|
Statoil, Staranger, Norway
Exploration drilling at Haltenbanken started in 1980 and to date 11 significant hydrocarbon discoveries have been made. The estimated total recoverable discovered reserves are 310 million SM3 of oil and condens and 305 billion SM3 of gas. Only 14 out of 25 proper wildcat wells turned out dry, giving a pure exploration success ratio of 44%. The major structural elements at Haltenbanken are (from East to West) the Troendelag Platform, the Halten Terrace and the Deep Moere Basin. The major hydrocarbon discoveries are located on the Halten Terrace and on the western margin of the Troendelas Platform.
After a long period of regional passive subsidence during Triassic and most of Jurassic the Halten Terrace was formed as a consequence of Late Jurassic east-west rifting and early/middle Cretaceous north-south Dextral wrenching. During late Cretaceous and Tertiary again regional passive subsidence dominated. In Miocene very deep waters had developed at Haltenbanken as well as over most of the Mid-Norwegian Shelf. In Pliocene uplift of the Fennoscandian Shield resulted in rapid westward shelf progradation and deposition of up to 1000 m of sediments.
This Pliocene burial had a great impact on the formation and entrapment of hydrocarbons at Haltenbanken. As the temperatures increased the rich Upper Jurassic marine and Lower Jurassic terrestrial source rocks passed through the oil window and are currently still producing large amounts of oil, condensates and gas.
The rapid Pliocene sediment loading also led to great overpressure in the Upper Jurassic to palaeocene line-grained sequence. In the Eastern areas this overpressure resulted in a perfect seal over the Middle and Lower Jurassic sandstone reservoirs. In the Western areas also the Jurassic reservoir sequence is severely overpressured leading to less favourable sealing conditions. On the other hand, the overpressure in the reservoirs in the Western areas seems to have reduced the rate of compaction, hence preserving the porosity and permeability better at great depths. Seven of the most important discoveries at Haltenbanken are discussed as case examples. The variations in type of hydrocarbons (oil, vs. gas or gas/condensates) entrapped in the individual fields are caused by subtle, but important variations in source rock maturity, migration conditions (including drainage area and storeage capacity), trapping conditions (fill-spill or fill-leak), timing and PVT-conditions.
