--> --> Paleogeographic Controls on Reservoir Presence and Trap Generation, offshore Namibia

AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Paleogeographic Controls on Reservoir Presence and Trap Generation, offshore Namibia

Abstract

With recent improvements in the quality of 3D seismic data, and through the acquisition and interpretation of large surveys, it has been possible to gain a greater understanding of turbidite reservoir architectures in deepwater settings in frontier basins. The Atlantic offshore basins of Namibia are currently experiencing renewed interest from the exploration sector with multiple new entrants into the country including small independents, national oil companies and supermajors, and multiple wells expected to be drilled in 2018-2019 by various operators. Through the interpretation of modern 3D seismic data, integrated with key offset wells and other regional geological data, paleogeography is observed to play a pivotal role in the sedimentary fill of the continental margin in the Walvis Basin at a range of scales, all of which influence the description of potential hydrocarbon reservoirs and traps. At a regional scale paleogeography has controlled the provenance, supply and distribution of sediments from the Aptian through to the end of the Cretaceous, including a structurally-pinned primary input. Inland tectonic uplift during the early Cenozoic shuts-off the supply of coarse clastic sediments and changes the sedimentary system drastically. Paleoshelf orientation and paleoslope gradient have controlled the stability of sediments on the shelf and have been fundamental controls on reservoir distribution. Carbonate sedimentation within the early drift sequence has changed the orientation and position of the shelf during the Early Cretaceous. These potential carbonate build-ups have controlled the lateral extent and position of Cretaceous sedimentary system inputs and the character of subsequent reservoir systems. Paleoshelf architecture has been a significant influence throughout the basin’s evolution and has led to the formation of highly prospective stacked reservoir systems. At a local scale paleogeography has determined the geometry and distribution of deepwater turbidite channel and fan systems. One such example is seen within Block 2312 where the Springbok igneous complex, a Barremian-age expression of syn-rift magmatism, has induced positive basin floor topography at the base drift surface in the order of 400m. The flow and preservation of sand-prone Cenomanian-Turonian age deepwater channels has been influenced by this structural paleo-high. The course of the deepwater channel systems have been constrained for some time leading to the aggradation of a significant thickness of reservoir before channel avulsion occurs and the channel system moves away from the paleohigh. In both this and other examples, large structural prospects form where channels partially drape positive paleo-topography and vertical closure has subsequently been enhanced through differential compaction. This paper explores the role of paleogeography at a basin-wide scale before examining its impacts at a prospect scale by using examples from 2016 3D seismic data. Extensive 3D seismic data exists around the world showing turbidite slope channel facies and geometry in remarkable detail within the Cenozoic however the Aptian-Albian and Cenomanian deepwater reservoir systems of the West Africa margin have rarely been resolved at this level of detail. The influence of basin floor highs, shelf architecture and slope gradient, are explored at multiple stratigraphic intervals and are considered to have been critical to the development of highly prospective structural traps within an emerging hydrocarbon play system.