Evolution of coal-bed methane in Southeast Qinshui Basin, China: insights from stable and noble gas isotopes

The late Carboniferous-early Permian coal seams of the Qinshui Basin in Shanxi Province are the most prolific producer of coalbed methane (CBM) in China. Methane formed in the late Triassic during deep burial and reheating in late Jurassic-early Cretaceous driven by magmatic underplating. Basin inversion brought the coal seams to 400-700 m from the surface in the mid-late Cenozoic. Here we present results of a study aimed at understanding the origin of the methane, and how it was affected by Cenozoic exhumation of the basin. Methane from a 12 km traverse perpendicular to the basin margin in the southeast part of the basin have stable isotope compositions (δ13C = -30.2 to -35.2‰, and δD = -155 to -194‰) indicating a thermogenic origin with limited biogenic input. They are, however, lighter than expected based on coal maturity, and C1/(C2+C3) (>1000) are significantly higher than typical thermogenic methane (<50). This is due to diffusive fractionation during commercial gas extraction. He-Ne-Ar isotopes are a mixture of crustal-radiogenic gas with air-derived noble gases. 4He concentrations (0.52 to 33.25 ppmv) and 4He/40Ar* ratios (0.06-1.74) are unusually low. He-Ne-Ar concentrations are consistent with the open system Rayleigh fractionation of noble gases derived from air-saturated water with 4He/40Ar* = 1 during gas extraction. The low 4He/40Ar*, compared with average crust (5) or local production (13) values, implies that more than 90% of the radiogenic 4He produced in the coals has been lost prior to equilibrium between gas and water phase in the reservoir. This likely occurred in response to gas loss process during rapid exhumation in Cenozoic, showing that the He and Ar content of natural gases is a sensitive indicator of gas loss event caused by recent basin inversion. The event may have led to the loss of up to 44% of the methane from the coal seams. This study demonstrates the importance of basin inversion on gas preservation in shallow CBM, and shows that, in contrast to δ13CCH4, the light noble gases are essential for tracing such a process

Reconstructing marine redox conditions for the Early Cambrian Yangtze Platform: evidence from biogenic sulphur and organic carbon isotopes

In order to constrain marine redox conditions for the sedimentary environments immediately preceding the rapid diversification of metazoans carbon and sulphur isotopes, as well as carbon, sulphur and iron abundances, were measured in shales, cherts, and carbonates from two Lower Cambrian (Nemakit-Daldynian to Atdabanian) sections belonging to a shallow shelf and deeper water basinal environment on the Yangtze Platform, South China.\ud \ud A largely anoxic and most probably sulphidic deeper water body for most of the Nemakit-Daldynian and lower Tommotian is supported by positive δ34S values for sulphide, as well as high degree of pyritization (DOP) and total organic carbon (TOC) values. Additionally, an isotope fractionation between organic and carbonate carbon of > 32‰ suggests that sulphate reducing and sulphide oxidizing bacteria flourished and added to the biomass. Furthermore a positive correlation between δ34S of sulphide and δ13C of organic and possibly inorganic carbon is observed. In the lower Tommotian, low δ13C values for organic and inorganic carbon together with lower δ34S values for pyrite, if compared to underlying and overlying samples, have been measured. This is indicative of bacterial sulphate reduction taking place not only in the interstitial waters but also in an anoxic, possibly euxinic water column, and thus leading together with chemoautotrophic bacteria to recycling of organic matter, superimposed on the temporal evolution of the global carbon cycle. In the upper Tommotian and Atdabanian, decreasing DOP and TOC and high δ34S values for pyrite and organic sulphur suggest that the water column became oxic and that bacterial sulphate reduction occurred primarily in the sediment, where sulphate limitation was soon established. Organic carbon and carbonate carbon isotope ratios increase, reflecting open marine conditions and oxic photosynthesis dominating carbon cycling in the shelf environment. In the basinal deeper water section, anoxic conditions were still present or re-established during the upper Tommotian. In the lower Atdabanian, δ34S is variable, but mostly high in the deeper section, with DOP indicating an oscillation between oxic and dysoxic conditions