The Sedimentological Development of the Fangst Group, Halten Terrace, Mid-Norway
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Cores from well 6506/12-1 of the Smørbukk Field (Halten Terrace, Mid-Norway) show a nearly complete 175 meter thick section comprising the Middle Jurassic Fangst Group and the upper part of the underlying Ror Formation of the Båt Group. The Fangst Group comprises three formations: The Ile Formation, the Not Formation and the Garn Formation. The Ile-and Garn formations are sand- dominated and constitute some of the most important reservoir units in the prolific hydrocarbon province of the Norwegian Sea. The aim of the present study was to thoroughly describe the Fangst Group of well 6506/12-1 and from this interpret its sedimentological development. Correlation with well 6506/11-6 and 6506/11-8 was carried out based on wireline logs and core photos, and this correlation led to increased understanding of sand body development in the study area and formed the basis for paleogeographic reconstruction. Rifting between Norway and Greenland initiated in the Late Carboniferous and occurred in pulses until the two landmasses finally separated in Early Cenozoic times. The Mid- Jurassic has been described as a period of nearly total tectonic quiescence on the Halten Terrace, but some faulting may have occurred which influenced sedimentation when the Fangst Group was formed. A total number of 18 facies or subfacies was recognized in well 6506/12-1, and have been grouped into five facies associations (FAs). FA1 represents a prograding low-energy offshore to prodelta succession (the Ror Formation) conformably overlain by prograding, tidally reworked mouth bar deposits. These mouth bars are interpreted as the delta front of the progradational/aggradational tide-dominated delta which was responsible for deposition of the Ile Formation. FA2 represents the lower delta plain, consisting of tidally-dominated facies comprising tidal flats and channel sandstones, with minor fluvial dominance toward the top of the interval. FA3 records the backstepping of the delta plain towards the end of Ile Formation deposition, and the delta plain was eventually flooded. This transgression left a wave ravinement surface and a significant transgressive lag. The overlying FA4 defines the Not Formation, with a maximum flooding surface defined within offshore black shales and marking the change from transgression to regression. Regression in the Not Formation is characterized by normal shoreface progradation. FA5 defines the Garn Formation, and unconformably overlies FA4. A significant hiatus has been interpreted at this sequence boundary in well 6506/12-1. The medium- to coarse-grained sands are interpreted as a progradational to aggradational stack of amalgamated braided river deposits, deposited in a braid delta. A transgression in mid-Garn flooded the braid delta and created shallow embayments on the former delta top, before renewed progradation brought the braided rivers closer to the location of well 6506/12-1 again. Correlation with wells 6506/11-8 and 6506/11-6 west of 6506/12-1 confirms the continuance of sand bodies of the Fangst Group. However, there are differences between the wells which aid the paleogeographic reconstruction. A roughly N-S oriented shoreline and seaway is interpreted for the Fangst Group. The Ile Formation is interpreted as a tide-dominated delta which prograded from...
UtgiverThe University of Bergen
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