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Book Chapter

Porosity, Diagenesis and Productive Capability of Sandstone Reservoirs

By
Edward D. Pittman
Edward D. Pittman
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Published:
January 01, 1979

Abstract

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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Contents

SEPM Special Publication

Aspects of Diagenesis

Peter A. Scholle
Peter A. Scholle
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Paul R. Schluger
Paul R. Schluger
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SEPM Society for Sedimentary Geology
Volume
26
ISBN electronic:
9781565761568
Publication date:
January 01, 1979

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