Polarcus, in collaboration with The Gambia’s Ministry of Petroleum and GeoPartners, has recently completed a 1,504 km2 RightBANDTM Multi-Client 3D seismic survey over Blocks A2 and A5, within the prolific MSGBC Basin offshore The Gambia. This modern broadband data volume lies in an area with no previous 3D seismic coverage, and where the commerciality of the SNE oil discovery in neighboring Senegal has recently been confirmed. Screening of the new 3D multi-client volume has identified a number of both structural and stratigraphic leads on trend with the Senegalese discoveries, confirming that the new deepwater clastic play concept extends into The Gambia. Polarcus’ Block A2-A5 survey provides optimum imaging to allow evaluation of this highly prospective area.
The survey was acquired in this geologically complex area using the Polarcus Alima towing a 12 x 100m x 8,100m ultra-quiet Sentinel® solid-streamer spread with an 18.75m flip flop shot interval and a nine-second record length. The data has been processed by DownUnder GeoSolutions through broadband pre-stack time and depth migration workflows.
The seismic line below appeared as a multi-page pull-out feature in our print magazine. To subscribe to the print magazine use the 'Print Subscription' feature in the menu at the top of this page.
Extension of a New Oil-Prone Fairway
The Polarcus Block A2-A5 survey is located in The Gambia’s offshore Casamance-Bissau sub-basin, located in the southern part of the greater Senegal Basin. The area has seen recent renewed industry focus due to three successive hydrocarbon discoveries on the adjacent Cairn-operated acreage in Senegalese waters to the north. The world class SNE-1 and FAN-1 oil discoveries in Albian-aged sandstones have opened up a new oilprone fairway with multiple play types. Similar plays along the West Africa margin are already in development or production, and the adjacent acreage in The Gambia provides a number of leads and prospects on-trend to these exciting discoveries.
Exploration for hydrocarbons in The Gambia dates back to the 1950s, but none of the companies involved in the early exploration of the area progressed beyond initial studies apart from Chevron, who drilled Jammah-1 in 1979. This is located in Block A2 and to date is the only well drilled offshore The Gambia. It was plugged and abandoned with gas shows at a TD of 3,020m. It targeted an Albian shelf-edge carbonate feature slightly to the east of the axis of a large north-south trending anticline and encountered viable sandstone and carbonate reservoir horizons but only minor petroleum shows. The well was drilled on the basis of 2D seismic data and post-well studies suggested it was dry due to erosion of potential Turonian source rocks at the location, though analysis of the new seismic data acquired by Polarcus in 2015 suggests it did not test a valid structural closure.
Studies have reviewed the offshore exploration potential and a range of play types including fan and slope channel systems, karstified reef build-ups, 4-way closures and clastic deposits onlapping the shelf edge have been identified. No further drilling has taken place offshore The Gambia.
A New Oil Province
In 2014 Cairn drilled two deepwater exploration wells offshore Senegal, SNE-1 and FAN-1; both wells discovered oil, opening a new hydrocarbon basin on the Atlantic Margin. FAN-1, the first well drilled in deepwater offshore Senegal, is located in 1,427m of water in the Sangomar Deep Block. It encountered high quality light oil in multiple stacked deepwater Cretaceous clastic fan bodies with 29m of net oil-bearing reservoir located in a combined structural and stratigraphic trap. The second deepwater exploration well, SNE-1, located in 1,100m of water approximately 24 km from the FAN-1 discovery, targeted Albian sandstones overlying Aptian carbonates. A 95m gross oil column with a gas cap was encountered in excellent quality Albian reservoir sands with 36m of net oil pay. However, no hydrocarbons were encountered in the deeper carbonates.
The SNE-1 discovery was appraised by the SNE-2 well which flowed up to 8,000 bopd on DST, confirming the high delivery of the principal reservoir unit and the connectivity of the principal reservoir with the discovery well. In March 2016 Cairn announced the results of further appraisal drilling with the successful testing of the SNE-3 well, which flowed at a maximum rate of 5,400 bopd on DST. The well confirmed similar reservoir quality and correlation of the principal reservoir units with SNE-1 and SNE-2, including similar oil-down-to and oil-up-to depths. Initial indications confirmed the same 32° API oil quality as seen in the previous wells.
Further exploration followed with BEL-1, which drilled the Bellatrix Prospect and tested the Buried Hills play, discovering gas in two good quality Cenomanian sandstone reservoirs. The well was deepened to further appraise the SNE discovery and to progress towards proving a minimum economic field size. It also confirmed the extension of reservoirs and oil column on the northern flank of the discovery. Estimates by the operator Cairn, post the SNE-2 well, suggest contingent reserves of 385 MMbo and further analysis of the resource is underway as further appraisal drilling is ongoing.
Gambia: The Aptian-Albian Shelf
The Block A2-A5 area lies immediately to the south and along structural trend with the recent discoveries in neighboring Senegal. The Jurassic to Aptian carbonates are overlain by a series of Albian sequences displaying large scale clinoforms. These occur as a number of discrete stacked systems, each prograding from east toward the west. The clinoforms can be seen to be laterally extensive across the area, providing the potential for reservoir sandstones across the shelf. As well as providing reservoir sands into a number of structural play types, the evolution of the delta-to-slope systems provides possible stratigraphic components to plays within shelfal distributary systems, shelf edge clinoforms and more basin-ward down dip sand-bodies.
Uplift and rotation due to the withdrawal of Triassic salt led to erosion of the Aptian-Albian shelf and the overlying Upper Cretaceous sequence. This has provided a further series of prospective play types with structural components. Gambia Blocks A2-A5 contain all the necessary components of a successful petroleum system as seen immediately to the north in Senegal, with potential existing for source rocks in several Jurassic to Cretaceous sequences both outboard the shelf-slope break and across the shelf area.
Multiple Play Types
Interpretation of the newly acquired 3D multi-client volume has identified a number of play types across the shelf and shelf edge areas including: 4-way dip closures, fault bounded 3-way dip closures, Buried Hills plays, and slope fan and stratigraphic plays. A number of examples are presented herein.
Structural 4-Way Dip Closures: Initial interpretation of the final PSTM data volume after a simple 1D conversion to depth shows a number of 4-way dip closures affecting the Albian sequence all along the shelf edge area across Blocks A2-A5. Interpretation of the full PSDM data volume, due in the third quarter of 2016, will allow proper evaluation of this exciting series of leads.
Structural Fault Bounded 3-Way Dip Closures: A series of rollovers into the prominent north-north-east to south-south-west trending faulting affecting the uppermost Albian sequence is observed. The 3D data reveals a number of closures at this stratigraphic level forming a series of leads along strike. Deeper within the sequence the Albian is less affected by this fault system and 4-way closures occur.
The Buried Hills Play
Analogs to the Cenomanian Buried Hills play as successfully drilled in the BEL-1 exploration well can be seen across the A2-A5 area. Erosion and incision of the Upper Cretaceous sequence has resulted in trapping of Cenomanian sand-bodies sealed by the overlying Tertiary and Cretaceous shales and claystones. The new 3D data enables interpretation of the depositional packages within the Cenomanian, allowing evaluation of a number of leads. The example below, which also displays a structural 4-way dip closure of the underlying Albian sequence, is analogous to the section across the SNE and BEL discoveries.