Geological characterisation of shallow marine-to-deltaic sandstone reservoir targets: Krossfjord and Fensfjord formations, Troll Field, Norwegian North Sea

Abstract

The sedimentological character and stratigraphic architecture of shallow-marine reservoirs are strongly controlled by the interplay of physical processes that occur at and near the shoreline (e.g. wave- vs. tide- vs. fluvial-dominated). These aspects can be further complicated by the interplay of tectonics in rift basins through fault block rotation, uplift, and subsidence. This thesis presents a subsurface case study from the Middle-to-Upper Jurassic “syn-rift” Krossfjord and Fensfjord formations, Horda Platform, offshore western Norway. The distribution, geometry, and connectivity of these sandbodies are poorly understood, as they have not been the focus of previous work. However, the formations form a significant oil and gas reservoir in the Troll and Brage fields, and a prospective reservoir in the Gjøa Field.

Analysis of core and wireline-log data from the Krossfjord and Fensfjord formations identified wave- and tide-dominated deltaic, shoreline and shelf depositional environments. The integration of biostratigraphic data enabled subdivision of the formations into ‘series’ bound by maximum flooding surfaces. The integration of 3D seismic data defined the gross stratigraphic architecture, specifically the stacking patterns of clinoform sets, and enabled further subdivision of the ‘series’. Seismic geomorphological analysis of clinoforms, calibrated using forward seismic models of outcrop analogues, aided interpretation of the shoreline process regime (e.g. relative influence of waves, tides and river-mouth processes) in the context of shoreline trajectories. Palaeogeographic reconstructions illustrate that a subaqueous delta was located over the Troll Field fronting a wave- and current-driven southerly-directed spit during Middle to Late Jurassic times. In conclusion, a robust understanding of the Krossfjord and Fensfjord formations is established in order to drive future exploration in these, and coeval, reservoirs. In addition, the novel forward seismic modelling methodology described herein has wide applications and the results are directly applicable to many other shallow-marine reservoir sandstones, for which the outcrops studied are considered to be sedimentological analogues.