Experimental Assessment of Porosity Reduction Through Ductile Grain Deformation in Sandstones

D. Joe Benson

The effect of ductile grain deformation on porosity in sandstones has been recognized for some time, but studies to date have been either qualitative or theoretical in nature. To test the quantitative significance of ductile grain deformation on porosity, a series of sands containing variable percentages (5-50%) of ductile grains mixed with equal-sized quartz were manufactured. These mixtures were compressed in a biaxial compression system at pressures from 4,000 to 20,000 psi simulating burial to depths of up to 20,000 ft.

Results indicate a strong negative correlation between porosity and depth of burial in sands with similar ductile grain content. At pressures less than 4,000 psi porosity, reduction was largely the result of packing readjustment and grain rotation. At pressures greater than 4,000 psi, however, there was progressive deformation of ductile components and a concurrent decrease in porosity. Porosity loss ranged from 1.2 to 2.1% per 1,000 psi.

A strong negative correlation also exists between porosity and ductile grain content. Porosity values of sands compressed to 10,000 psi ranged from 24% in samples with 20% ductile grains to 9% in samples with 50% ductile grains. A roughly linear trend is apparent, with porosity loss ranging from 0.6 to 1.9% for each additional 5% of ductile grains.

Experiments with various grain sizes and additional ductile components indicate these variables also influence porosity reduction, although not as greatly as pressure or ductile grain content.

These studies indicate ductile grain content can be an important control on porosity reduction in sandstones, particularly those which have been deeply buried. They also suggest there is a definable relationship between ductile grain content, depth of burial, and ultimate porosity. This relationship may be useful in predicting reservoir quality in deeply buried sandstones.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.