Quantitative characterization of reservoir space in the Lower Silurian Longmaxi Shale, southern Sichuan, China

Based on the drilling data of the Upper Ordovician Wufeng Shale and the Lower Silurian Longmaxi Shale in southern Sichuan Basin, the construction of matrix pores and the development condition of fractures in a marine organic-rich shale are quantitatively evaluated through the establishment of the reservoir petrophysical models and porosity mathematical models. Our studies show that there are four major characteristics of the Longmaxi Shale confirmed by the quantitative characterization: (1) the pore volume of per unit mass is the highest in organic matter, followed in clay minerals, finally in brittle minerals; (2) the porosity of the effective shale reservoir is moderate and equal to that of the Barnett Shale, and the main parts of the shale reservoir spaces are interlayer pores of clay minerals and organic pores; (3) the porosity of the organic-rich shale is closely related to TOC and brittle mineral/clay mineral ratio, and mainly increases with TOC and clay mineral content; (4) fractures are developed in this black shale, and are mainly micro ones and medium-large ones. In the Longmaxi Shale, the fracture density increases from top to bottom, reflecting the characteristics with high brittle mineral content, high Young's modulus, low Poisson's ratio and high brittleness at its bottom.