Petroleum Geochemical Characterisation of the Lower Congo Coastal Basin Bucomazi Formation

Abstract

An integrated biostratigraphic and geochcmical study of the Lower Congo Coastal basin Bucomazi Fm. has been performed on the type section penetrated in well CABGOC 86-1. Contributing in large part to a reserves estimate of 4.5 billion BO (recoverable) in the coastal states (northern Angola, Cabinda. Congo and Zaire), suich source quality rocks were deposited under lacustrine conditions accompanying the rift phase opening of the South Atlantic margin. Reaching thicknesses up to 1.8 km of Pre-Salt2 sediments, localised graben/half graben initiated subsidence provided the focus for accumulation of organic-rich sequences during the latter Early Cretaceous. Paleo-environmental fauna/flora (ostracod spp.) from 86-1 suggested a limited late Barremian-Aptian age range and an initially freshwater, evolving into a saline, depositional regime. With an anticipated diversity in depositional style reflecting salinity, water depth and paleoclimatic control, the Bucomazi Fm. reduces to a generic sedimentological term. Lower Congo oils reflect compositional diversity with previous geochemical characterisation of the Bucomazi Km. amply confirming a high level of vertical and lateral heterogeneity in these sediments. Embracing both source potential, organofacies development and kerogen kinetic considerations, such factors can exert important constraints on hydrocarbon generation and the realisable prospectivity of a basin. Investigation of these factors has been pursued as a regional benchmark study to systematically evaluate the source characteristics of the "type section" sequence. High density Rock-Eval based analyses (3-m frequency) revealed considerable source inhomo- geneity, this being recognised in terms of variable organic richness, kerogen type and kinetic parameters. Similarly, zones of enhanced source richness, delineated by conspicuous carbon isotope excursions to l3C-enriched kcrogens, reflected periods of intense biomass turnover deriving from stimulated carbon dioxide budget and nutrient influx. Stratigraphic deliniation of the section, incorporating these data and supporting bitumen biomarker signatures, allowed segregation of the section into two major sequences comprising four subordinate sub-members. The lower. Middle Bucomazi "organic-rich" zone corresponding to "basin fill" sedimentation, exhibited a fine structure of cyclical dcpositional events with the accumulation of kerogens of higher activation energy and variable carbon isotopic signature. By contrast, the "sheet drape" Upper Bucomazi sediments, although lacking in persistent organic richness, were isotopically more homogeneous and revealed kerogens with lower, gencratively more labile, activation energies. As illustrated by a simple subsidence model, this dichotomy in activation energy has important implications in understanding the sequential maturation and subsequent hydrocarbon expulsion from these heterogeneous source sub- units. In this way, a mechanism is available for the mixing of different charges, the aggregate oils thus formed accounting for the inherent diversity of the emplaced petroleums, all ostensibly from a common source. Application of these concepts has led to a more detailed understanding of the Ponta Vermelha Trough hydrocarbon habitat and the character of the coastal Zaire oils.