Author: Michael Treloar
Product Owner for FairwayFinder
The Republic of Gabon’s ongoing and long-running 12th Shallow and Deep Water Licensing Round has been impacted by series of delays related to the continuing global pandemic. However, the country’s 60-year history of oil production and cluster of recent deep-water discoveries clearly demonstrate the opportunity for future success. In this analysis we have used an integrated assessment of subsurface geology to offer commentary on the key plays in the area, their future potential and associated exploration risk.
The analysis presented in this article is based on a synthesis of Halliburton Landmark’s Neftex® Predictions, a DecisionSpace® 365 Solution, which provided a comprehensive synthesis of the region’s petroleum geology, underpinned by models of plate tectonics, sea level change, paleoclimate and geological structure.
Integration of the various datasets and interpretations included in the Neftex Predictions product suite was conducted using FairwayFinder, a DecisionSpace 365 cloud application. By leveraging sophisticated processing methods and cloud technology, FairwayFinder was used to accelerate the play screening workflow to approximately three minutes per play identified offshore Gabon.
The petroleum systems of offshore Gabon can broadly be divided into plays that lie above the thick Aptian Ezanga evaporites, and those that lie below. The primary source rock for the post-salt plays are Cenomanian-Turonian marine black shales. Pre-salt plays are sourced by thick, lacustrine black shales deeper in the Early Cretaceous syn-rift stratigraphy.
Late Cretaceous slope and fan sandstones form the primary reservoir interval for proven post-salt plays. Commercial discoveries have also been made in Miocene slope channels, Paleocene-Eocene fans and Aptian-Albian marine carbonates. Reservoirs for the pre-salt plays are fluvial, deltaic and lacustrine sandstones related to syn-rift deposition. Gas has also been discovered in pre-salt lacustrine carbonates in the onshore portion of the basin.
Proven traps offshore Gabon are dominantly structural, relating to salt movement and extensional tectonics.
Activity to Date
The North Gabon Basin provides the majority of the nation’s offshore oil production. It is characterized by a thick post-salt package of passive margin sediments. Most of the proven fields are situated in the shallow waters and produce from Late Cretaceous slope and fan sandstones. With some exceptions, such as the Nyonie Deep Field, pre-salt plays have yielded limited success in this basin to date.
The South Gabon Basin features a substantially thicker pre-salt section, having acted as the main depocenter of the Dentale Delta during the Early Cretaceous. This extensive clastic succession hosts the basin’s commercial discoveries. Initial pre-salt deep-water discoveries in the centre of the basin, Diaman and Leopard, encountered gas, whereas more recent discoveries further south, Ivela and Boudji, contain oil, but questions remain over the commercial viability of these discoveries.
North Gabon Basin Fairway
An integrated analysis of our subsurface data and interpretations, including Gross Depositional Environment maps, well data and tectonic elements, suggests that the Late Cretaceous fan sandstones are likely to extend into the deeper waters of the North Gabon Basin. This is supported by Neftex Predictions’ Earth System Science modelling of the Late Cretaceous, which incorporates paleotopography, drainage, tidal, and climate models to predict high sediment input into the basin from river systems draining much of Central Africa at the time.
The Cenomanian-Turonian source kitchen is also predicted to extend into the deep-water (Figure 1). This prediction has been made by integrating a regional-scale depth framework, constrained by well data, seismic lines and a range of other data types, with geothermal gradient data to calculate subsurface temperature. However, the presence of traps in the deep water is an uncertainty, particularly once the Continent-Ocean boundary has been crossed and salt structures or well-developed fault blocks are unlikely. Replicating commercial discoveries made in Paleocene–Eocene and Miocene reservoirs in the deep-water will face similar challenges. More subtle stratigraphic traps in the prolific Late Cretaceous sands and younger clastic reservoirs may offer new potential in the deep-water blocks on offer.
Though the proposed Early Cretaceous pre-salt source rock has yet to be penetrated in the North Gabon Basin, the Nyonie-deep field produces gas and condensate from pre-salt reservoirs and indicates that the interval is both present and effective. Analysis of subsurface structure and geothermal gradient suggests that the Early Cretaceous source kitchen is likely to be gas mature to overmature in the south of the basin, with some potential for oil maturity in the centre of the basin (Figure 2).
The proven Dentale/Gamba sands reservoir is predicted to be present over the majority of the basin (Figure 2). However, targeting valid traps and seals in this interval has proven a challenge to date; for example, the Padouck Deep-1 well drilled in 2014 is thought to have failed due to a lack of seal integrity.
Figure 1: LEFT: Kitchen map for the Cenomanian-Turonian source rock interval. RIGHT: Reservoir presence chance map for the Late Cretaceous reservoir interval, with discoveries in the interval overlaid in black. Both maps were generated using the FairwayFinder cloud application.
South Gabon Basin Fairway
The maturity of the Early Cretaceous source interval represents a major uncertainty concerning the extension of the proven play concepts into the deep waters of the South Gabon Basin, as explorers move further out towards oceanic crust. Oil is likely to be the more commercially desirable phase in such settings, but recent drilling has yielded a mixture of small oil and gas discoveries. An integrated assessment of our depth framework for the area and our model for geothermal gradient predicts that an oil-prone, pre-salt fairway has the potential to exist over much of the southern section of the basin (Figure 2).
The post-salt succession in the South Gabon Basin is thin compared to its northern counterpart. Therefore, the widespread Cenomanian–Turonian source interval is predicted to be immature over much of the area (Figure 1). Post-salt plays carry the risk that they may be reliant on vertical migration from the pre-salt source rock, through the thick evaporate layer, which to date has yet to be proven.
Furthermore, our interpretations indicate that the Late Cretaceous reservoir interval, that hosts the majority of fields in the North Gabon Basin, is unlikely to be as extensively distributed in the area (Figure 1). In contrast to the North Gabon Basin, Neftex Predictions’ Earth System Science models predict that drainage basins feeding into the South Gabon Basin during the Late Cretaceous are likely to be small, and in arid paleoclimates.
Figure 2: LEFT: Proxy maturity map for the Early Cretaceous source rock interval. RIGHT: Reservoir presence chance map for the Dentale/Gamba reservoir interval, with discoveries in the interval overlaid in black. Both maps were generated using the FairwayFinder cloud application.
Offshore Gabon has many attractive features for would-be explorers – two proven, prolific pretroleum systems, a long history of production with the associated topside infrastructure, and recent discoveries that extended proven plays into the deep-water.
Neftex Predictions provided rapid access to key insights across a range of scales, from understanding the gross architecture of the basin, through characterising the petroleum systems to predicting the distribution of individual play components. FairwayFinder streamlined the collation and integration of these insights into an evaluation of the potential fairways.
Our studies suggest the assessed fairways have the potential to be relatively widespread across the blocks on offer, however, hydrocarbon phase, trap size, and seal integrity pose key risks. Given the potential for new oil discoveries in a relatively untested deep-water setting, that is close to established infrastructure, it is clear why the ongoing license round could be of high interest to many.
For further information on the themes discussed, and to learn how a synthesis of the Neftex Predictions product suite and FairwayFinder could be useful in other, similar exploration scenarios, contact us today.