Compensatory Stacking Patterns within Turbidite - Channel Lobe Systems and the Impact to Resource Distribution, Reservoir Architecture and Connectivity: Example from the Polecat Discovery, UKCS

Dutton, David M.; Oudit, Karize; Theophilos, Andrew; Sweetman, Stuart

The Polecat discovery, 40km east of the Buzzard Field, UK, was made in 2005 by Nexen Petroleum U.K. Ltd when the 20/04a-8 well found 40ft of net sour pay within turbidite reservoirs of Kimmeridigian to Volgian age. An extensive 6 well appraisal campaign (including side-tracks) was subsequently undertaken to prove up a commercial accumulation and understand the upside potential of the discovery. The well results confirmed: (1) that the seismic data cannot directly inform the geological model given the sands are typically of sub-seismic resolution; (2) all reservoir sands are not in communication; (3) Polecat is a stratigraphic trap.

Further insight into the reservoir architectures has been gained from the integration of extensive chemostratigraphic correlations, biostratigraphic analysis, core analysis and RCI pressure information. In particular, the chemostratigraphic analysis has facilitated a correlation framework that has been interpreted to demonstrate the discontinuous nature of the different reservoir units across the sub-basin. A lobe interpretation has been made on the basis that the chemostratigraphic data suggests an aspect ratio of at least 600:1 for the main reservoir unit at Polecat. Examination of this data allows an interpretation of the depositional model to be postulated whereby a number of discrete lobe elements have entered a weakly confined sub-basin in a sequential manner. It possible to infer the stacking geometries of these lobes through geologic time as the accommodation space (seafloor topography) changes and subsequent turbidite flows react to pre-existing topography and develop a compensatory and offsetting pattern of deposition. Similar styles of deposition are readily observed from outcrop studies of the Carboniferous Ross Formation, Ireland and can be used to provide a reservoir scale analogue.

This paper demonstrates how reservoir complexity can be unravelled through the use of a fully integrated geological approach and how reservoir stacking patterns have implications for reservoir connectivity with the pay at Polecat being restricted to the uppermost turbidite element.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013