A Review of Carbonate Continental Systems in Active Rift Settings (Offshore Angola) – A Combined Subsurface and Outcrop Study for Derisking Reservoir Presence

Abstract

Early Aptian carbonate continental systems have been recently reviewed at the subsurface in Kwanza Basin (Offshore Angola, South Atlantic realm), deposited in a transitional zone (marginal highs) between the necking zone (slightly thinned continental crust) and the exhumation basin (crust hyper extension period). Low access to rare well data in this domain forces explorationists to look for near onshore fossil analogues to increase the comprehension of regional facies heterogeneities, stacking patterns and reservoir geometries among other petroleum elements (Guiraud et al., 2010; Held et al., 2016; Gindre-Chanu et al., 2016). Conventional petrographic examination (i.e. microfacies analysis and early post-depositional diagenetic cements) of sidewall core and cuttings thin sections, in combination with continuous wireline log data (especially bore-hole images), provided detailed information on the vertical stratigraphic evolution of these deposits. A chrono-stratigraphic scheme has been established based on palynomorph and ostracod occurrences. An early post-rift seismic mound was drilled which revealed to be a nearly pure carbonate system showing no evidence of any marine incursions throughout the Pre-salt section. The upper part is dominated by the presence of sparitic fibrous radiaxial calcite and shrubs-like components. Lower mound is characterized by two silica-rich zones showing two major silicification events, separated by a lacustrine muddy and fine-laminated µm-scale dolomitic interval with some ostracod occurrences. This carbonate system overlies several hundred meters of syn-rift siliciclastic, volcaniclastic and volcanic deposits, highly intruded by sill and dyke bodies. Magma-poor margins with punctual high magmatic budgets at different phases of the opening, provides an important influx of hydrothermal mineralizing fluids. It can in turn develop aggrading sedimentary bodies with complex mineral phases (calcium and magnesium carbonates, iron and silice predominantly; Pinto et al., 2015) deposited in competition with intercalated tufa and lacustrine environments. The aim of this integrated transverse work is to combine new rock data from time-equivalent onshore Angola deposits, in order to decipher the role of main actors such as climate, volcanism and physiography (with respect to structural evolution), thereby providing critical information for derisking reservoir presence in frontier exploration domains.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017