Application of noble gas geochemistry to the quantitative study of the accumulation and expulsion of lower Paleozoic shale gas in southern China

Abstract

The noble gases He and Ar are trace components in natural gas. Although their genesis is significantly different from that of hydrocarbon gases, they can provide key information on the accumulation and preservation of natural gas. In this paper, the relative and absolute abundances of He and Ar in lower Paleozoic shale gas samples from different areas of southern China were systematically analyzed, and the relationships between the concentrations of noble gases and the accumulation and expulsion of shale gas were evaluated. The evolution of He and Ar concentrations in shale gas system can be divided into three phases: the early accumulation stage, the hydrocarbon dilution stage and the preservation and enrichment stage. The generation of a large amounts of methane in the shale leads to the lowest relative concentrations of He and Ar in the hydrocarbon dilution stage. After that, the tectonic uplift causes the partial loss of the shale gas, and methane is no longer generated, but He and Ar are still produced. The concentrations of He and Ar present in shale gas are mainly controlled by the conditions of the preservation and enrichment stage. Tectonic uplift is the main reason for the loss of shale gas, and later uplift often results in higher gas content, which also leads to relatively low concentrations of He and Ar. The absolute abundance of residual Aratm and the He/Ar ratio of a shale gas can indirectly reflect the preservation conditions of that shale gas.