Diagenesis of shale and its control on pore structure, a case study from typical marine, transitional and continental shales

Due to discrepancies in pore structure, the productivity of shale gas reservoirs under different diagenesis stages varies greatly. This study discussed the controlling of sedimentation and diagenesis on shale pore structure in typical marine, transitional, and continental shales, respectively. Continental shale samples from the Shuinan Formation, Jiaolai Basin, transitional shale samples from the Taiyuan, Shanxi and Xiashihezi Formations, Ordos Basin, and marine shale samples from the Longmaxi Formation, Sichuan Basin, were collected. Scanning electron microscope with argon ion polishing, high-pressure mercury injection, and low-temperature nitrogen adsorption experiments were conducted to acquire pore structure parameters. And the diagenetic stage of the reservoir was classified according to thermal maturity, organic geochemical parameters, and mineral composition. Our results exhibit that continental, transitional, and marine shales are period A, period B of the middle diagenetic stage, and the late diagenetic stage, respectively. For pore structure, micropore (0-2 nm) and mesopore (2-50 nm) controlled pore volume and specific surface area of transitional and marine shales, and specific surface area of continental shale have similar results, while micropore, mesopore, and macropore (>50 nm) all have a significant proportion of pore volume in continental shale. The pore structure characteristics and controlling factors exhibit a pronounced difference in different diagenesis stages, the compaction and cementation in period A of the middle diagenesis stage is relatively weak, intergranular pore and interlayer pore of clay minerals are well preserved, and moldic pore and dissolved pore developed as well; organic matter is in high maturity in period B of the middle diagenesis stage, organic matter pore developed correspondingly, while the intergranular pore developed poorly affected by compaction, notably, the carbonate is negligible in transitional shale, and the interlayer pore of clay minerals are well preserved with weak cementation; while dissolution and metasomatism controlled the pore structure in the late diagenesis stage in marine shale, the primary pores were poorly preserved, and the organic matter pore and carbonate dissolved pore developed. Results from this work are of a specific reference for shale gas development under different diagenesis stages.