Geochemical Inversion - A Modern Approach to Inferring Source-Rock Identity from Characteristics of Accumulated Oil and Gas

Abstract

In recent years, petroleum geochemists have been re-focusing their efforts on developing practical means for inferring, from hydrocarbon chemistry and geologic constraints, the “provenance” of hydrocarbon accumulations, seeps or stains. This capability, referred to here as “Geochemical Inversion”, can be invaluable to the explorationist in deriving clues as to the character, age, identity, maturity and location of an accumulation's source rocks and evaluating a petroleum system's hydrocarbon supply volumetrics. Geochemical inversion is most useful where pertinent source-rock information may be absent because exploratory drilling focused strictly on structural highs and failed to penetrate the deeply buried, effective basinal source facies. Advances in chemical analysis technology over the last decade have facilitated the development of powerful geochemical methods for unravelling of complex chemistries of crude oil and natural gas at the molecular and subatomic levels to extract specific information on the hydrocarbons' source. Inferences on such factors as organic matter make-up, depositional environment, lithology, age and maturity of the source can frequently be drawn. These inferences, together with a sound analysis of the geologic and architectural constraints on the system, can supply clues as to the identity and location of the probable source sequence. This paper describes the principles underlying geochemical “inversion” and provides applications in exploration and exploitation settings. In addition, this paper demonstrates inversion of geochemical characteristics of migrated hydrocarbon fluids to specific attributes of the source. The paper also illustrates the use of systematic variations in fluid chemistry within a geologic setting to infer source location, degree of hydrocarbon mixing and relative migration distance.