ABSTRACT: Sandstone Permeability Viewed as a Function of Intergranular Macroporosity: Garn Formation, Haltenbanken, Mid-Norwegian Continental Shelf

S. N. Ehrenberg

The Middle Jurassic Garn Formation is one of the principal hydrocarbon reservoir units of the Haltenbanken area. This coastal regressive fan delta sandstone layer averages around 60 m in thickness and consists mainly of well-sorted, medium-grained, subarkosic arenite.

The Garn Formation follows an approximately linear porosity-depth trend similar to that for sandstones from the Viking Graben province of the North Sea. Permeability, however, does not follow any simple trend with depth. Average permeabilities remain >1 d down to at least 2.7 km below the sea floor, but between 3.3-4.0 km depth there is major and seemingly erratic reduction in permeability.

Modal analysis of 270 horizontal plug samples from 16 wells shows that permeability is a function of percent intergranular macroporosity, which is interpreted to be a measure of effective porosity. The distinctly nonlinear correlation observed between permeability and percent intergranular macroporosity can be explained using the Kozeny-Carman equation, which describes the dependence of permeability on porosity and surface area. This equation predicts behavior similar to that observed: at higher porosity values permeability is less sensitive to porosity variation, but as the effective porosity falls below around 10%, permeability drops drastically.

The abundance of intergranular macroporosity in the Garn Formation is controlled mainly by two depth-dependent (temperature-dependent) diagenetic processes. The first is quartz cementation, which steadily reduces total porosity with increasing depth. The other is reaction of K-feldspar and kaolinite to illite, which occurs as the sandstone reaches a critical thermal threshold at 3.7 km below the sea floor. Illitization and associated feldspar dissolution result in major reorganization of the pore system and decrease in the ratio of effective to ineffective porosity.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990