Exploiting the geochemical attributes of migrated oils for an improved understanding of their sourcing facies : Lessons learned from the southeastern Caribbean and the Norwegian Barents and North Seas

Abstract

Assessing the geochemistry of oils and the source rock facies from which they originated is crucial in petroleum systems analysis. Crude oils are usually available for geochemical characterization, but pertinent source rock data is often scarce or absent because exploratory drilling mostly targets reservoirs in structurally elevated areas and fails to sample the deeply buried, prolific basinal source facies. Explorationists, unable to perform direct oil-to-source correlations, must rely on source rock inferences drawn from oil geochemistry, a practice named geochemical inversion. This work focuses on organic geochemical investigations in the southeastern Caribbean-Atlantic margin and selected areas of the Norwegian Continental Shelf. These geologically distinct regions were selected as natural laboratories because their different data sets and geochemical commonalities offer a unique arena to explore the successes and pitfalls of selected geochemical proxies in geochemical inversion. A variety of techniques are employed to: (1) Review the accuracy of stable carbon isotopes in assessing organofacies in source rocks and oils; (2) Explore the utility of inverting oil biomarker composition to source rock organofacies and associated kinetic; and (3) Appraise the validity of selected biomarker ratios employed in age and lithofacies determination. Paper I combines maceral descriptions, TOC and Rock-Eval, carbon isotopes, and palaeogeographical considerations. Integration of this data resulted in a subregional to regional characterization of organofacies and sedimentary environment changes through the Upper Jurassic to Lower Cretaceous Hekkingen Formation in the Norwegian Barents Sea. Importantly, this paper documents a poor correlation between maceral compositions and stable carbon isotopes of source rock extracts. It is tentatively explained as the result of varying degrees of diagenetic alteration of the organic matter. Paper II builds on Paper I and evaluates the petroleum generation potential of the Hekkingen Formation using pyrolysis gas chromatography, bulk kinetics, and micro-scale sealed vessel pyrolysis analyses. This article demonstrates how the source rock thermal stability and the character of the generated hydrocarbon blend varies with regard to maceral assemblages. Paper III collects Upper Jurassic source rocks across the northern North Sea. The samples were analysed for TOC and Rock-Eval, carbon isotopes, and maceral compositions, and the results are integrated to produce a regional organofacies interpretation. The isotopic and biomarker composition of 120 oils distributed across the North Sea is utilized for organofacies assessment of their source facies and subsequent comparison with the interpreted source rock variability. This work finds that carbon isotopes of both source rocks and oils become progressively heavier with increasing terrestrial detritus as per macerals and biomarkers, respectively. Paper IV uses biomarker and isotope data to perform a geochemical characterization of oils onshore Barbados. Interpretations suggest that these liquids were derived from a Cretaceous deep marine shale source rock. This manuscript also compares the Barbados dataset with publicly available data to establish possible relationships between the facies sourcing the Barbados oils and well-understood source rocks in the region. The studied oils, as well as most oils used for the regional comparison, exhibit high concentrations of extended tricyclic terpanes. This publication argues that the elevated abundance of these markers in the region may be circumstantial evidence to date Upper Cretaceous-sourced oils. Paper V evaluates the molecular and carbon isotopic composition of fifteen heavy oils onshore Suriname. Oils produced from Cenozoic reservoirs (Group A) possess compositional attributes characteristic of oils generated from a distal marine shale, possibly the Upper Cretaceous shales of the Canje Formation. These oils possess varyingly high abundances of extended tricyclic terpanes. In contrast, Group B oils occur in Late Cretaceous beds and have biomarker relationships diagnostic of oils derived from a proximal marine depositional system rich in terrestrial organic matter. These oils exhibit isotopic values that are in average 3‰ heavier than the Group A oils. This thesis provides clear observations, conclusions, and recommendations that can help optimize the use of selected geochemical proxies in inverting oil compositions to source rock attributes. The results have direct implications for understanding source rocks in the southern Caribbean-Atlantic margin and elsewhere when pertinent data is absent.