The Adventdalen Group
Middle Jurassic to Lower Cretaceous (Bathonian to Cenomanian)
The Adventdalen Group is subdivided into the Fuglen, Hekkingen, Knurr, Kolje and Kolmule formations, with its type area in the northern part of block 7120/12 in the Hammerfest Basin and in 7119/12 in the eastern part of the Ringvassøy-Loppa Fault Complex. The thickness varies from more than 900m in the Bjørnøyrenna Fault Complex (7219/8-1S) to 300m north of the Troms-Finnmark Fault Complex. Nevertheless, the thickness decreases to approximately 60m or less on structural highs in the centre of the Hammerfest Basin, reflecting the effect of Upper Jurassic tectonic movements. The group is dominated by dark marine mudstones, locally including deltaic and shelf sandstones as well as carbonate.
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The Hekkingen Formation is an important hydrocarbon source rock. Both the Fuglen and Hekkingen formations constitute good cap rocks. The Hekkingen Fm (Upper Oxfordian–Tithonian) has been drilled in the Hammerfest Basin, the eastern part of the Bjørnøya Basin (Fingerdjupet Sub-basin) and the Bjarmeland Platform. The lower boundary is defined by the transition from carbonate cemented and pyritic mudstone to poorly consolidated shale in the Fuglen Formation. The upper boundary in the reference well (7120/12-1) is defined towards the thin sandy limestone of the Knurr Formation. The thickness in the type well (7120/12-1) is 359m, and in the reference well (7119/12-1) the thickness is 113m. Within the Hammerfest Basin the thickest sequence is found in the type well, thinning northwards to less than 100m. Very high thicknesses are interpreted along the eastern margins of the Harstad Basin and Bjørnøya Basin, as seen in well 7219/8-1S in the southern part of the Bjørnøyrenna Fault Complex (856m thickness). Thin sequences are found on the Bjarmeland Platform. The dominant lithology in the formation is shale and mudstone with occasional thin interbeds of limestone, dolomite, siltstone and sandstone. The amount of sandstone increases towards the basin margins. The formation was deposited in a deep shelf with partly anoxic conditions.
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Cretaceous
Berriasian to Cenomanian
The Cretaceous sucsession is subdivided into three formations: The Knurr, Kolje, and Kolmule Formations. The dominant lithology of the Knurr, Kolje and Kolmule formations is dark to grey-brown shale with thin interbeds of siltstone, limestone, dolomite and local sandstone. The thickness is in the order of 1000-1400m in the type area (blocks 7119/12 and 7120/12). Thicknesses within the Hammerfest Basin are closely related to Upper Jurassic structural development. The formations are thickest along basin margins and thin towards the central part of the Hammerfest Basin. In our study we have focused on the Knurr Formation, as this may represent thief sands in relation to the main Mesozoic aquifers. The Knurr Formation (Berriasian/Valanginian to lower Barremian) is distributed over the southwestern part of the Barents shelf, mainly in the Hammerfest Basin, the Ringvassøy-Loppa Fault Complex and the Bjørnøyrenna Fault Complex. A thin Knurr section is also found locally on the Bjarmeland Platform. The thickness of the Knurr Formation is 56m in the type well (7119/12-1) and 285m in the reference well (7120/12-1). The thickest drilled section so far is 978m (well 7219/8-1S) in the Bjørnøyrenna Fault Complex east of Veslemøy High. The base is defined by a thin sandy limestone overlying the Hekkingen Formation, and the upper boundary is defined with a presence of dark brown to grey shale in the Kolje Formation.
Although the formation shows similar lithology in most wells, the sand content is higher close to the Troms-Finnmark Fault Complex and in the Ringvassøy-Loppa Fault Complex. The sandstones are located in the lower part of the formation, pinching out laterally into the Hammerfest Basin and Bjørnøya Basin. The formation was deposited in an open and generally distal marine environment with local restricted bottom conditions.
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