LAUSR

Abstract Bay-head deltas are important paralic sedimentary systems at the interplay between fluvial and marine processes. The resulting sand accumulation can be significant for the origin of hydrocarbon reservoirs with extraordinary properties and contribute with additional volumes in near-field situations or as secondary reservoirs. Nonetheless, this type of depositional system is barely captured in existing field descriptions and has received little attention from a sedimentological point of view, although our understanding of related sedimentological processes has increased recently by the investigation of comparable modern systems. The Middle Jurassic (Bathonian-Callovian) Hugin Formation in Block 15/3 (Gudrun Field area) of the Norwegian North Sea is up to 245 m thick and consists mainly of laminated mudstone deposited in a restricted (lagoonal) embayment. The succession contains numerous sand bodies with various thickness and restricted lateral continuity, each representing a shallowing-upward trend and facies types characteristic of prograding bay-head deltas. A stratigraphic analysis of seven wells across a ca. 10 km long NW-SE transect shows the diachronous nature of the Hugin Formation, with early deposition close to the axis of the South Viking Graben (West) and gradually later deposition towards the graben shoulder (East). The bay-head deltas were fed by river systems (the laterally equivalent Sleipner Formation) from the Utsira High in the East and responded with back-stepping as a consequence of the overall transgression from the North (the overlying Heather Formation). Enhanced understanding of the depositional system of the Hugin Formation and its comparison with modern and outcrop analogues has increased the predictability of the heterogeneity and compartmentalisation in the Middle Jurassic reservoir of the Gudrun Field area.