72
3 PARTICLES, PORES, AND PERMEABILITY
Fracture porosity can also form from atectonic processes. It is often found immediately beneath unconformities. Here fractures, once formed by weathering, may have
been enlarged by solution (especially in limestone paleokarst) and preserved without
subsequent loss of their porosity (see Section 9.2.7). Fracture porosity is extremely important in both aquifers and petroleum reservoirs. This is because a very small amount
of fracture porosity can give a very large permeability; the fractures connecting up many
pores of other types that might otherwise be ineffective. There is, therefore, a very large
literature on fractures (e.g., Aguilera, 1980; Reiss, 1981; Van Golfracht, 1982; Nelson,
1986; Atkinson, 1987).
3.2.2.3.7 Summary
The preceding account shows something of the diverse types and origins of pores. Essentially there are two main genetic groups of pore types. Primary porosity is formed
when a sediment is deposited. It includes intergranular or interparticle porosity, which
is characteristic of sands, and intraparticle porosity found in skeletal carbonate sands.
Secondary porosity forms after sedimentation by diagenetic processes. Recrystallization, notably dolomitization, can generate intercrystalline porosity. Solution can generate moldic, vuggy, and cavernous pores. Because such pores are often isolated from one
another permeability may be low. Fractures form in both unconsolidated and brittle
sediments. In the first instance the fractures remain closed, but in brittle rocks fracture
porosity may be preserved, enlarged by solution, or diminished by cementation. Fracture porosity occurs not just in indurated sediments, but also in igneous and metamorphic rocks.
It is important to note that many sedimentary rocks contain more than one type of
pore. The combination of open fractures with another pore type is of particular significance. The problems generated by such dual pore systems negating the assumptions
of Darcy's law have been noted earlier. Fine-grained rocks, such as shales, microcrystalline carbonates, and fine sands, have considerable porosity. They often have very low
permeabilities because of their narrow pore throat diameter and concomitant high capillary pressure. The presence of fractures, however, can enable such rocks to yield their
contained fluids. The success of many oil and water wells in such formations often depends on whether they happen to penetrate an open fracture. Recognition of the significance of fractures in producing fluids from high-porosity, low-permeability formations
has led to the development of artificial fracturing by explosive charges that simultaneously wedge the fractures open with sand, glass beads, etc. Similarly, the productivity of
fractured carbonate reservoirs can be increased by the injection of acid to dissolve and
enlarge the fractures. Figure 3.22 summarizes the relationship between pore type, porosity, and permeability.
3.2.3 The Origin of Primary Porosity
The porosity and permeability of a sediment are controlled by its textural characteristics at the time of deposition and by subsequent diagenetic changes, including compaction, cementation, and solution.The effects of diagenesis on the porosity and permeability of sandstones and carbonates are discussed in Chapters 8 and 9, respectively.
3 PARTICLES, PORES, AND PERMEABILITY
Fracture porosity can also form from atectonic processes. It is often found immediately beneath unconformities. Here fractures, once formed by weathering, may have
been enlarged by solution (especially in limestone paleokarst) and preserved without
subsequent loss of their porosity (see Section 9.2.7). Fracture porosity is extremely important in both aquifers and petroleum reservoirs. This is because a very small amount
of fracture porosity can give a very large permeability; the fractures connecting up many
pores of other types that might otherwise be ineffective. There is, therefore, a very large
literature on fractures (e.g., Aguilera, 1980; Reiss, 1981; Van Golfracht, 1982; Nelson,
1986; Atkinson, 1987).
3.2.2.3.7 Summary
The preceding account shows something of the diverse types and origins of pores. Essentially there are two main genetic groups of pore types. Primary porosity is formed
when a sediment is deposited. It includes intergranular or interparticle porosity, which
is characteristic of sands, and intraparticle porosity found in skeletal carbonate sands.
Secondary porosity forms after sedimentation by diagenetic processes. Recrystallization, notably dolomitization, can generate intercrystalline porosity. Solution can generate moldic, vuggy, and cavernous pores. Because such pores are often isolated from one
another permeability may be low. Fractures form in both unconsolidated and brittle
sediments. In the first instance the fractures remain closed, but in brittle rocks fracture
porosity may be preserved, enlarged by solution, or diminished by cementation. Fracture porosity occurs not just in indurated sediments, but also in igneous and metamorphic rocks.
It is important to note that many sedimentary rocks contain more than one type of
pore. The combination of open fractures with another pore type is of particular significance. The problems generated by such dual pore systems negating the assumptions
of Darcy's law have been noted earlier. Fine-grained rocks, such as shales, microcrystalline carbonates, and fine sands, have considerable porosity. They often have very low
permeabilities because of their narrow pore throat diameter and concomitant high capillary pressure. The presence of fractures, however, can enable such rocks to yield their
contained fluids. The success of many oil and water wells in such formations often depends on whether they happen to penetrate an open fracture. Recognition of the significance of fractures in producing fluids from high-porosity, low-permeability formations
has led to the development of artificial fracturing by explosive charges that simultaneously wedge the fractures open with sand, glass beads, etc. Similarly, the productivity of
fractured carbonate reservoirs can be increased by the injection of acid to dissolve and
enlarge the fractures. Figure 3.22 summarizes the relationship between pore type, porosity, and permeability.
3.2.3 The Origin of Primary Porosity
The porosity and permeability of a sediment are controlled by its textural characteristics at the time of deposition and by subsequent diagenetic changes, including compaction, cementation, and solution.The effects of diagenesis on the porosity and permeability of sandstones and carbonates are discussed in Chapters 8 and 9, respectively.