Crustal-Scale Architecture and Heat Flow Account in Ultra-Deep Water Lower Congo Basin

Abstract

Abstract In the present study the integration between interpreted seismic reflection profiles, available seismic refraction data, 2D gravity modelling, 2D structural restoration and sandbox analogue modelling are used to depict the crustal-scale architecture and elucidate heat flow issues in the ultra-deep water Lower Congo Basin in order to provide a constrained input to the subsequently performed thermo-tectonic and petroleum system modelling of the investigated region. The integrated analysis provides a state-of-the-art of the large-scale structural and basin evolution of the ultra-deep water Lower Congo Basin. Locally hyper-extended and denser continental crystalline crust is revealed in the area of study. Structural restoration indicates anomalously extensive subsidence at a short time-interval (112–98 Ma) just after breakup. Considerable stretching ß-factors are revealed within the ultra-deep water region at two modelled transects. Processes that governed the pre-breakup extension have been constrained through analogue modeling. Based on the above multi-fold analysis potential geothermal-gradient scenarios have been considered in petroleum system modelling that shown important effect on the generation history of a pre-salt source rock potentially present in the deep-water. Introduction Heat flow account and its petroleum system efficiency implications are key risk factors in hydrocarbon exploration. Increasing exploration interest for deep seated pre-salt targets in the South Atlantic, far from any well penetration control points (deep water and ultra-deep water), has brought to attention the importance of correctly modelling the heat flow and its reconstruction through the South Atlantic rifting process and basin evolution. Eni has been active on the West-Africa pre-salt Play since the early 60's and recently has been on the front row in trying to unravel un-explored potential of this Play in frontier areas (deep waters). The extensional margin evolution from rift through breakup rapture of the continental lithosphere to progressive oceanic crust formation remains controversial (e.g. Rosendahl et al., 2005). Complicating the issue, the deep crustal structures along several margins are partially or totally masked by evaporate deposits and/or by magmatic materials which deteriorate the seismic reflection resolution at depth. Furthermore, the complex interaction of structural and magmatic relationships during continental rifting and breakup results in a wide variety of margin styles, ranging from narrow to wide, and from "magma-dominated" to "magma-poor" margin rift-systems with different heat flow evolution. The South Atlantic passive margins formed during Mesozoic time as a result of lithospheric extension followed by breakup of the Paleozoic Gondwana super-continent. The opening of the South Atlantic which started in the southern portion and propagated towards the North, resulted in considerable diachronic deformation. All recent studies show that although the South Atlantic Central Segment experienced some volcanism during breakup, magmatic products were not sufficiently voluminous to form seaward dipping reflections (SDRs), and thus the Central Segment conjugate margins exhibit "magma-poor" affinity (e.g. Moulin et al., 2009; Lentini et al., 2010; Huismans and Beaumont, 2011).