--> Evolution of Gas Isotopes in Petroleum Exploration

AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis

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Evolution of Gas Isotopes in Petroleum Exploration


Isotopic analyses of gases emerged as an additional dimension in petroleum systems evaluations about a half century ago. Analyses were carried out initially on methane (C1), soon to be followed by the other gaseous alkanes, as well as carbon dioxide, hydrogen sulfide, etc. Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) technology catalyzed the widespread use of stable isotopic composition of carbon (13C) and later hydrogen (H). Early on, it became apparent that the mechanism of formation controls the isotopic composition of these gases. The first clear distinction appeared between the pyrolytic gaseous hydrocarbons (traditionally called thermogenic) and biogenic C1, and innumerable studies addressed biogenic and thermogenic gas mixing in the subsurface to build understanding of charge history. Furthermore, observations in both natural gas accumulations and in laboratory pyrolysis experiments show that thermal maturity of kerogen controls isotopic composition of thermogenic gases. Moreover, the kerogen type and initial isotopic composition are also important controls on carbon isotope of gas, which impacts maturity trends. Using these principals, empirical models of expected 13C of pyrolytic C1-C5 composition in closed and open systems have been developed. Post-generative processes can also shift the initial isotopic composition of hydrocarbon gases and create predictable molecular and isotopic overprints on the original gas composition. Fluid composition-altering processes include mixing, diffusion (via solubility or through microporous media), adsorption/desorption, secondary cracking, biodegradation, etc. Full suite isotopic analyses of pressurized downhole samples from worldwide petroleum exploration wells provide in-depth understanding of these in-situ spot-samples, and relevance to petroleum system history. The development of continuous isotope logging (initially of 13C-C1) and gas molecular composition rig- site logs provides further elucidation of spatial distribution of gases of different origins, and the extent of alteration processes across sedimentary basins. Meanwhile, it was evident that mud gas spot-samples or logs need a thorough understanding of drilling and mud gas extraction-related artifacts, which impact their molecular and isotopic composition. One of the most impactful drilling artifacts can be drill bit metamorphism, which occurs due to drill bit overheating during inefficient drilling. Further complications of interpretation of isotopic composition of gaseous hydrocarbons can arise from contribution of gases derived from abiogenic processes (not involving organic matter at any stage) in certain hydrothermal and metamorphic settings. Gaseous hydrocarbons are also useful in geothermometry. Recent developments use the clumped isotope analysis of C1 to characterize temperature of gas formation. Therefore, the interpretation of results depends on methane being pure, unaltered and from a single source. However, there are a many possible source and alterations pathways for C1. Petroleum exploration and development studies benefit from using isotopic composition in tandem with reservoir-representative logs of gas molecular composition in real time. This approach provides full-well basic-geochemical context on distribution of fluid types and alteration risks. This allows optimizing placement of stations for downhole fluid analysis and sampling. Such strategically placed downhole samples can be then studied in detail (e.g. C1-C5 isotopic composition of solution gas, phase behavior and flow assurance studies, fluid geochemical fingerprint and properties, and other key reservoir fluid information).