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Advanced Modeling of Gas (Methane Through Pentane) Compositions and Carbon Isotopes in Petroleums and Petroleum Mixtures


A number of organic geochemical tools (models, cross-plots and classification schemes) have been proposed to help understand the origin and maturity of methane to pentane components (PC1-5) in petroleum. Most tools have focused on understanding the C and H isotopic evolution of each petroleum component while gas composition has remained largely the reservoir engineer's problem. Many tools relegate composition to a simple ‘dryness’ (inverse ‘wetness’) parameter reflecting the proportions of PC1 relative to one or more of the PC2 to PC5 components. This is unhelpful as PC1 is formed in every gas-generating process: bacterial methanogenesis, kerogen breakdown, oil (PC6+) to gas cracking, and wet gas (PC2-5) to lean gas (PC1-4) cracking. These tools lack full systematics of changing composition with increasing maturity, other than the (unsubstantiated) assumption that dryness continuously increases with increasing maturity. Without a compositional model for all components, including iso- and normal PC4 and PC5, (un)mixing of isotope values can't be performed for the whole compositional range: (un)mixing is limited to individual molecules, e.g. subtraction of biogenic PC1 from a biogenic-thermogenic PC1-5 mixture. Also, inability to understand the effects of instantaneous vs. cumulative capture has led some workers to invoke mixing when none is actually needed! t!Ps has built a forward model t!PsMIX2016TM of the compositional and isotopic evolution of the 7 acyclic PC1-5 compounds (excluding neo-pentane, present in very low concentrations and rarely reported) during bacterial methanogenesis, kerogen breakdown, and oil to gas cracking. Generation and cracking processes overlap during the later heating history of a source rock, so the model is guided by output from a kinetic model of petroleum generation, cracking and expulsion to assign the respective mass proportions of PC1-5 with increasing maturity. This is important since oil to gas cracking generates wet (PC2-5 -rich) isotopically light gases at the same time as the yield from the kerogen is becoming increasingly dry (PC1-rich) and isotopically heavy. Applications include the ability to: (un)mix gas mixtures from multiple sources; recognize partial capture signatures; recognize thermogenic contributions to biogenic gases that are too subtle to be revealed by PC1 isotope excursions; perform advanced mud gas logging interpretation while drilling, including prediction of reservoir fluid GOR, properties and phase.