Source and seal coupling mechanism for shale gas enrichment in upper Ordovician Wufeng Formation - Lower Silurian Longmaxi Formation in Sichuan Basin and its periphery

Abstract

Abstract In recent years, theoretical geologic research has made important progress in shale gas exploration and development, and new discoveries, such as the Jiaoshiba, Weiyuan and Changning shale gas fields, provide geological cases to define key controls on shale gas accumulation in Wufeng Formation-Longmaxi Formation marine shale strata. A mechanism coupling source and seal development for shale gas enrichment is proposed in this study based on detailed research results of isochronous stratigraphic distribution of source rock and seals and source-seal dynamic evolution. Great thickness, wide distribution and high total organic carbon content (TOC) of sources are key controls on the shale gas content, and storage capacity in organic matter-hosted pores, micro-cracks and bedding fissures is intensively developed. The Wufeng Formation and the lower part of the first member of the Longmaxi Formation (WF2-LM4 graptolitic zone) are favorable sources that feature a slow deposition rate, well graptolitic zone, favorable organic organism type and high TOC. The organic matter type is benefit to the development of organic-matter pores because the planktonic algae tends to generate and expulse large amounts of hydrocarbon and form many organic-matter pores. Both high biogenic quartz and high TOC ensure development of abundant organic matter-hosted pores and result in a three-dimensional network of interconnected organic matter pores that allows shale gas storage and flow. The seal can be divided into direct and indirect seals. The direct seal refers to the middle and upper parts of the first member of the Longmaxi Formation (LM5 and higher graptolitic zone) that are characterized by low TOC and low porosity. The indirect seal refers to the regionally distributed Lower-Middle Triassic gypsum layer(s) and mudstone that maintain regional geopressure systems. The shale gas fields of Jiaoshiba, Fushun-Yongchuan and Changning have high geopressure coefficient factors (1.5, 2.0 and 1.5, respectively). In the absence of the Lower-Middle Triassic gypsum layer(s) and mudstone regional seals, the high-pressure system was destroyed and resulted in low-normal geopressure shale gas reservoirs, for instance, the Pengshui and Zhaotong wells. The static match and dynamic match of source and seal constrain the location of shale gas reservoirs and the degree of shale gas enrichment. The model of a mechanism coupling source and seal for shale gas enrichment provides new ideas and methods for the assessment of shale gas. This model promotes the evaluation of shale gas from the static parameters approach to the source and seal dynamic approach. In the Sichuan Basin and its peripheral areas, the promising areas for shale gas accumulation are those with favorable sources in the WF2-LM4 graptolitic zone and the presence of the Lower-Middle Triassic gypsum layer(s) and mudstone regional seal.