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Origin and Maturity Estimates of Generated Gas Using Gas Isotopic Analyses - Examples From Experimental Pyrolysis Using Natural Oils and Gas Samples Collected From the New Albany Shale, Illinois Basin


Keywords: gas isotopes, gas analyses, experimental pyrolysis

Gas isotopic analyses are used in hydrocarbon resource assessment and exploration to determine origin of the generated gas and provide estimates on the maturity of the source horizon (or oil reservoir) from which it generated. The ratios 13C/12C carbon and 2H (D)/H hydrogen isotopes are the standard isotopic ratios used in maturation estimates. Isotopic fractionation in petroleum systems is complex and gas isotopic ratios determined from wellhead samples, core/cuttings (canister desorption) or isotubes reflect the generation, migration, leakage, and reservoir history of the gas sample. Isotopic fractionation occurs in-situ as gas is generated directly from the kerogen (source rock) or via secondary cracking of oil within the source interval. As gas and/or oil migrates and is expulsed from source horizons, thermal fractionation continues. Cumulative fractionation and further isotopic modification may also occur with bacterial mediation of associated gas or segregative leakage.

Closed-system experimental pyrolysis of natural oils generates gas isotopes that are solely the result of thermogenic bond-breaking cracking reactions. Comparison with New Albany Shale (NAS) gas samples illustrates complexities in interpreting maturation estimates from natural datasets. The salinity of connate water (or water added in the pyrolysis chamber) can skew δ13C and δD values such that generated gas of thermogenic origin may be mistakenly interpreted as microbial. Carbon isotopic values of ethane (C2) and propane (C3) are less susceptible to segregative and microbial fractionation effects than methane. However, anomalous δ13C values were measured in NAS desorbed gas which may indicate microbial mediation and/or simultaneous generation from kerogen and in-situ cracking of branched alkane homologs and naphthenes. Maturity estimates using C2 and C3 values determined from experimental pyrolysis generally follow calculated Ro (vitrinite reflectance) trends while gas generated from the NAS consistently contain isotopes which indicate anomalously low thermal maturities. Reasons for the deviation may involve microbial mediation and fractionation associated with generation from low-maturity shales. Maturity estimates using gas isotopic data requires evaluation in context with data derived from kerogen, oil and recovered water from the interval of interest.