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Application of Gas Isotope Signatures to Estimate Source Rock Thermal Maturity in Unconventional Gas Plays

Abstract

The ability to use gas isotope signatures to determine the source rock thermal maturity in a shale gas play could be a powerful tool, especially where vitrinite reflectance may not be either available or reliable. Previous studies and modern gas isotope models have attempted to relate natural gas isotope signatures to vitrinite reflectance equivalents in conventional plays. These studies have indicated that the practice of determining vitrinite reflectance equivalents from gas isotope signatures is kerogen dependent, adding a level of complexity. Because of the differences in the mechanisms by which gases accumulate in unconventional systems (principally primary cracking from kerogen and secondary cracking of generated bitumen, oil, and wet gas) and the gas signatures that are retained, the relationship used to estimate vitrinite reflectance equivalent based on gas isotope signatures needs to be modified and tested. Isotopic signatures of methane and propane when compared against those of ethane demonstrate a partial and full isotope reversal, where the isotopic composition becomes lighter than expected. These signatures have been compared with corresponding reflectance data, and suggest the approximate thermal maturity at which the partial isotope reversal begins (the “isotope rollover effect”), and at what point a full reversal in the gas isotope signatures has occurred. Our study suggests that the position of this reversal varies but may be as low as a vitrinite reflectance of 1.5%. Thus proposed relationships used to estimate the thermal maturity of gas need to be re-examined in order to take into account a number of the issues encountered in shale gas systems, which may have masked or overprinted their isotopic composition because of their advanced level of thermal maturity. These issues include the overprinting of gas signatures by secondary hydrocarbon cracking, and calibration of the isotope signatures to a specific play. The inclusion of secondary cracking in our assessment of gas isotope data has yielded reasonable results, while the use of kerogen-specific parameters for each play has also improved the robustness of the method and could provide an additional means of estimating thermal maturity in where more routine measurement of thermal maturity in unconventional shale gas plays is lacking.