Abstract: Porosity, Diagenesis, and Productive Capability of Sandstone Reservoirs

Edward D. Pittman

Four basic types of porosity occur in sandstones: intergranular, dissolution, microscopic, and fracture. The first three are related to rock fabric and can be considered end members of a ternary classification diagram. Fracture porosity may be associated with any other porosity type.

All sandstones initially have intergranular porosity, which commonly is associated with good permeability, large pore apertures, and prolific hydrocarbon production. Dissolution porosity, which may consist of isolated pores, results from leaching of feldspar, carbonate, sulfate, or other soluble material. Sandstones with dissolution porosity may have adequate porosity for a reservoir, but poor permeability unless pores are interconnected. Sandstones with significant clay minerals have abundant microporosity, high surface area, small pore apertures, low permeability, and high irreducible water saturation. Fracture porosity, which contributes no more than a few percent of voids to storage space, will enhance the deliverability of any reservoir. Open fractures, either natural or induced, are essential for adequate deliverability in reservoirs with predominantly dissolution porosity and/or microporosity.

Mineral diagenesis modifies pore geometry of sandstones. Porosity type and/or geometry change with diagenesis; for example, macropores become micropores, minerals dissolve to create voids, and pores are partly to completely occluded by precipitation of minerals.

Porosity type seldom is homogeneous in rocks. As a result, problems may occur in interpreting logs of sandstones containing significant micropores and interconnected macropores. Micropores may hold irreducible water, whereas macropores hold producible oil, gas, or water, depending on height above the oil or gas-water contact. Log calculations may indicate high water saturation and a nonproductive interval, although the reservoir may be capable of water-free hydrocarbon production because the water is not producible.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado