Geochemical comparison between gas in fluid inclusions and gas produced from the Upper Triassic Xujiahe Formation, Sichuan Basin, SW China

Natural gas in the Xujiahe Formation of the Sichuan Basin is dominated by hydrocarbon (HC) gas, with 78-79% methane and 2-19% C2+ HC. Its dryness coefficient (C-1/C1-5) is mostly < 0.95. The gas in fluid inclusions, which has low contents of CH4 and heavy hydrocarbons (C2+) and higher contents of non-hydrocarbons (e.g. CO2), is a typical wet gas produced by thermal degradation of kerogen. Gas produced from the Upper Triassic Xujiahe Formation (here denoted field gas) has light carbon isotope values for methane (delta C-13(1): -45% to -36%) and heavier values for ethane (delta C-13(2): -30% to -25%). The case is similar for gas in fluid inclusions, but delta C-13(1) = -36% to -45% and delta C-13(2) = -24.8% to -28.1%, suggesting that the gas experienced weak isotopic fractionation due to migration and water washing. The field gas has delta C-13(CO2) values of -15.6% to -5.6%, while the gas in fluid inclusions has delta C-13(CO2) values of -16.6% to -9%, indicating its organic origin. Geochemical comparison shows that CO2 captured in fluid inclusions mainly originated from source rock organic matter, with little contribution from abiogenic CO2. Fluid inclusions originate in a relatively closed system without fluid exchange with the outside following the gas capture process, so that there is no isotopic fractionation. They thus present the original state of gas generated from the source rocks. These research results can provide a theoretical basis for gas generation, evolution, migration and accumulation in the basin. (C) 2014 Elsevier Ltd. All rights reserved.