A dry well west of the Mandal High drilled 2 years ago has reduced the reservoir potential in this part of the Central Graben.
Well 2/9-5S was drilled in production license 494 in 2014. The license was awarded in January 2009 to Dana, Det Norske oljeselskap and Bridge through APA2008.
Several commercial discoveries
The Exploration history in the graben/terrace system west of the Mandal High has proven several commercial oil and condensate accumulations in the chalk (Valhall, Ekofisk) and pre-Cretaceous (Embla) reservoirs. The commercial fields are located in the deeper part of the basin and along the Lindesnes Ridge.
Prolific mature source rock is present in the basins surrounding basinal highs and terraces west of the Mandal High. However, only traces of hydrocarbons have been encountered in wells drilled on the Piggvar Terrace.
A technical oil discovery was made within the chalk and fractured basement in well 2/6-5 located on the Mandal High.
Well 2/9-5S was drilled to test the Heimdalshø Prospect on a down-faulted terrace to the west of the Mandal High. The primary objective of the well was the test the prospectivity of a Jurassic sequence, and to explore a possible migration pathway towards the high.
The basement structural high was believed to be a sediment source for sands deposited on the adjacent terraces. The depositional model suggested that the Upper Jurassic sandstones observed in wells down-flank of Heimdalshø (2/9-2 and 2/9-3) represented distal parts of the expected reservoir sandstones in Heimdalshø.
Refined evaluation prior to drilling, resulted in a reservoir model comprising an Upper Volgian sand deposited by gravity flows overlying a reservoir sandstone of a shallow marine shoreface origin of Kimmeridgian to Oxfordian age. The kitchen for hydrocarbons to the Heimdalshø Prospect was the Upper Jurassic Mandal and Farsund formations on the Piggvar Terrace.
Risk on reservoir was considered intermediate, and low risk was associated with the presence of a valid trap and a working migration pathway.
Well 2/9-5S penetrated only minor sands in the Upper Jurassic section and no hydrocarbon shows.
Drilling of Heimdalshø resulted, however, in increased understanding of the structural and stratigraphic development of the area.
The Mandal High was probably not a major sediment source for reservoir sands to the terraces on the western flank of the high as earlier postulated.
In addition, well 2/9-5S revealed shales deposited under anoxic conditions, suggesting that the terrace and possibly parts of the high were drowned during the Late Jurassic. In this setting, the sand deposition on the terrace was limited in the 2/9-5S position.
As a consequence of the well, reservoir potential and hence migration pathways in a close proximity to the Mandal High, is now considered to have a higher risk compared to the pre-drill view.
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