From spit system to tide-dominated delta: integrated reservoir model of the Upper Jurassic Sognefjord Formation on the Troll West Field

Abstract

The Troll Field in the Norwegian North Sea is one of the largest offshore gas fields in the world. Its western part contains a thin but exploitable oil leg (11–26 m) below the thick gas column, with the majority of the oil located in the late Jurassic Sognefjord Formation. The reservoir geology of the Troll Field was discussed in a few papers prior to production start-up in 1995, but a comprehensive account of the geological model of the Late Callovian – Late Oxfordian Sognefjord Formation has not as yet been published. The present paper reviews the depositional setting and architecture of this reservoir unit based on integration of results from disciplines such as sedimentology, seismic data analysis and stratigraphy. The Troll West reservoir formed on the edge of the Horda Platform during the Late Jurassic rift event; it consists of numerous stacked and generally offlapping sandstone units with intervening finer-grained deposits. The reservoir succession contains three composite sequences, of which the lower two belong to the Sognefjord Formation and the upper one is part of the Draupne Formation. Within this framework, five basic sequences and fifteen reservoir zones occur at the systems tract scale. The lower to middle parts of the studied succession reflect southwestwards growth and decay of a coastal spit system flanked to the east by a tidal backbasin. Brackish water facies have been identified in the eastern parts of the field through the use of detailed palynology. The extensive well database (including 15 cored production wells) and the high quality of the seismic data in the Troll area provide a unique opportunity to gain insight into the evolution of such a major spit-backbasin to tide-dominated deltaic system. A typical spit/strandplain progradation episode results in a clinoform succession comprising bioturbated sands of offshore transition origin overlain by lower shoreface sandstones which pass upward into clean, generally coarser-grained sands of upper shoreface and foreshore origin. The bases of such regressive sandbodies are often sharp, due to rapid facies translations during forced regression, but they may also be characterized by up to 40 m thick coarsening-upwards successions containing alternating siltstones and sandy event-beds generated by storm winnowing of the spit platform. Eastwards, these shallow-marine sandstone units finger into heterogeneous coastal plain deposits. On seismic data, this lateral transition is portrayed by the change from clinoform units in the west to undulatory seismic patterns in the east, as seen on maps based on classification of trace shape. A change in coastal morphology from wave- to tide-dominated took place in the Troll West area in the Late Oxfordian, perhaps related to an increase in tidal range. In the tidal facies tract, heterolithic inshore deposits formed in tidal channels and flats are replaced, in a seaward direction, by more sand-dominated high-energy tidal deposits such as mouth-bars and sand-ridge complexes. Further seawards, muddy shelf or prodelta deposits similar to those found in the wave-dominated facies tract are seen. In the Kimmeridgian, relative sea-level rise and eastward tilting of the Horda Platform caused reworking of the upper part of the Sognefjord Formation and localized hanging-wall shoreline sands formed as the uppermost reservoir intervals on Troll West.