Source rock characterisation and correlation with oils using free and macromlecularly-bound biomarkers (Offshore Angola)

Abstract

This study addresses the information potential of biological marker compounds of the free aliphatic hydrocarbon fraction and the biomarkers bound into the macromolecular organic matter fractions of oils and source rocks, within an exploration framework of Offshore Angola. Bound biomarkers are released by hydropyrolysis, and applied to overcome problems of drilling mud contamination and biodegradation, and are also investigated from the context of being an additional quantitatively significant 'pool' of source information to enhance oil to source rock correlations. Early Cretaceous rift and drift events associated with the separation of the African and South American continents led to the development of palaeo-depositional centres for organic-rich lacustrine sediments (Burwood, 1999). This rifting later evolved into the drift phase opening of the South Atlantic (Burwood et al., 1995) which led to the development of highly prolific petroleum systems involving deposition of extensive marine source rocks in the Cretaceous and Tertiary (Cole et al., 2000). This study addresses a suite of oils from the Angolan Margin, and source rock samples from four wells covering almost the whole stratigraphic section. Bulk geochemical analysis shows that almost the entire Cretaceous/Tertiary section is composed of source rocks with oil or gas-prone hydrocarbon potential, although lateral variations in hydrocarbon potential occur between wells. Quantitative analysis of the biomarkers in the macromolecular fractions of the oils and source rocks compared to the free fractions, shows that the bound biomarkers are quantitatively less important, representing 10 - 45% for kerogens (proportional to the thermal maturity of the well) and typically less then 1 % in the asphaltene and polar fractions. The biomarkers released from the macromolecular fractions of both the oils and source rocks show evidence for compositional fractionation of biomarkers between the free and bound biomarker fractions. This is illustrated by the absence of certain compounds in the bound fractions which can be found in the free fractions (e.g. oleanane and BNH), and also variations exist in the percentage of bound hopanes relative to steranes. From an oil-source rock correlation perspective the asphaltene-bound fraction of the oils appears to be most comparable to the kerogen-bound biomarkers of the source rocks, at least in terms of maturity parameters. The biomarkers released from the macromolecular fractions (particular the kerogen-bound) provide a good replacement for the free biomarkers, and from an exploration perspective may prove useful where contamination by drilling mud has occurred. Although absolute values of individual ratios calculated for the biomarkers in the kerogen-bound and bitumen fractions are not directly comparable, the values do not differ greatly. The biomarkers in the free hydrocarbon fraction of the marine oils and source rocks show good correlation, as do those for the lacustrine samples. The kerogen-bound biomarkers in the source rocks show good correlation with the asphaltene-bound biomarkers in the oils for the marine facies; however, the lacustrine oil does not correlate well with the lacustrine source rocks possibly as a result of poor sample representation. To determine the specific marine stratigraphic group(s) from which the marine oils are sourced is more difficult. On the basis of their higher C35αβ hopane and C29

sterahe abundances the Pinda and Malembo Gps. have been ruled out as contributing to the marine oils. Detailed oil-source rock correlation using biomarkers in both the free and bound fractions of the marine oils and source rocks shows that the oils are most likely derived from either a pure labe or pure Landana source, or a source rock section comprised of both labe and Landana source rocks.