Characteristics and Controlling Factors of Micro- and Nano-Scale Reservoirs in Continental Shale Sequence of Western Sichuan Depression, China

摘要

The continental shale sequence in Western Sichuan Depression (WSD) is well developed. However, low porosity and permeability, complex lithologies and their combination, and the strong heterogeneity of such kind of nano-scale reservoir rocks commonly restrict the exploration and development of natural gas. In this study, reservoir petrology composition, microscopic pore structure, and nanometer pore quantitative distribution of the fifth member of Xujiahe Formation of the Upper Triassic (T(3)x(5)) continental shale are comprehensive analyzed by experiments including the whole rock X-ray diffraction, field emission scanning electron microscopy, N-2 and CO2 gas adsorption, and high-pressure mercury injection. The result shows that the T(3)x(5) shale sequence are rich in organic matter and significant changes in the mineral composition, pore space type and reservoir physical properties with the variety of the lithology. The porosity and permeability of shale are higher than those of siltstones and fine sandstones in the overall densification background. Mesopores (2-50 nm in diameter) are common in the shale sequence, whereas micropores (<2 nm) and the macropores (5 nm-1 mu m) are rare in almost all of the samples covering shale, siltstone and fine sandstone. Weak hydrodynamic sedimentary condition and strong diagenesis are identified to be the main reasons leading to poor physical properties. The variation of reservoir physical properties and pore distribution among different lithologies might be attributed to several geological factors, including the abundance of TOC, mineral composition, pore types, and distribution characteristics of different levels of apertures.