70
3 PARTICLES, PORES, AND PERMEABILITY
with "cold seeps" where gases and liquids, often including methane and petroleum,
emerge on the sea bed (see Section 5.3.5.7).
3.2.2.3.4 Moldic porosity
A fourth type of secondary porosity, generally formed later in the history of a rock than
fenestrae and stromatactis, is moldic porosity. Molds are pores formed by the solution
of primary depositional grains generally subsequent to some cementation. Molds are
fabric selective. That is to say, solution is confined to individual particles and does not
cross-cut cement, matrix, and framework. Typically in any rock it is all the grains of one
particular type that are dissolved. Hence one may talk of oomoldic, pelmoldic, or biomoldic porosity where there has been selective solution of ooliths, pellets, or skeletal
debris (Fig. 3.19C, Plate 1D). The geometry and effective porosity and permeability of
moldic porosity can thus be extremely varied. In an oomoldic rock, pores will be subspherical and of similar size. In biomoldic rocks, by contrast, pores may be very variable
in size and shape, ranging from minute apertures to curved planar pores where shells
have dissolved, and cylinders where echinoid spines have gone into solution.
3.2.2.3.5 Vuggy porosity
Vugs are a second type of pore formed by solution and, like molds, they are typically
found in carbonates. Vugs differ from molds though because they cross-cut the primary
depositional fabric of the rock (Fig. 3.19D, Plate 1E). Vugs thus tend to be larger than
molds. They are often lined by a selvage of crystals. With increasing size vugs grade
into what is loosely termed "cavernous porosity." Choquette and Pray (1970, p. 244)
proposed that the minimum dimension of a cavern is a pore which allows a speleologist to enter or which, when drilled into, allows the drill string to drop by more than half
a meter through the rotary table. Large-scale vuggy and cavernous porosity is commonly developed beneath unconformities where it is referred to as "paleokarst" (see
Section 9.2.7). This serves as a petroleum reservoir in a number of fields such as Abqaiq
in Saudi Arabia (McConnell, 1951), the Dollarhide field of Texas (Stormont, 1949), and
the Casablanca field of offshore Spain (Watson, 1982).
3.2.2.3.6 Fracture porosity
The last main type of pore to be considered is that formed by fractures. Fractures occur
in many kinds of rocks other than sediments. Fracturing, in the sense of a breaking of
depositional lamination, can occur penecontemporaneously with sedimentation. This
often takes the form of microfaulting caused by slumping, sliding, and compaction. Fractures in plastic sediments are instantaneously sealed, however, and thus seldom preserve
porosity. In brittle rocks fractures may remain open after formation, thus giving rise to
fracture porosity (Fig. 3.19E, Plate 1F). This porosity type characterizes rocks that are
strongly lithified and is, therefore, generally formed later than the other varieties of porosity (Fig. 3.21). It is important to note that fracture porosity is not only found in wellcemented sandstones and carbonates, but may also be present in shales and igneous
and metamorphic rocks.
Fracture porosity is much more difficult to observe and analyze than most other pore
3 PARTICLES, PORES, AND PERMEABILITY
with "cold seeps" where gases and liquids, often including methane and petroleum,
emerge on the sea bed (see Section 5.3.5.7).
3.2.2.3.4 Moldic porosity
A fourth type of secondary porosity, generally formed later in the history of a rock than
fenestrae and stromatactis, is moldic porosity. Molds are pores formed by the solution
of primary depositional grains generally subsequent to some cementation. Molds are
fabric selective. That is to say, solution is confined to individual particles and does not
cross-cut cement, matrix, and framework. Typically in any rock it is all the grains of one
particular type that are dissolved. Hence one may talk of oomoldic, pelmoldic, or biomoldic porosity where there has been selective solution of ooliths, pellets, or skeletal
debris (Fig. 3.19C, Plate 1D). The geometry and effective porosity and permeability of
moldic porosity can thus be extremely varied. In an oomoldic rock, pores will be subspherical and of similar size. In biomoldic rocks, by contrast, pores may be very variable
in size and shape, ranging from minute apertures to curved planar pores where shells
have dissolved, and cylinders where echinoid spines have gone into solution.
3.2.2.3.5 Vuggy porosity
Vugs are a second type of pore formed by solution and, like molds, they are typically
found in carbonates. Vugs differ from molds though because they cross-cut the primary
depositional fabric of the rock (Fig. 3.19D, Plate 1E). Vugs thus tend to be larger than
molds. They are often lined by a selvage of crystals. With increasing size vugs grade
into what is loosely termed "cavernous porosity." Choquette and Pray (1970, p. 244)
proposed that the minimum dimension of a cavern is a pore which allows a speleologist to enter or which, when drilled into, allows the drill string to drop by more than half
a meter through the rotary table. Large-scale vuggy and cavernous porosity is commonly developed beneath unconformities where it is referred to as "paleokarst" (see
Section 9.2.7). This serves as a petroleum reservoir in a number of fields such as Abqaiq
in Saudi Arabia (McConnell, 1951), the Dollarhide field of Texas (Stormont, 1949), and
the Casablanca field of offshore Spain (Watson, 1982).
3.2.2.3.6 Fracture porosity
The last main type of pore to be considered is that formed by fractures. Fractures occur
in many kinds of rocks other than sediments. Fracturing, in the sense of a breaking of
depositional lamination, can occur penecontemporaneously with sedimentation. This
often takes the form of microfaulting caused by slumping, sliding, and compaction. Fractures in plastic sediments are instantaneously sealed, however, and thus seldom preserve
porosity. In brittle rocks fractures may remain open after formation, thus giving rise to
fracture porosity (Fig. 3.19E, Plate 1F). This porosity type characterizes rocks that are
strongly lithified and is, therefore, generally formed later than the other varieties of porosity (Fig. 3.21). It is important to note that fracture porosity is not only found in wellcemented sandstones and carbonates, but may also be present in shales and igneous
and metamorphic rocks.
Fracture porosity is much more difficult to observe and analyze than most other pore