Shale gas accumulation mechanism in a syncline setting based on multiple geological factors: An example of southern Sichuan and the Xiuwu Basin in the Yangtze Region

Abstract

The determination of favorable areas for shale gas enrichment under a syncline background is an important problem that needs to be addressed. The research objects in this study are typical shale gas blocks of the southern Sichuan Basin and the Xiuwu Basin in the Yangtze region with Lower Cambrian and Upper Ordovician–Lower Silurian shales. We carried out permeability tests parallel and perpendicular to the bedding surface, permeability tests with different overburden pressures, and permeability tests and isothermal adsorption experiments under different osmotic pressures before and after the adsorption of methane parallel to the bedding surface. In addition, along with the analysis of drilling data statistics, the enrichment of marine shale gas under a syncline background was studied with respect to dip angle and burial depth. The results show that the preservation conditions of shale gas are better when the dip angle is below 10°, while they are relatively poorer at angle more than 20°. This is due to the migration of natural gas, mainly in parallel direction to the bedding surface, leading to favorable shale gas preservation with decreasing angle. Based on double effects of the overburden pressure and gas adsorption (both increase) parallel to the bedding surface on the permeability of shale, the permeability decreases. When the overburden pressure increases to 10–15 MPa, the permeability parallel to the shale layer changes suddenly and decreases to a low value, which means that under the syncline background, the drilling depth with good shale gas production capacity should be at least 1019–1529 m.