3.2 POROSITY AND PERMEABILITY
67
porosity type. It is present initially in almost all sediments. Intergranular porosity is generally progressively reduced by diagenesis in many carbonates, but is the dominant porosity type found in sandstones. The factors that influence the genesis of intergranular
porosity and which modify it after deposition are discussed at length in a later section
(see Section 3.2.3.1).
3.2.2.2.2 Intragranular or intraparticle porosity
In carbonate sands, particularly those of skeletal origin, primary porosity may be present within the detrital grains. For example, the cavities of mollusks, ammonites, corals,
bryozoa, and microfossils can all be classed as intragranular or intraparticle primary
porosity (Fig. 3.18B, Plate 1B). This kind of porosity is often diminished shortly after
deposition by infiltrating micrite matrix. Furthermore, the chemical instability of the
carbonate host grains often leads to their intraparticle pores being modified or obliterated by subsequent diagenesis.
3.2.2.3 Secondary or Postdepositional Porosity
Secondary porosity is that which, by definition, formed after a sediment was deposited.
Secondary porosity is more diverse in morphology and more complex in genesis than
primary porosity. It is more commonly found in carbonate rocks than in siliciclastic
sands. This is because of the greater mobilty of carbonate minerals in the subsurface,
compared to quartz. The following main types of secondary porosity are recognizable.
3.2.2.3.1 Intercrystalline porosity
Intercrystalline porosity occurs between the individual crystals of a crystalline rock
(Fig. 3.19A, Plate 1C). It is, therefore, the typical porosity type of the igneous and highgrade metamorphic rocks, and of some evaporites. Strictly speaking, such porosity is
of primary origin. It is, however, most characteristic of carbonates that have undergone
crystallization and is particularly important in recrystallized dolomites. Such rocks are
sometimes very important oil reservoirs. The pores of crystalline rocks are essentially
planar cavities which intersect obliquely with one another with no constrictions of the
boundaries or throats between adjacent pores.
3.2.2.3.2 Fenestral porosity
The term "fenestral porosity" was first proposed by Tebbutt et al. (1965) for a "primary
or penecontemporaneous gap in rock framework, larger than grain-supported interstices." This porosity type is typical of carbonates. It occurs in fragmental carbonate
sands, where it grades into primary porosity, but is more characteristic of pellet muds,
algal laminites, and homogeneous muds of lagoonal and intertidal origin. Penecontemporaneous dehydration, lithifaction, and biogenic gas generation can cause laminae to
buckle and generate subhorizontal fenestral pores between the laminae (Fig. 3.19B).
This type of fabric has been termed loferite by Fischer (1964) on the basis of a study
67
porosity type. It is present initially in almost all sediments. Intergranular porosity is generally progressively reduced by diagenesis in many carbonates, but is the dominant porosity type found in sandstones. The factors that influence the genesis of intergranular
porosity and which modify it after deposition are discussed at length in a later section
(see Section 3.2.3.1).
3.2.2.2.2 Intragranular or intraparticle porosity
In carbonate sands, particularly those of skeletal origin, primary porosity may be present within the detrital grains. For example, the cavities of mollusks, ammonites, corals,
bryozoa, and microfossils can all be classed as intragranular or intraparticle primary
porosity (Fig. 3.18B, Plate 1B). This kind of porosity is often diminished shortly after
deposition by infiltrating micrite matrix. Furthermore, the chemical instability of the
carbonate host grains often leads to their intraparticle pores being modified or obliterated by subsequent diagenesis.
3.2.2.3 Secondary or Postdepositional Porosity
Secondary porosity is that which, by definition, formed after a sediment was deposited.
Secondary porosity is more diverse in morphology and more complex in genesis than
primary porosity. It is more commonly found in carbonate rocks than in siliciclastic
sands. This is because of the greater mobilty of carbonate minerals in the subsurface,
compared to quartz. The following main types of secondary porosity are recognizable.
3.2.2.3.1 Intercrystalline porosity
Intercrystalline porosity occurs between the individual crystals of a crystalline rock
(Fig. 3.19A, Plate 1C). It is, therefore, the typical porosity type of the igneous and highgrade metamorphic rocks, and of some evaporites. Strictly speaking, such porosity is
of primary origin. It is, however, most characteristic of carbonates that have undergone
crystallization and is particularly important in recrystallized dolomites. Such rocks are
sometimes very important oil reservoirs. The pores of crystalline rocks are essentially
planar cavities which intersect obliquely with one another with no constrictions of the
boundaries or throats between adjacent pores.
3.2.2.3.2 Fenestral porosity
The term "fenestral porosity" was first proposed by Tebbutt et al. (1965) for a "primary
or penecontemporaneous gap in rock framework, larger than grain-supported interstices." This porosity type is typical of carbonates. It occurs in fragmental carbonate
sands, where it grades into primary porosity, but is more characteristic of pellet muds,
algal laminites, and homogeneous muds of lagoonal and intertidal origin. Penecontemporaneous dehydration, lithifaction, and biogenic gas generation can cause laminae to
buckle and generate subhorizontal fenestral pores between the laminae (Fig. 3.19B).
This type of fabric has been termed loferite by Fischer (1964) on the basis of a study