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1 Shell Expro, 1 Altens Farm Road, Nigg, Aberdeen, UK
2 Conoco (UK) Ltd, Park House, 116/129 Park Street, London W1Y 4NN, UK
3 Koninklijke/Shell Exploratie en Produktie Laboratorium, Postbus 60, 2280 AB Rijswjik ZH, Netherlands
Image analysis has been used to quantify the pore structure of dolomite-cemented sandstones in the Ribble Member of the Fulmar Field, UK North Sea. This technique was used because the data obtained from conventional petrography did not enable quantification of the controls on permeability. Three-quarters of the sandstones in the Ribble Member have high measured porosity and permeability (ø17–29%, Kh40–1800mD). In these poorly cemented sand-stones, intergranular porosity predominates and permeability reflects (depositional) sediment texture. One quarter of the sandstones contain > 20% dolomite. The dolomite (mean
Backscattered electron imaging and automated image analysis have enabled quantification of the differences in pore structure between the poorly cemented and cemented sandstones. Permeability is not only related to the amount of porosity present, but also to the amount of dolomite cementation. The degree of cementation influences average pore area and pore connectivity, which in turn controls permeability. Permeability is lower in the cemented sandstones because the dolomite rhombs have occluded the intergranular pores, creating restricted flow paths through small intercrystalline pores. As a result of these differences in pore structure, hydrocarbon production from the cemented sandstones will be slower than from the poorly cemented sandstones.
The permeability of the cemented sandstones cannot be predicted from porosity logs alone but also requires knowledge of the amount of dolomite cement present. In uncored wells this information should be obtainable using modern wireline log evaluation techniques. This study demonstrates the importance of combining a detailed understanding of the rock’s pore structure with a sound geological model in order to predict reservoir behaviour.
13C PDB +0.8
, mean 18O PDB –2.3) formed from sea water trapped and buried within the sediments and occurs as patches or nodules that should not form reservoir scale barriers to fluid flow. These cemented sandstones are less permeable (ø15–25%, Kh0.4–600mD). They contain grain-dissolution pores created by the dissolution of sponge spicules and intercrystalline pores between the dolomite rhombs.
