Porosity and Effective Porosity

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热心网友 时间：2024-01-08 17:35

Total porosity is defined as the part of rock that is void space，expressed as a percentage:

w here Vvis the void volume and VTis the total volume. A related parameter is termed the void ratio，designated as e，and stated as:

expressed as a fraction，w here Vsis the solid volume. As total volume is the sum of the void and solid volume，the follow ing relationships can be derived:

Figure 5. 1 show s some typical kinds of porosity associated w ith various rocks. The term primary porosity is reserved for interstitial porosity ( Figures 5. 1 a － d) ，and the term secondary is used for fracture or solution porosity ( Figures 5. 1 e － f) . Interstitial porosity w as investigated by Graton and Fraser in 1935，w ho demonstrated that its value can range from about 26% to 47% through different arrangements and packing of ideal spheres. In actuality，the porosity of a sedimentary rock w ill depend not only on particle shape and arrangement，but on a host of diagenetic features that have affected the rock since deposition.

Figure 5. 1 Relation between texture and porosity

Porosity can range from zero or near zero to more than 60% ( Table 5. 1) . The latter value is reflective of recently deposited sediments w hereas the former value is for dense crystalline rocks or highly compacted soft rocks such as shales. In general，for sedimentary materials，the smaller the particle size，the higher the porosity. This is best demonstrated by comparing the porosity of coarse gravels w ith fines，and the total gravel assemblage w ith silts and clays.

Table 5. 1 Range in values of porosity

An important distinction is the difference betw een total porosity，w hich does not require pore connections，and effective porosity，w hich is defined as the percentage of interconnected pore space. Many rocks，crystallines in particular，have a high total porosity，most of w hich may be unconnected. Effective porosity implies some connectivity through the solid medium， and is more closely related to permeability than is total porosity. Some data on effective porosity are show n in Table 5. 2. As noted，effective porosity can be over one order of magnitude smaller than total porosity，w ith the greatest difference occurring for fractured rocks.

Table 5. 2 Range in values of total porosity and effective porosity

Heath in 1982 recognized five types of porosity in dominant w ater-bearing bodies at or near the Earth's surface in the United States and attempted to map their distribution ( Figure 5. 2) . There are some difficulties w ith this map because of the necessity of mapping a single type of opening in areas w here tw o or more types are present. How ever，this is a useful presentation and one to which we will refer frequently in this chapter. Each pattern on Figure 5. 2 is associated with one or more major water-yielding formations in the United States. Thus，the solution- enlarged openings in carbonate rocks that make up the Florida peninsula are known as the Floridian system; the sands and gravels stretching from New Jersey into Texas are sediments of the Atlantic and Gulf coastal plains; the sand and gravel in the Midwest represents glacial deposits; the sandstones in the northern mid-continent represent several formations，including the Dakota sandstone and the Cambrian-Ordovician system; the sands and gravels in the western part of the nation occupy alluvial basins，whereas the occurrence of these same sediments in central United States represents the remnant of a giant alluvial apron that formed on the eastern slope of the Rocky Mountains; and the basalt in western United States occupies the Columbia River Plateau.

Figure 5. 2 Map of conterminous United States showing types of water-bearing openings in the dominant aquifers