Oil–source rock correlations – Limitations and recommendations

Abstract

A rigorous, defensible and causal oil–source rock correlation definitively ties an individual source rock sample to an individual crude oil using genetically-based, internally consistent parameter matches. Such a correlation is a relationship established between the two components which is consistent with all known chemical, geochemical and geological information, and meets three criteria: (a) the relationship must be causal – the oil must arise (at least in part) from the specified source rock(s); (b) chemical data used in the correlation must be comparable – the elemental, molecular and isotopic data derived from the source rock must be of the same type as that derived from the oil; and (c) all available geological data must be supportive – clear geological evidence must exist which allows the proposed source rock to have sourced the oil. The three points of this definition are satisfied, and the correlation is successful, if we solve one analytical and three geological problems. Natural extraction of crude oil from its source rock – i.e., “expulsion” – differs mechanically and temporally from laboratory extraction, leading to a correlation problem referred to as “extraction differences”, and representing the single greatest analytical uncertainty in correlation efforts. Geologically, three aspects confound oil–source correlations: occurrence of multiple source units and/or source units at differing maturity levels; lateral and vertical depositional variations in source unit(s) organic matter; and lateral and vertical variation in-reservoir unit(s) organic matter. These concerns are overcome by taking five actions for each oil–source correlation effort. (1) Select representative samples using statistically defensible methods. (2) Establish the inherent compositional variability – laterally and temporally – due to depositional and maturation processes in each prospective source unit. (3) Assess the extent of migration-induced changes in oil composition, including post-migration changes such as in-reservoir alteration. (4) Support each correlation with migration histories derived from 4d models. (5) Iterate correlation results with new data gathered from ongoing exploration efforts. These five actions will (a) allow correlation success to be measured more objectively, (b) establish risking parameters for direct use in basin assessment, and (c) provide baseline criteria for use in assessing the reliability of oil–source rock correlations. Although these approaches will improve conventional oil–source rock correlation efforts, ultimately they may not be needed if inversion methods become more reliable. The ability to invert compositional data for crude oil – i.e., predict the age, lithology, maturity, and other characteristics of a source rock solely through chemical analysis of its expelled oil – could eventually result in successful oil–source rock “correlations” in which source rock analysis for oil–source rock correlation purposes is less critical.