Angola: source rock control for Lower Congo Coastal and Kwanza Basin petroleum systems

Abstract

The purpose of this paper is to provide an overview of petroleum occurrence and provenance for the 1000 km West African Atlantic Margin from Cabinda to mid-Angola. Over this margin the Lower Congo Coastal and Kwanza provinces cumulatively account for reserves of c. 6 gigabarrels oil recoverable (GBOR). These are dominantly reservoired in Pinda carbonate traps of the former basin. However, with production from a range of aggradational wedge, carbonate platform and pre-salt reservoirs, a diversity in oil character presupposes complex hydrocarbon habitats charged by multiple sourcing. Each of these two major Atlantic margin salt basins constitutes a different, source rock driven, hydrocarbon habitat. As classic passive margin pull-apart basins, Early Cretaceous initiated rift events (Pre-rift, Syn-rift I, II, etc.) evolved into the drift phase opening of the southern Atlantic. A striking feature of this progression was widespread evaporite deposition of the Aptian Loeme salt. This separates two distinct sedimentary and tectonic domains of the Pre- and Post-Salt. The core Lower Congo Coastal habitat is dominated by the Pre-Salt Bucomazi Formation sourced ‘poly’ petroleum system. These lacustrine, often super-rich, sediments reveal considerable organofacies variations between their basin fill (Syn-rift I) and sheet drape (Syn-rift II) development, accounting for the compositional diversity in their progenic petroleums. Of crucial impact is a cognate diversity in their kerogen kinetic behaviour. This controls the conditions and timing of generation and realization of charge potential. With the Lower Congo Coastal habitat extending southwards towards the Ambriz Spur, the Bucomazi facies proper appears restricted to the northern and deeper proto-lake trend. Over the more weakly subsident margins such troughs host inferior sheet drape potential. Elsewhere, Upper Cretaceous-Palaeogene marine clastic Iabe Formation sourced petroleum systems are hydrocarbon productive, yielding petroleums of unique, and/or mixed Pre-Salt hybrid, source provenance. A less attractive Pre-Salt source system is reflected in the Kwanza habitat. Consequential effects of salt movement have proved important for halokinetic induced structuration, Cretaceous block rafting and pseudograben (Tertiary trough) source deposition. Marginal reserves of Infra-Cuvo and Cuvo derived petroleums are known. With limitations in charge potential at this level, the modest Kwanza reserves (0.1 GBOR) have a Post-Salt provenance deriving primarily form the Eocene Margas Negras and subordinate Aptian Middle Binga sourced petroleum systems. Cases of multi-sourced hybrid petroleums abound. In addition to the organic-rich Albian micrites, Upper Cretaceous black shales provide unproven possible offshore potential.

Palaeoenvironmental control on primary fluids characteristics of lacustrine source rocks in the Autun Permian Basin (France)

“…On the other hand, the presence of medium-and heavy-paraffins in the pyrolysates are directly linked to the concentration of terrestrial OM in the kerogen, and probably to depositional environment conditions allowing optimal OM preservation. However, this may not explain the dominance of C 14þ compounds in other lacustrine formations, such as the GRS one and the Bucomazi Formation (Angola) where there is a lack of terrestrial OM (Burwood, 1999;Ruble et al, 2001).…”

“…n-alkanes with a number of carbon > 14, also called n-C 14þ [Behar et al, 2010;Burwood, 1999;Derenne et al, 1994;Ruble et al, 2001;Tissot and Welte, 1984]). However, past studies of lacustrine source-rocks showed that a variation of medium-heavy paraffins concentration (i.e.…”

“…However, past studies of lacustrine source-rocks showed that a variation of medium-heavy paraffins concentration (i.e. n-C 14þ /n-C 6þ ) may be encountered within a single source-rock formation (Burwood, 1999;Burwood et al, 1992;Gaglianone and Trindade, 1988;Tissot and Welte, 1984). Furthermore, as lakes contain much smaller volumes of water and sediments than oceans, their environments is much more sensitive to climate, accommodation and river discharge changes (Bohacs et al, 2000;Johnson et al, 1987;Perlmutter and Matthews, 1990).…”

Methylhopanoids: Molecular indicators of ancient bacteria and a petroleum correlation tool

“…4). This is not surprising given the thick sequences of source rocks in this area, including presalt (Early Cretaceous syn-rift lacustrine) and postsalt (mid Cretaceous to Tertiary postrift marine) sources (Burwood, 1999). The oils are classified as being of marine, lacustrine or "mixed" source, on the basis of their C 26 /C 25 tricyclic terpane and hopane/sterane ratios ( Fig.…”

“…6. Classification of the offshore West African oils using a cross-plot of the C 26 /C 25 tricyclic terpane and hopane/sterane ratios (Burwood, 1999;Schiefelbein et al, 1999). The hopane/sterane ratio uses Ts, Tm, 28,30-bisnorhopane, C 29 ␣␤, C 29 ␤␣, C 30 ␣␤, C 30 ␤␣, and C 31 to C 35 ␣␤ 22Sϩ22R hopanes, and C 27 to C 30 ␣␤␤ 20Sϩ20R steranes.…”