The Importance of Pre-Salt Stratigraphy
By Colin Saunders
Senior Regional Geoscientist
Part of the Spotlight Series
Acreage offshore Gabon has recently been launched, with current plans for the round to close by the end of September 2019.
The Neftex® Insights portfolio allows a rapid assessment to be made of the basin, with an understanding of geological risk. Thus, decisions about seismic acquisition and forward plans can be made quickly when assessing opportunities presented within a licensing round.
Recent key discoveries in the basin
The pre-salt petroleum system was proven in the South Gabon Basin in the 1960s and exploration is currently focused on the productive syn-rift succession immediately below the salt seal.
In 2013, the gas and condensate Diaman discovery opened up the pre-salt in the deep-water area. A non-commercial deep-water discovery at Sputnik-1 in the South Gabon Basin and a number of failures in the deep-water of the North Gabon Basin hampered further interest in the deeper offshore basins for a number of years. In 2018, oil was discovered at the Boudji-1 well (over 2800 m water depth) in pre-salt sandstones, closely followed by the discovery of oil at the Ivela-1 well, again in pre-salt sandstones, although, these are now thought to be non-commercial.
Pre-Salt Play Risks
Improvements in seismic imaging below the salt in recent years have allowed exploration to progress from the onshore and nearshore areas into the deep water. However, over the last five years, issues have arisen associated with exploration stepping farther outboard, especially when drilling beyond the Atlantic hinge zone. The key issues to address are:
Our assessment of hydrocarbon phase along the margin showed that oil is more likely to be present in the southern and south-western parts of the South Gabon Basin (Figure 1). This is supported by the first oil discoveries in the deep water offshore Gabon, which were made by the Ivela-1 and Boudji-1 wells in 2018.
Further key issues are structure size and leaking faults (Figure 1). An understanding of the controls on structure size and timing of activation is needed to enable commercial, and not just technical, success. Leaking faults are thought to be the reason for the limited hydrocarbon columns at Ivela-1 and Boudji-1. It appears that a number of discrete fault types and trends were involved in the creation of structures in the pre-salt offshore Gabon. These include hinge lines, fracture zones, and normal faults. Public domain seismic data across the region suggest that the salt seal might be breached over the largest structures.
The final key risk to address is reservoir quality, which is variable along the margin and between the onshore and deep-water. The Dentale and Gamba formations are the primary reservoir targets for the South Gabon Basin. Whilst they both comprise continental sandstones, there are marked differences in the quality of the reservoirs associated with these formations. The Dentale Formation is more heterogeneous in its reservoir properties, due to the range of environments in which it was deposited. The Gamba Formation, where developed, is more homogeneous, of higher reservoir quality, but it is thinner.
Whilst both formations have been proven commercial in shallow water offshore Gabon, as exploration progresses into deeper waters, it is likely that the higher-quality reservoirs of the Gamba Formation will be needed to ensure commerciality, certainly for oil. The distribution of these higher quality sandstones in the deep water is unclear, but a detailed stratigraphic study of the wells drilled to date would provide a predictive model for future exploration targets. Mapping of the reservoir in the shallow water and reports from the deep water wells do, however, favor the south-western area.
Opportunity this Round Offers
In terms of volumetric potential for oil, the pre-salt ‘Gamba’ fairway remains perhaps the most attractive, particularly in the south-western deep-water area.
Beyond the pre-salt we see a combination of other opportunities in this license round, including mature stage exploration in the post-salt fairway in the North Gabon Basin, possible deep-water extensions to that post-salt fairway and exploration for deeper plays in the shallow-water pre-salt blocks, particularly in the South Gabon Basin.
(Map shows the blocks on offer in the 12th Shallow and Deep Water Licensing Round for offshore Gabon, with the likely hydrocarbon phase of each block, key well results from the last 5 years and the key considerations associated with each area).