--> Onshore to Offshore in the South Atlantic: Insights From Outcrops for New Plays along the Continental Margins

2018 AAPG International Conference and Exhibition

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Onshore to Offshore in the South Atlantic: Insights From Outcrops for New Plays along the Continental Margins

Abstract

Outcrop work has been undertaken by CGG Robertson along the underexplored conjugate margins of the South Atlantic in southern Angola, Namibia, Uruguay and South Africa. New analysis has found evidence of oil seeps, hydrocarbon migration pathways, thermally mature and immature source rocks, together with reservoir facies in siliciclastic and carbonate lithologies. These occur in Permo-Triassic and Lower Cretaceous sequences, thus our reconstructions of the pre-rift, rift and early break-up relationships across the conjugate margins provide useful exploration insights. Spectacular outcrops exposed over 100km along the Namibe Basin of southern Angola demonstrate tectono-stratigraphic relationships between rift boundary faults, volcanics, source and carbonate reservoir facies. These provide useful constraints on the character of the pre-salt eastern margin of the Santos Basin offshore Brazil, and suggest that equivalent plays may also be developed amongst fault blocks close to shore along the margin of the Namibe Basin. In northern Namibia, outcrops indicate that hydrocarbons from Permian source systems have migrated through Cretaceous pre-volcanic sandstones and conglomerates. Interpretation of seismic data suggests that Permo-Triassic strata may also be present beneath the Cretaceous section offshore. This extends Namibian Cretaceous-Tertiary offshore play potential into parts of the margin where overburden is too thin for maturation of Cretaceous source facies. On the conjugate margin in Uruguay, outcrops and shallow boreholes demonstrate the presence of equivalent Permo-Triassic source facies, and an additional section of Devonian marginally mature oil-prone source facies. Good quality pre-Cretaceous volcanic sandstone reservoirs are widely distributed at outcrop in Uruguay. Our analysis of exploration wells and available seismic data proves the presence of a down-faulted Palaeozoic section, and evidence for migrant thermogenic hydrocarbons in all offshore wells drilled to date. New outcrop and analytical work in South Africa suggests that the play potential of a Mesozoic sedimentary section deposited in the Orange and Outeniqua Basins prior to Cretaceous rifting and break-up may not have been properly considered during exploration undertaken to date.