Geochemical characteristics and possible origin of shale gas in the Toolebuc Formation in the northeastern part of the Eromanga Basin, Australia

Abstract

The geochemical features of shale rocks and shale gas samples, which were collected from six wells in the northeastern part of the Eromanga Basin, were analyzed in detail. The results show that these shale rocks are enriched in thermally immature Type II organic matter and shale gases are dominated by CH4 (65%–83%) with minor C2H6 and C3H8. Their δ13CCH4 (−53.9‰ to −51.1‰) and δDC1 (−236‰ to −205‰) values indicate that the shale gas samples represent a mixture of thermogenic and biogenic gases. This inference is also supported by the fact that the actual measured δ13CCH4 values are significantly lighter than the theoretical δ13CCH4 values (−47.31‰ to −45.57‰). Biogenic methane is the primary biogenic gas, which is generated by CO2 reduction. Secondary biogenic gas has been ruled out due to the presence of non-biodegraded saturated hydrocarbons in the core extracts from shale rocks and the fact that their δ13CCO2 values are lighter than 2‰. According to our estimation, the proportions of primary biogenic and early thermogenic methane range from 22.2% to 31.4% and 68.6% to 77.8%, respectively. Therefore, the shale gas studied here is dominated by early thermogenic gas with the mixing of a small portion of biogenic gas. The relatively stable tectonic setting and good sealing capacity of the shale system likely play key roles in controlling the preservation of shale gas in the Toolebuc Formation in the study area.