--> Compound Specific Carbon Isotope Analysis of Palaeo Gases Trapped in Fluid Inclusions: Implications for the Source of Gas in the Browse Basin, Australia

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Compound Specific Carbon Isotope Analysis of Palaeo Gases Trapped in Fluid Inclusions: Implications for the Source of Gas in the Browse Basin, Australia

Abstract

Carbon isotopic signatures of natural gases have been widely used to study their origins and maturities, and to establish gas-source correlations. However, natural gas accumulations in reservoirs can have mixed sources, or may be affected by secondary alteration processes such as leakage, water washing and biodegradation. In contrast, palaeo gases trapped in fluid inclusions (FIs) can be expected to be free of any secondary alteration that may have occurred in the reservoir. FIs are formed during mineral growth, which can trap fluids such as oil, gas or water. FIs are normally less than 10 μm in diameter. Hence, it is a significant challenge to measure compound-specific carbon isotope ratios in such trace level gases. The methods described in the literature are limited by either the large dead volume in typical crushers, or the maximum amount of sample that can be crushed, with the result that they are not suitable for lean samples. An online crushing-trapping system developed in this study can quantitatively transfer the released gases from the crusher to a micro-trap to achieve compound specific carbon isotope analysis. This novel design is sensitive and imparts no isotopic or molecular fractionation during cryo-trapping or subsequent thermal release from the trap. The Browse Basin, located offshore in the Timor Sea region of Australia's North West Shelf, is highly prospective for hydrocarbons with giant accumulations of natural gas. Although there is extensive ongoing exploration activity, the charge history of the gas accumulations within the Caswell sub-basin remains a matter of debate with gases originating from multiple sources. Evidence for palaeo-oil columns in many gas zones further complicates the filling history of the gas reservoirs. The newly developed technique in this study has enabled the compound-specific carbon isotope analysis of palaeo-gases (hydrocarbon gases C1-C5 and CO2) trapped fluid inclusions to be measured. Samples (Heywood-1, Argus-1, Dinichthys-1 and Titanichthys-1) previous analysed for molecular composition of FI oil has been selected to investigate carbon isotope of FI gases. These will add further constraints to the source of the gases in the Browse Basin.