Upper Triassic-Lower Jurassic reservoir rocks in the Tampen Spur area, Norwegian North Sea

Abstract

Alluvial sandstones of Late Triassic to Early Jurassic age form prominent reservoir rocks in the Statfjord, Snorre, Gullfaks, Gullfaks Sør and Visund fields of the Tampen Spur area. The Rhaetian-Sinemurian Statfjord Formation is a major oil reservoir in all fields. The Lunde Formation is the principal oil reservoir in the Snorre Field and is an important gas reservoir in the Gullfaks Sør and Visund fields. The underlying Lomvi Formation and the sand-rich parts of the Teist Formation are the reservoirs of potential play types. In all fields the hydrocarbons are trapped within crest segments of rotated fault blocks that are eroded to various depths and capped by Lower Jurassic to Cretaceous mudstones and shales. Faults and fractures cut and complicate the reservoirs, especially in the east flank of the Statfjord Field and in the Gullfaks and Gullfaks Sør fields. In the Tampen Spur area, the Triassic to Early Jurassic rocks are up to 2500 m thick. The succession was deposited within a wide alluvial plain during the thermal subsidence phase subsequent to Permian-Early Triassic rifting. Channel and channel belt sandstone bodies interchange with floodplain mudstones and nonchannelized sandstones. Five main types of stream systems are suggested to explain the variation in facies association, geometry and dimension of sandstone bodies. The depositional architecture has been controlled by the interaction of base-level changes and variation in rate of sediment input, besides autocyclic processes. The Triassic-Lower Jurassic reservoir rocks are heterogeneous from a field-wide scale to a microscopic scale. Permeability and porosity reflect sedimentary facies and textural variations modified by diagenesis. A major challenge in the reservoir characterization of these rocks is to model the 3D geometry of the fluvial sandstone bodies and make reliable well-to-well correlations. Stochastic as well as descriptive modelling have been used in the reservoir description. Data obtained from outcrop studies of analogue fluvial formations and modern fluvial systems have been of great value in the modelling procedures and also in the application of sequence stratigraphical and allostratigraphical approaches in correlation within the alluvial successions. In the Statfjord Field, the reservoirs in the Statfjord Formation have been in production since 1979. The production results from this unit verify a predominant sheet geometry of the major sandstone bodies with very good reservoir qualities. Persistent shales of floodplain, lacustrine or lagoonal origin form effective pressure barriers. Experiences from production within the Statfjord Formation on the Snorre Field since August 1992 are generally in accordance with those obtained from the Statfjord Field. The upper member of the Lunde Formation on the Snorre Field has been in production since April 1993 and the results so far confirm the reservoir model. For all of the Triassic-Lower Jurassic reservoirs of continental sandstones in the Tampen Spur area, methods for IOR presuppose detailed reservoir description for 3D modelling of sandstone body geometry and heterogeneity. Integration of high-resolution seismics, sedimentological data and production results within the framework of modern conceptual models on alluvial stratigraphy is recommended. In further exploration on Triassic-Lower Jurassic fluvial play types in the northernmost North Sea area refined models for basin development and facies distribution for this time interval should be evolved.