VERTICAL SEALING MECHANISM OF SHALES AND ITS EFFECT ON SHALE GAS ENRICHMENT: A CASE STUDY OF MARINE SHALES FROM SICHUAN BASIN IN THE UPPER YANGTZE AREA IN SOUTH CHINA

Compared with the North American continent, China’s plate included many small plates and has undergone many tectonic movements, which is very important to research the preservation conditions of shale gas. Therefore, how to clarify vertical sealing mechanism of shales and its effect on shale gas enrichment is a significant problem to be solved.

This paper took research objects of the Upper Ordovician Wufeng Formation, the first member of the Lower Silurian Longmaxi Formation and the Lower Cambrian Qiongzhusi Formation, which all existed in the representative shale gas blocks in the Sichuan basin of the Yangtze region. Studied by statistical drilling data, this paper focuses on the detailed analyses of the JiaoYe-1, Ning-201 and Wei-201 well. Meanwhile, the experiments were carried out, such as the porosity test, the TOC content analysis and the experiment of overburden permeability, permeability experiments before and after adsorption of methane under different osmotic pressure, and isothermal adsorption experiments. Combined with formula deduction, the experiments results are applied on studying the effect of shale and its roof and floor on shale gas enrichment.

The results show that in eastern Sichuan, the roof of the organic-rich shale of the Wufeng Formation and the first member of Longmaxi Formation is the assemblage of argillaceous siltstone and silty shale, while in southern Sichuan, it is the assemblage of gray shale and silty shale. And in both areas, The floor is nodular limestone and limestone in both areas. The combination of roof and floor, with low porosity, can form a quality roof and floor sealing ability in the absence of fracture development. Its sealing ability is stronger than that in the middle and upper part of the Qiongzhusi Formation, considering its roof and floor composed of argillaceous siltstone and silty shale. The floor of the organic-rich shale at the bottom of the Qiongzhusi Formation is a dolomite reservoir with good storage capacity. This two kinds of shale are unconformablly contacted, which made the worst sealing ability among the three intervals. With the increase of burial depth, the overlying pressure of shale increases. With the increase of TOC content and thickness, the amount of adsorption gas of shale increases.