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AAPG Bulletin

Abstract

AAPG Bulletin, V. 95, No. 11 (November 2011), P. 19371958.

Copyright copy2011. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/03071110122

Diagenetic controls on reservoir quality in Middle to Upper Jurassic sandstones in the South Viking Graben, North Sea

Tom Erik Maast,1 Jens Jahren,2 Knut Bjorlykke3

1Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway; [email protected]
2Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway; [email protected]
3Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway; [email protected]

ABSTRACT

The deeply buried synrift play of the South Viking Graben is characterized by highly variable reservoir quality. An integrated approach incorporating petrophysics, petrography, and one-dimensional basin modeling methods was applied to investigate these variations. Analysis shows that average porosities below 4000 m (gt13,123 ft) (vertical depth below sea floor) range from approximately 5% to as much as about 25% in comparable quartz arenitic sandstones. From porosity-depth trends, three porosity categories can be recognized (normal-, low-, and high-porosity sandstones). Normal-porosity sandstones fall along the regional average porosity-depth trend. Low-porosity sandstones have been subject to extensive quartz cementation as a consequence of a higher degree of thermal maturity and plot below the regional porosity-depth trend. High-porosity sandstones plot above the regional porosity-depth trend. Here, quartz cementation has been inhibited by grain-coating microquartz, and thus porosity has been preserved. Hydrocarbon emplacement has previously been thought to have inhibited quartz cementation in the study area, but this study concludes that the reservoirs are mainly water-wet, allowing for continued quartz cementation despite the presence of hydrocarbon pore fluids. Predicting the distribution of microquartz-coated sandstones and the degree of thermal maturity is therefore fundamental for successful exploration in the deeply buried parts of the synrift play. This study presents a regional and stratigraphic framework for such predictions that may be incorporated into play models in the area.

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