Regressive and transgressive cycles in a rift-basin: Depositional model and sedimentary partitioning of the Middle Jurassic Hugin Formation, Southern Viking Graben, North Sea

Abstract

The Jurassic Hugin Formation consists of shallow-marine sandstones that belong to a significant hydrocarbon reservoir in the Sleipner area in the Norwegian North Sea. The formation encompasses coarsening-upward units of mouth bar and shoreface facies, interpreted to record delta outbuilding during regression; and fining-upward units with tidal channel, dune, and tidal flat facies interpreted as part of an estuary environment during transgression. The correlations reveal that the studied part of the Hugin Formation consists of 8 sequences, each with a transgressive and a regressive unit, representing the transgressive systems tract and the highstand systems tract respectively. The sequences are stacked retrogradationally landward as a result of rapid tectonic subsidence and rifting of the Viking Graben. Rifting led to the development of an elongate graben where tidal currents were amplified, wave-action damped and longshore drift (as sediments supply) reduced or absent. Lowstand and forced regressive systems tracts are not identified, and their absence is interpreted to reflect suppression of relative sea level falls in a rapidly subsiding basin where the basin subsidence rate outpaced any potential fall in eustatic sea level. Through facies interpretation and sequence-stratigraphic correlations between wells, these regressive and transgressive units are shown to exhibit characteristic thickness trends in the form of sigmoidal-shaped wedges, stacked in an offset manner in a landward to basinward orientation. These thickness trends illustrate sediment partitioning within the sequences and are explained by the relationship between accommodation versus sediment supply in terms of mass-balance. During regression, the focus of sedimentation was pushed basinward, and during transgression it was pushed landward as sediments were trapped there. The mapping of these sequence-stratigraphic units serves as input to reservoir models and to help increase recovery and identify new exploration targets.