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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 Ar atm and the He/Ar ratio of a shale gas can indirectly reflect the preservation conditions of that shale gas. • No evidence was found of external sources of He or Ar in the shale gas systems. • The relative abundances of He and Ar can reflect the accumulation and expulsion process of the shale gas. • The He/Ar ratio is related to the time available for diffusion and the sealing capabilities of the shale system. [ABSTRACT FROM AUTHOR]

Fourteen gases and their associated oils were collected at the surface from production wells in the Shahejie Formation, Dongying Formation, and Archean in six zones around the Qingshui Sub-sag of the Liaohe Depression, Bohai Bay Basin. The molecular compositions and carbon and hydrogen isotopic compositions of natural gas were measured. The origin, migration, accumulation, and secondary alteration of natural gas were studied based on the lateral and vertical comparison of natural gas geochemical characteristics. The natural gases include humic-type (δ13C 2 > −29.0‰, δ13C 1 > −43.0‰), sapropelic-type (δ13C 2 < −29.0‰, δ13C 1 > −43.0‰), and secondary microbial (δ13C 1 of −48.5‰, C 1 /(C 2 +C 3) of 46.3, δ13CO 2 of −1.5‰) gases. The humic-type and sapropelic-type gases were derived from the source rocks in the third and fourth members of the Shahejie Formation, respectively. The humic-type gases dominate. The biodegradation level of natural gas in the study area varies, and biodegraded gases are characterized by high Δ13C(C 3 –C 2), low C 3 /C 2 , and low C 3 /iC 4 and are associated with biodegraded oils. The vertical migration of natural gas in the west and east of the Qingshui Sub-sag is limited. There are good additional exploration prospects for humic-type gas, derived from high-maturity source rocks of the third member of the Shahejie Formation in the deep layers (depth >4.5 km) of the Qingshui Sub-sag. Secondary microbial gas is a potential exploration target in the shallow layers (depth <2.5 km). • Humic-type gas is from the source rock of the 3rd member of Shahejie Formation. • Sapropelic-type gas is from the source rock of the 4th member of Shahejie Formation. • Biodegraded gas: 13C-rich propane, depletion of propane and n-butane. • Secondary microbial gas: 13C-rich carbon dioxide and 13C-depleted methane. • Natural gas mainly accumulates near the source rock. [ABSTRACT FROM AUTHOR]