--> Interpretation of Natural Gases: Historical Developments, Present State-Of-The-Art, and Future Research Directions

AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis

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Interpretation of Natural Gases: Historical Developments, Present State-Of-The-Art, and Future Research Directions


Petroleum geochemists interpret molecular and isotopic composition of natural gases to understand their origin (microbial, thermogenic, abiotic), source rocks organofacies (marine shales, coals etc.) and post‐generation processes (mixing, biodegradation, thermochemical sulfate reduction etc.). These interpretations are commonly based on the empirical gas genetic diagrams first proposed in 1970‐ 1980s. The diagram of δ13C‐CH4 versus CH4/(C2H6+C3H8) presented by Bernard et al. (1976, 1977) is most commonly used to distinguish microbial gases (δ13C‐CH4<‐55‰ and CH4/(C2H6+C3H8)>1000) from thermogenic gases (δ13C‐CH4>‐52‰ and CH4/(C2H6+C3H8)<1000). Schoell (1980, 1983) proposed to interpret microbial and thermogenic origins of CH4 using the genetic diagram based on its carbon (δ13C) and hydrogen (δ2H) isotopes. Whiticar et al. (1986) further revised this diagram by simplifying the field of thermogenic CH4 and separating microbial CH4 formed during CO2‐reduction (relatively enriched in 2H) from microbial CH4 generated through methyl‐type fermentation (relatively depleted in 2H). Both versions of this diagram are now widely used in petroleum and environmental studies. Gutsalo and Plotnikov (1981) published the first gas genetic diagram based on δ13C of CH4 and CO2and outlined fields of abiotic, microbial and thermogenic gases. Although there are other genetic schemes utilizing noble gases and various gas ratios (e.g., Ballentine and O’Nions, 1994; Lorant et al., 1998), petroleum geochemists most often use three diagrams described above. However, as a large amount of gas geochemical data became available since the publication of those diagrams in 1970‐1980s, the originally defined genetic fields on these diagrams became partly inadequate. Recently, Milkov and Etiope (2018) revised these diagrams using a large global dataset of >20,000 gas samples from various geological settings around the world. They redefined the genetic fields of primary microbial gas and thermogenic gas, formalized the fields of secondary microbial gas and abiotic gas previously proposed in various papers and summarized compositional changes during various gas alteration processes. The diagrams of Milkov and Etiope (2018) cover most hydrocarbon‐ containing gases known to exist in nature, and represent an essential tool for gas interpretations. The authors also emphasized the importance of holistic integration of geochemical and geological data for reliable gas interpretations. Further improvements in gas interpretations shall come from more sophisticated analytical and interpretation techniques. Recent studies demonstrated the utility of clumped isotopes to constrain the origins and formational temperatures of natural gases (Stolper et al., 2018; Clog et al., 2018). As most natural gases are complex mixtures, new algorithms are needed to determine relative contributions of gases of various origin, from multiple sources and affected by different processes. Modern tools of Machine Learning may assist in such advanced gas interpretations.