interaction with organic matter, the development of shale pores is promoted within a certain content range. When the content exceeds this range, the development of micropores is inhibited. The conversion threshold in the Dingshan area is 5.0%.