An integrated study of the garn and melke formations (middle to upper jurassic) of the smorbukk area, halten terrace, mid-norway

Abstract

This contribution presents a re-evaluation of the shallow marine reservoir sandstones of the Garn Formation and their relationship to heterolithic shelf facies of the Not and Melke formations in the Smorbukk area, Halten Terrace, mid-Norway. Most of the existing interpretations of the Garn Formation suggest development as a relatively homogeneous and simple sheet-like sandstone, with a lateral extent of 10s of km. Deposition has been previously interpreted to have occurred during a period of tectonic quiescence. This interpretation is largely based on the lithostratigraphic correlation of remarkably uniform wireline log profiles supported by regional biostratigraphic correlations. In contrast, an integrated study of seismic, seismic attribute, core and biostratigraphic data in the Smorbukk-Smorbukk South area indicate that the Garn Formation is strongly age and facies diachronous, often over short distances (5-10 km). Furthermore, it can be demonstrated that sediment dispersal and stratigraphic architecture were influenced by fault-created physiography. The lower part of the Garn Formation comprises up to 50 m of aggradationally stacked shallow marine sandstones which are in disconformable contact with underlying heterolithic shelf facies of the Not Formation. These lower sands onlap onto tectonically active structural highs (Smorbukk fault block and Smorbukk South field) and are of limited geographical extent, only being present in down-dip depocentres. The overlying upper Garn sandstones comprise a series of aggradational to progradational shelf-shoreface cycles, with individual cycles progressively backstepping and onlapping onto structural highs. Collectively, these cycles define a retrogradational stacking pattern. Seismic and well log correlation indicate that these upper sands are coeval with Melke Formation shelf/offshore mudstone facies in down-dip depocentres. There is a clear and hitherto largely overlooked relationship between fault-controlled basin physiography, facies distribution, sediment dispersal and stacking patterns. These new data have important implications for understanding palaeogeographic development and reservoir geometry of the Garn Formation.