ABSTRACT: Sedimentology, Depositional Environment, and Diagenesis--Controls on Reservoir Quality in the Gull Lake South Voluntary Unit No. 1, Southwest Saskatchewan

WILKINSON, KENT, University of Regina, Regina, Saskatchewan, Canada

The Gull Lake South Voluntary Unit No. 1 is located within the Middle Jurassic Upper Shaunavon oil field trend of the Swift Current area, Saskatchewan. The Shaunavon Formation in this area consists of upper and lower members and is a part of a mixed siliciclastic/ carbonate/evaporite sequence that is typical of the Middle Jurassic in southern Saskatchewan.

In the study area, the Upper Shaunavon consists of five subunits recognizable by their assortment of lithologies and ichnofaunas. They include interbedded sand and shales, fine-grained, variably bioturbated, quartz sandstone, and carbonates that range from lime mudstone to coquinod molluscan grainstones. The biota in these rocks include gastropods, pelecypods, echinoderms, fish scales, and tubular algae as well as the ichnofossils Chrondrites, Teichichnus, and Planolites. The vertical sequence of ichnofossils and lithologies suggest multiple fluctuations in sea level on a shallow shelf setting.

Distribution of porosity and permeability appears to be controlled by facies with a diagenetic overprint. The main factor influencing porosity is a late-stage ferroan calcite cement as identified by potassium ferricyande-alizarin red-S staining. This cement occludes interparticle porosity in allochem-rich layers but is poorly distributed in quartz-rich intervals, these being the major reservoir rock. Significant moldic porosity has developed in some allochemic-rich layers, which forms a secondary reservoir rock.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)