Types and Origin of Nanoscale Pores and Fractures in Wufeng and Longmaxi Shale in Sichuan Basin and Its Periphery

In Sichuan Basin and its periphery, the effective shale gas reservoir space of Wufeng and Longmaxi formations mainly consists of pores with diameters smaller than 1 μm and fractures with widths smaller than 1 μm, i.e., nanometer pores and fractures. By means of argon ion milling, scanning electron microscopy and CT scanning, the types and origins of these nanoscale pores and fractures are investigated and summarized in this paper. The nanoscale pores mainly include organic matter pores and mineral pores. The organic matter pores are formed via thermal maturation, conversion of organic matter, and expulsion of hydrocarbon products from the shale. They are predominantly distributed in the nanoscale pores of biological residue, kerogen and asphalt. The mineral matrix pores are residual pores between and inside mineral particles after shale compaction, cementation, recrystallization, replacement, dissolution and other diagenesis. These pore types constitute the major reservoir space of shale gas. The nanoscale fractures are mainly developed inside and on the periphery of organic matter, between brittle mineral crystals, inside crystals and clay mineral particles, generally with the widths from only a few nanometers to hundreds of nanometers and lengths from a few micrometers to hundreds of micrometers. They are formed by vertical stretching due to unloading uplift, horizontal tectonic deformation, expansion caused by hydrocarbon generation, and diagenetic shrinkage of clay minerals. The nanoscale fractures constitute the main percolation pathways of shale reservoirs.