Main Control Factors of Shale Gas Differential Vertical Enrichment in Lower Cambrian Qiongzhusi Formation, Southwest Sichuan Basin, China

Multiple shale gas intervals occur in the Qiongzhusi Formation in the Southwest Sichuan basin,where gas contents in upper shale section are significantly higher than those in the lower shale section. Main control factors of shale gas differential vertical enrichment in Qiongzhusi Formation were understood through analyzing geochemical feature,reservoir and sealing conditions with rock pyrolysis,physical experiements,XRD diffraction,and field-emission scanning electron microscopy. The main control factors can be summarized as follows. (1) TOC content. It is higher and enriched in the upper shale section but lower and scattered in the lower shale section,resulting in the difference in hydrocarbon generation,organic pores and micro-pore growth. (2) Mineral component and reservoir property. The upper shale is characterized by high clay mineral content,especially I/S content,which contributes to the absorbed gas. The porosity of upper shale is higher than that of lower shale,where organic pores are the main pore space. The lower shales are mainly composed of clay mineral pores,intergranular pores and micro-fractures. Compared with the lower shales,the clay mineral content in the upper shale is higher,which is dominated by I/S mixed layer that can make an important contribution to the enrichment of adsorbed gas. (3) Sealing conditions. The top and floor of the upper shale section are well developed,however,the bottom of the lower shale section is relatively weak due to the existence of paleo-weathering carbonate crust at the bottom of the shale. Therefore,high TOC,high clay mineral content and well-developed organic pores,good sealing conditions contribute to the enrichment of shale gas at upper shale section.