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

Abstract

The industry relies on a number of organic geochemical tools (models, cross-plots and classification schemes) to help understand the origin and maturity of methane to pentane (petroleum components with 1 to 5 carbon atoms; t!Ps nomenclature: PC01-05) in petroleum reservoir fluids. Most of these tools were developed before - or else were not aligned with - modern kinetic models of petroleum forming processes. To-date, most have focused on understanding the C and H isotopic evolution of individual PC01-05 components; 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 PC01 relative to one or more of the PC02 to PC05 components. This is unhelpful as PC01 is formed in every gas-generating process: bacterial methanogenesis, kerogen breakdown, oil (PC06+) to gas cracking, and wet gas (PC02-05) to drier gas (PC01-04) cracking. These tools lack the full systematics of changing composition with increasing maturity, other than the (unsubstantiated and incorrect) assumption that dryness continuously increases with increasing maturity. Without a compositional model for all components, including iso- and normal butanes and pentanes, (un)mixing of isotope values can’t be performed for the whole fluid: (un)mixing is limited to individual molecules, e.g. subtraction of biogenic PC01 from a biogenic-thermogenic PC01-05 mixture. Also, inability to understand the effects of instantaneous vs. cumulative capture in these models has led some workers to invoke mixing to explain ‘unusual’ isotopic profiles where none is actually needed! t!Ps has built a tool t!PsMIX2016TM to forward model the compositional and isotopic evolution of the seven acyclic PC01-05 compounds (excluding neo-pentane, present in very low concentrations and rarely reported in legacy datasets) 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 leverages output from a kinetic model of petroleum generation, cracking and expulsion to assign the respective mass proportions of PC01-05 from these sources with increasing maturity. This is important since oil to gas cracking generates “wet” (PC02-05-rich) isotopically light gases at the same time as the yield from the kerogen is becoming increasingly dry (PC01-enriched) 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 PC01 isotope excursions; perform advanced mud gas logging interpretation while drilling, including prediction of reservoir fluid GOR, properties and phase. Model capabilities are demonstrated by modeling the PC01_d13C vs. dryness trajectory of fluid expelled from the Carboniferous coal source of the giant Groningen gas field in the Netherlands. The Groningen reservoir fluid is a cumulative mixture of expelled gas components from: breakdown of lipid Organofacies D/E kerogen; breakdown of lignin-derived Organofacies F kerogen; and cracking to gas of un-expelled oil sorbed in the organic matter.

AAPG Datapages/Search and Discovery Article #90337 ©2019 AAPG Middle East Region GTW, Regional Variations in Charge Systems and the Impact on Hydrocarbon Fluid Properties in Exploration, Dubai, UAE, February 11-13, 2019