Extent and Continuity of Carbonate-Cemented Zones in Sandstone Reservoirs

P. C. Smalley, H.-H. Stolum, P. E. Overli, H. Johansen, N.-M. Hanken

Three Norwegian oil fields--Draugen, Brage, and Frigg--have been studied in an attempt to pinpoint possible flow barriers due to cementation. We made use of core description, isotope data (Sr, O, C, D/H), and cathodoluminescence of thin sections. Three different types of information were extracted from the interpretation of such data: relative age of cementation, thickness of cemented zone, and facies and depositional mechanisms. These were combined to a method for evaluating lateral extent and continuity (tightness) of the cemented zone.

According to the underlying geological model, cementation can take place in two phases until the oil window is reached. One is early and facies dependent. Calcium is biogenic or supplied from seawater. Cementation either occurs very near the surface at a redox boundary layer or is related to a wedge of meteoric water. The result tends to be a thin but extensive layer of cemented nodules.

The second phase is late, occurring generally at a temperature of 60°-70°C. Calcium is probably supplied from shales. If no early cement is present in the sand, late cement will nucleate at random to form concretions. Due to the resulting diffusion gradients in the pore water, such concretions are likely to stay isolated. If, however, early cement is present, late cement will tend to use the nodules of the early cement as nucleation sites. Where these are concentrated in a layer, the result is commonly an amalgamated cement zone which is extensive as well as tight.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.