Observed link between folded Seaward Dipping Reflectors (SDRs) and large-scale morphology and architecture of the Early Cretaceous carbonate build-up and platform in the Orange Basin

Spectrum’s Anongporn Intawong¹*, Paolo Esestime¹ and Karyna Rodriguez¹ demonstrate an Early Cretaceous carbonate build-up reef structural closure prospect, in Volume 37 of First Break Magazine.

The Orange Basin is located in the volcanic-rifted margin of Namibia and South Africa and was formed during the break-up of Gondwana in the Late Jurassic to Early Cretaceous period (e.g. Nurnberg and Muller, 1991; Karner and Driscoll, 1999). Rifting and opening of the Orange Basin is believed to have occurred in an already extended and possibly inverted basement (Clemson et al., 2002). A general tectono-stratigraphic chart of the Orange Basin is presented in Figure 1.

Three major hydrocarbon accumulations have been discovered to date within the Orange Basin; a near shore oilfield A-J and two gas and condensate fields, Kudu and Ibhubesi. The near South African shore A-J oilfield discovered 36 degree API oil in a Hauterivian lacustrine and fluvial sandstone reservoir within a half-graben on the basin’s shelf (Crown Energy, 2013). Farther north and more out-board, the Kudu field is situated in the northern part of the Orange Basin in Namibia and has two productive reservoir intervals in aeolian sand dunes of Late Hauterivian or Early Barremain age. The main reservoir is approximately 20 m thick, with high net-to-gross ratio and an average porosity of 15%, mainly secondary porosity generated by the dissolution of widespread calcite cement. These sands are intercalated by flood basalts within the clastic prone Seaward Dipping Reflectors (SDRs) sequence, which have been proven to act as local seals. The Ibhubesi and AF-1 gas fields on the South African Orange basin shelf have reservoirs in Albian channel and fan sandstones displaying good porosities (16-25%), and are tied to anomalously high amplitude seismic reflectors.

Four potential source rocks have been identified in the Orange Basin. These comprise, firstly, localized syn-rift Late Jurassic to Berriasian lacustrine shales (believed to be the source for the A-J Field) and, secondly, Valanginian to Hauterivian lacustrine shales deposited within clastic prone Seaward Dipping Reflectors (SDRs) (believed to be the source for the Kudu field). Highly cracked condensate (oil cracking more than 95%) of lacustrine saline mixed with less mature non-cracked condensate marine derived oil types have been reported from Kudu-4 and -5 condensate samples analysis (Mello et al., 2012).

Thirdly, post-rift Barremian to Aptian restricted marine shales and lastly, high TOC is found along the margin in Cenomani-an-Turonian organic-rich open marine shales. The Barremian to Aptian restricted marine source rocks have been encountered in a number of wells penetrated along the margin including the DSDP 361, South African and Kudu wells, and exploration wells drilled by HRT, with up to 10% TOC. This Barremain-Aptian source rock is also a contributor to the Kudu gas and condensate field and other gas discoveries made on the shelf offshore South Africa. Regional mapping of the Barremian-Aptian source rock based on Spectrum’s extensive 2D seismic data in the Namibia and South Africa margin tied to Aptian markers from wells demonstrate the presence of the Aptian source rocks in the Orange Basin (Intawong et al., 2015). The source rock is ubiquitously distributed over the Orange Basin with some variation in thickness over two main depocentres divided by a NW-SE trending Outer High (Intawong et al., 2015). The Cenomanian-Turonian source rock has been penetrated by exploration wells 2012/13-1, 2513/08-1, Kudu and 2515/15-01 with up to 7% TOC. The Cenomanian-Turonian source rock lies below sedimentary gravity driven Mass Transportation Deposits (MTDs), where it may provide a decollement surface accommodating the gravity flow.

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Observed link between folded Seaward Dipping Reflectors (SDRs) and large-scale morphology and architecture of the Early Cretaceous carbonate build-up and platform in the Orange Basin