Abstract
The Barremian-Cenozoic depositional sequences in the northern Orange Basin, SW, South Africa, were investigated using the principles of seismic stratigraphy to understand the interplay of tectonics and sedimentary processes in the distribution of potential hydrocarbon reservoirs. A seismic stratigraphic workflow (seismic sequence, seismic facies and lithofacies analysis) was completed by utilising three seismic lines (L1, L2 and L3) tied to Wireline data (gamma, checkshots and sonic) in two exploration wells (A1 and A2). Seven depositional sequences were mapped followed by the creation of lithofacies log interpreted from the gamma-ray log (GR) by setting maximum GR value at 60 API for Sandstone, 60–100 API for Siltstone and above 100 API for Shale. Six seismic facies units are recognised based on internal geometry and configurations of the seismic reflectors; Tangential-Oblique (SF1), Hummocky (SF2), Wavy-Parallel (SF3), Chaotic (SF4), Sub-parallel/parallel (SF5) and Divergent (SF6). SF4 is dominant within the Barremian-Aptian sequence and expressed in an incised valley fill, suggesting mass transport deposition accompanied by strong hydrodynamic conditions. Evidence of sedimentary basins progradation is seen within the Late-Albian-Turonian sequences, because of the occurrences of SF2, SF6 and SF 4 facies. SF5 facies is prominent in the Maastrichtian/Campanian sequence, indicating that the deposition of sediments may have been accompanied by uniform margin subsidence after the Late-Cretaceous uplift of the Africa margin. The occurrence of SF1 and SF4 facies within the Cenozoic sequence indicates terrigenous pro-deltaic deposits and mass transport deposits, respectively. Further results from seismic-lithofacies modelling reveal that sand deposits of Barremian-Aptian (SF4 facies unit) and Albian sequences (SF2 and SF6 facies units) are potential stratigraphic reservoirs in this part of the basin.
Highlights
As passive margin terrains are structurally less complex than active tectonic regimes, they preserve sedimentary records which can be investigated
Whilst exploration continues within the syn-rift pre-salt sequences of the Santos Basin (Scotchman et al 2010), further exploration is essential in northern Orange Basin for the delineation of potential reservoirs for a drilling programme
These depositional sequences are divided into packages within a chronological framework (Fig. 2) separated by regional unconformities and are coded from SQ1–SQ7, where SQ 7 (Barremian) is the oldest sequence and SQ1 (Cenozoic) the youngest sequence mapped (Fig. 4)
Summary
As passive margin terrains are structurally less complex than active tectonic regimes, they preserve sedimentary records which can be investigated. Two of the recent hydrocarbon discoveries in conjugate passive margin settings were from the Albian-age sandstone reservoirs of the southern Orange Basin, South Africa (Petroleum Agency SA report 2010), and in Brazil, from the Barremian-Aptian syn-rift nonmarine microbial carbonates of the Santos Basin (Scotchman et al 2010). In both cases, the reservoir quality is controlled by a multi-phase diagenesis Whilst exploration continues within the syn-rift pre-salt sequences of the Santos Basin (Scotchman et al 2010), further exploration is essential in northern Orange Basin for the delineation of potential reservoirs for a drilling programme
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