WAFR Salt Basins:A Regional Perspective of Crustal Architecture and Play Developments
The West Africa salt basins which includes South Gabon, Lower Congo, Kwanza and Namibe Basins have been the focus of E&P companies in their race to find the next giant discovery. Working petroleum systems have been identified throughout these salt basins in both Pre-Salt and Post-Salt sequences with majority of the discoveries coming from the onshore and shallow water region. However, despite a proven and working Post-Salt petroleum system in the deep water, the syn-rift Pre-Salt sequences remains underexplored. From the regional perspective, there is a gap of geological knowledge to be filled with respect to: 1) crustal and structural evolution during the syn-kinematic period, 2) changes in source rock and reservoir provenances in response to crustal evolution, 3) tectonostratigraphic implications on play developments. The objective of this study is to fill these knowledge gaps and improve our understanding on the crustal architecture and play developments of West Africa salt basins via an integrated regional G&G evaluation which includes regional 2D & 3D seismic and well data in the area.
Methodology This study is based on large 2D and 3D seismic data set, which spans from north Gabon to south Kwanza basins. For our regional study, deep seismic events were interpreted using the regional depth-migrated 2D seismic line (Congo Span). Three deep seismic events (>30 km) were interpreted namely the Top of Basement, Deep Reflector A and Deep Reflector B. The reflectors were picked at regional scale to establish the geometry of the crust and to better understand the crustal evolution of the margin. Additionally, the interior of the basins (10 horizons) were mapped based on the major unconformities and correlative conformities following the convention proposed by Mitchum et. al., (1977). Following the horizons interpretation, an integrated geological/geophysical database was utilized to define the sedimentary records in terms of gross depositional environment (GDE) maps.
West Africa Crustal Evolution Regional analysis of 2D & 3D deep seismic reflection (this study) and refraction profiles (Moulin et al., 2005) across the West Africa salt basins suggests wide tectonostratigraphic variations along the strike (N-S) and dip (E-W) lines of the margin. In the strike direction, the observation from deep discontinuities (Intra-crustal & Moho) and mapping of basement shows gradual change in rifting style in northward direction. Three segments with varying crustal thickness and stretched domains can be distinguished from the crustal isopach (Fig 1). Similarly, the distinct crustal thickness shift from ~20 km to 7 km can be interpreted as the necking zone which demarcates the boundary between deep water outboard and shallow water inboard basins along the margin.
Tectonostratigraphic Implications on Play Developments Currently, several lines of evidence from seismic interpretation indicate that Rift 1/Early Rift and Rift 2/Mid-Rift Pre-Salt sequences are confined within the inboard sub-basin and their corresponding surfaces terminate at the Atlantic Hinge Zone. Our seismic interpretation in the outboard sub-basin suggests a thick sequence of Rift 3/Late Rift that could possibly be deposited throughout the Dentale trough (Fig 2). Similarly, unexplored carbonate platforms and microbial build-ups have also been mapped within Kwanza basin suggesting the need to better understand its Pre-Salt play fluid distribution and carbonates evolution within the basins.
AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018