Jurassic reservoirs; field examples from the Oseberg and Troll fields: Horda Platform area

Abstract

Abstract The name Horda Platform as used in this paper covers an area between 60 and 61°N, and is limited to the west by the North Viking Graben. Giant oil and gas accumulations exist in the Oseberg and Troll fields. These giants contain about 69% of the oil and 97% of the gas resources discovered on the Horda Platform. Large accumulations also occur in the Veslefrikk, Huldra, Brage, Beta, and the southern Oseberg areas. The reservoir rocks are found in eastward-rotated fault blocks, and include sandstones of both Lower, Middle and Upper Jurassic age. The Oseberg Field is located 140 km northwest of Bergen. Hydrocarbons are trapped in three major east-northeasterly tilted fault blocks: Alpha, Alpha North, and Gamma. Gas caps exist in all three structures. The vertical gas column in Alpha is 380 m and the underlying oil column is 210 m. Production started December 1988. The reservoir pressures are maintained mainly by injection of associated gas and imported gas from Troll and Gamma North. Measures taken to control gas front movements include drilling of observation wells, production logging, seismic monitoring and pressure analysis. The reservoir rocks belong to the Middle Jurassic Brent Group: Oseberg, Rannoch, Etive, Ness and Tarbert formations. The Oseberg Formation is the main reservoir. It represents a local fan delta progradation from east to west. The apparently massive sands of the formation have a complex internal buildup reflected in the vertical permeability distributions. Permeabilities range from 1 to 4 darcies. Porosity is around 24%. The Rannoch-Etive, Ness and Tarbert formations represent the respective delta front-shoreline, delta plain and abandonment phase of the Brent delta that migrated northwards across the Oseberg Field. Excellent reservoirs occur in the Etive-lowermost Ness and Tarbert formations. A major challenge at this stage is to map deterministically more isolated fluvial channel sands of the Ness Formation. These sandstones account for about 25% of the field's oil in place, but only 10% of the reserves. Studies have shown that high-resolution seismic data combined with inversion techniques, attribute processing and geological evaluation might be feasible for deterministic mapping of channel sands. The Troll Field is located 65 km northwest of Bergen. The hydrocarbons, being mainly gas, are trapped in three main easterly- and southeasterly-tilted fault blocks. The maximum gas column is about 250 m. The oil column in the western fault block is 22–26 m, in the central block 12–14 m and in the eastern block 0–4 m. The reservoir rocks belong to the late Jurassic Viking Group: Fensfjord, Heather B and Sognefjord formations. They consist entirely of sandstones occurring in either coarsening-upwards or blocky units. The permeabilities are about 1 to 15 darcies and porosities about 20–35%. A biostratigraphical framework was established for the area at an early stage. These data were later incorporated into a sequence stratigraphic model. Sedimentological, ichnological and dipmeter studies have been integrated to establish a depositional model for the area and thereby to determine the reservoir geometry. The sediments represent delta and coastal progradations over a shallow marginal marine shelf. Frequent calcite cemented horizons occur in all parts of the reservoir. These are mainly of early diagenetic origin connected to abandonment and erosion surfaces. Some of them are widely correlatable.