The Heidrun field is located in the Haltenbanken region on the mid-Norwegian continental shelf in water depths of approximately 350 m. It lies 190 km from shore. The field was discovered in 1985, and appraisal drilling has subsequently confirmed a giant Cimmerian structure with hydrocarbons trapped in Jurassic sandstone reservoirs. The field has 750 million bbl of recoverable oil reserves, with associated gas reserves of 0.45 tcf and free gas reserves of 1.32 tcf. The accumulation extends over 37 km2 and was unitized in 1989 with 75% of the field in block 6507/7 and 25% in block 6507/8.
Block 6507/7 was acquired by Conoco Norway and partners in 1984 in the eighth round of licensing offered by the Norwegian government. The acquisition was based on extensive in-house exploration regional studies, which identified a Jurassic rift element in Haltenbanken and the potential for the generation of liquid hydrocarbons.
The Heidrun structure is a large southwest-plunging horst block on the southwest flank of the Nordland ridge and was formed during the Cimmerian extensional tectonic phase in the Late Jurassic-Early Cretaceous. The Heidrun reservoirs are severely truncated at the northern edge of the structure and are sealed by Cretaceous shales.
The Heidrun Jurassic reservoir rocks, the Fangst Group, and the Tilje and Åre formations were deposited on the southeastern flank of the developing northeast Atlantic rift domain. Despite an over-all transgressive regime, the interval was characterized by a high, coarse, clastic influx from the elevated rift shoulders. The shallow depth of burial (less than 2500 m) has limited compaction effects. Reservoir quality in the clean Fangst sands is somewhat enhanced by dissolution, and the sands exhibit maximum permeabilities higher than 10 darcys and porosities in excess of 30%.
The primary source of the petroleum is the Upper Jurassic Spekk Formation, which is mature in the downdip areas 5 to 15 km southwest and west of Heidrun. The Are Formation coaly beds are also potential source rocks in the same downdip areas. Therefore, fairly long petroleum migration paths are inferred.
A 5000 km high resolution 3D seismic survey was acquired in 1986. State-of-the-art processing has produced a data set far superior to previous 2D data. The discovery well and appraisal wells were extensively cored through the Middle and Lower Jurassic. The 3D data set forms the basis for detailed geologic and reservoir models. These models were used in the application for development consent, submitted to the Norwegian government in late 1989 by the Heidrun unit owners.
The current development plan proposes that exploitation of the Heidrun field will be carried out using an innovative concrete tension leg platform, offshore oil loading, and transportation of the gas to shore by pipeline for onshore use.