Sandstone reservoir rocks

Sands and sandstones are formed as a result of processes of sedimentation in which mineral fragments of assorted sizes and shapes are gradually fitted together under water, later to be compacted by the weight of super imposed strata. Tabular faces of grains assume similar orientation, developing parallel bedding planes. Small grains are deposited in the interstices between large grains and thus, a condition approaching minimum porosity for unconsolidated sand is attained.

Limestone Reservoir Rocks. Pores in limestones are much less uniform in shape and size than those of sands and sandstones. The rock openings are in many cases formed by solution and weathering and are extremely irregular in shape and distribution. Formed at prehistoric times, they are now found below younger formations resting unconfortably upon them.

Porosity of Reservoir Rocks. Porosity may be defined as the per cent of voids or pore space per unit volume. For example, in one hundred cubic feet of sandstone having an average porosity of 16 per cent, there would be sixteen cubic feet of pore space. When the term porosity is used, reference is usually made to the effective and not the absolute porosity, hence we do not ordinarily consider the very fine or unconnected openings, fractures or voids.

High porosity in a granular rock results from uniformity in size and shape of grains and is influenced also by their arrangement. It can be demonstrated mathematically that true spheres of uniform size, packed as closely as possible, will have a porosity of 25 per cent, irrespective of the diameter of the spheres. In nature, however, sand grains are not spherical. They occur in a variety of shapes and in many graduated sizes. As a result, during the process of deposition and sedimentation the small grains and fragments may shift in between the larger and posssibly more angular grains, thus materially reducing the effective porosity. In coarse-grained oil sands high initial production and rapid recovery are common.

Fine-grained oil sands having a porosity of less than 7 or 8 per cent rarely, if ever, produce oil in paying quantities. This is probably due to the fact that the interstitial openings are so small that they approach subcapillary size, and the pore friction or flow resistance established will not permit the oil and gas to move rapidly enough to be produced in paying quantities. Also, in coarse-grained sands the grains may have such angularity and be deposited in such a manner as to cause the connection between large pore spaces to be so small that they become subcapillary in size, and thus greatly retard fluid movement.