Abstract

The Thelma field is the most southerly of three fields located in Block 16/17 of the U.K. sector of the South Viking Graben and is operated by CNR International (U.K.) Ltd. The Thelma field comprises two submarine fans, the northern Thelma fan and the southern Southeast Thelma (SET) fan, revealed by subsurface mapping. The two fans consist of both Upper Jurassic Brae (upper Oxfordian–lower Volgian) and sand-shale (middle Volgian) members of the Kimmeridge Clay Formation. The two fan depocenters are separated by a low net-to-gross central region high, which was overtopped or diverted around by turbidity currents that formed the younger sand-shale member. The Brae member contains conglomerates, pebbly sandstones, sandstones, and thin-bedded turbidites (TBTs) of deep-water association. Seismic data are generally poor in this area, and conceptual and reservoir geological models are primarily based on oil field core, wireline logs, and dynamic reservoir data.

Three conceptual models were created to propose geological scenarios for field development: Concept A has the SET fan fed independently from the southwest, Concept B has the SET fan fed as an extension of the Thelma fan from the north, and Concept C has an SET fan fed by bypass across the central region high. Each concept has different implications for fan connectivity. Using detailed core descriptions from the Thelma field, state-of-the-art deep-water facies analysis, a new facies scheme, and known deep-water facies associations on the modern seafloor and outcrop analogs, we test the conceptual models to validate the geology.

The large-scale organization of the architectural elements of the Brae and sand-shale members is not well understood. By reviewing coarse-grained deep-water processes and the rock fabrics that they produce as a framework for rock property distribution in coarse-grained deep-water systems, we aim to investigate their architectural style. We recognize a series of facies associations from detailed facies descriptions of the cored intervals and, by distilling these sedimentological trends, ascribe the associations to uncored intervals and use these as a basis for correlation and reservoir modeling in the Thelma–SET area. The model is built from cored sedimentary facies, and facies associations recognizable on core-calibrated well logs, and then matched with dynamic well data, to test conceptual models.

By combining the conceptual geological models with dynamic well testing of reservoir behavior, we have a more robust understanding of the two fields. Thelma is a higher energy system, with strong aquifer support facilitated by channelized reservoir architecture and poor lateral shale continuity, whereas SET is a lower energy system with poor aquifer support because of persistent lateral continuity of shale caps on sandstone lobes, and with no deep channeling recognized.