Four basic types of porosity occur in sandstones: intergranular, dissolution, micro and fracture. The first three types are related to rock texture 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, if not destroyed, often is associated with good permeability, large pore apertures, and prolific hydrocarbon production. Dissolution porosity results from leaching of carbonate, felaspar, sulfate, or other soluble material. Sandstone reservoirs with dissolution porosity range from excellent to poor, depending on amount of porosity and interconnection of pores. Isolated dissolution pores result in low permeability. Sandstones with significant amounts of clay minerals have abundant microporosity, high surface area, small pore apertures, low permeability, high irreducible water saturation, and an increased sensitivity to fresh water. Fracture porosity, which contributes no more than a few percent voids to storage space, will enhance the deliverability of any reservoir. Open fractures, either natural or induced, are essential for economic deliverability rates from reservoirs with essentially only micropores or isolated dissolution pores.
Porosity type and/or pore geometry change with diagenesis: macropores become micropores, minerals dissolve to create voids, and pores are partly to completely occluded by precipitation of minerals. It is important to have an understanding of pore geometry, that is the size, shape, and distribution of pores in a reservoir. Pore geometry influences the type, amount, and rate of fluid produced.
Porosity type seldom is homogeneous in rocks. As a result, log interpretation problems may occur in sandstones containing significant micropores and interconnected macropores. Micropores may hold irreducible water while macropores may 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.
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Aspects of Diagenesis
There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.