Abstract:

Continental shale sequences are well developed in the Sichuan Basin, but the characteristic of complex petrology and lithology combination, strong heterogeneity and unclear reservoir property restricted the gas exploration and development process. Using experimental methods such as whole rock Xray diffraction, FESEM, high pressure mercury injection, isothermal adsorption, this study investigated the reservoir geological characteristics of the 5th Member of the Triassic Xujiahe Formation continental shale according to the petrologic features, micropore structure and physical property, and the major geological factors influencing reservoir development. On this basis, this study also clarified the controlling effects of reservoir on the natural gas content and occurrence state. The research shows that there are three kinds of superpositiontype reservoir structure in the 5th Member of the Xujiahe Formation (T3x5) in vertical, which are sandrich type, interbeded type and mudrich type. The densification of the reservoir is very serious, but the reservoir physical property of mudstone is better than that of siltstone and fine sandstone. The reservoirs in T3x5 have a small amount of primary and intergranular dissolution pore, which was caused by strong diagenesis and the sedimentary environment from distant part of the delta to shore shallow lake with low hydrodynamic energy. The large quantitative of flake intergranular pores, organic pore, and microfractures in shale make it being main contribution to the reservoir property. Besides, the continental shale gas also has mixing features of two kinds of gases, i.e., adsorption gas and free gas. However, adsorption gas is dominant, accounting for 84.9% of the total gas content. Gas occurrence state and gas content are controlled by the reservoir lithology, microscopic pore structure and reservoir physical property.

Key words: continental shale, reservoir characteristics, Western Sichuan Depression, gasbearing properties, pore structure characteristics