Oxfordian-Kimmeridgian (Late Jurassic) Reservoir Sandstones in the Witch Ground Graben, U.K North Sea

HARKER, STUART D., Occidental Petroleum (Caledonia) Ltd., Aberdeen, U.K, KENNETH A. MANTEL, Narwhal, London, U.K, DAVID J. MORTON, Deminex U.K Oil and Gas Ltd., London, U.K, and LESLIE A. RILEY, Paleoservices, Watford, U.K

Oil-bearing Late Jurassic Oxfordian-Kimmeridgian sandstones of the Sgiath and Piper formations are of major economic importance in the Witch Ground Graben. They form the reservoirs in Scott, which in 1993 will be the largest producing North Sea oil field to come on stream for more than a decade.

Together with Scott, the Piper, Saltire, Tartan, Highlander, Petronella, Rob Roy, and Ivanhoe fields contained almost 2 Bbbl of recoverable reserves in these formations. The Sgiath and Piper represent two phases of Late Jurassic transgression and regression, initially represented by paralic deposited sand culminating in a wave-dominated delta sequence. The history of the Sgiath and Piper formations is reviewed and lithostratigraphic and biostratigraphic correlations presented to illustrate the distribution of the reservoir sandstones.

A three-fold subdivision of the widespread Late Oxfordian Sgiath Formation is formally proposed with well 15/21a-15 in the Scott field as the designated reference well. The basal Skene Member consists of thinly interbedded paralic carbonaceous shales, coals, and sandstones and is overlain by marine shales of the Saltire Member. The uppermost Scott Member consists of shallow marine sandstones, a precursor to the more areally restricted, wave-dominated delta sequences of the Piper Formation. The Sgiath/Piper formational contact is represented by a basinwide transgressive shale ("I Shale") that generally acts as an effective barrier to fluid communication between the Sgiath and Piper reservoir sandstones. Miscorrelation between Sgiath and Piper reservoirs can result in erroneous explorat on and production models that markedly influence development economics and planning.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)