Geology Reference
In-Depth Information
Box 7.4.
Terms used to describe carbonate porosity in thin sections.
The arrangement within the list follows the porosity
classification of Choquette and Pray (1970), see Fig. 7.5.
Fabricselective pores
Interparticle (intergranular) porosity:
Porosity between individual particles or grains of a sedimentary rock. Corre-
sponds generally to depositional primary porosity, but also includes secondary porosity (e.g. resulting from partial
dissolution of aragonitic ooid cortices). Interparticle porosity is high in modern mud-free carbonate sands (up to
about 50%) as well as in modern mud-bearing sediments (about 40 to about 75%). The preservation of open interpar-
ticle pores in ancient carbonates is promoted by the absence of water in the pores in dry climates, by a protective seal
of clay or evaporites, or by early oil emplacement. Common pore sizes are 0.05 to 1 mm. Pl. 13/6; Pl. 29/5, 6, 7; Pl.
34/1, 2, 4; Pl. 40/5; Pl. 41/1; Pl. 61/1, 2; Pl. 80/2; Pl. 93/5; Pl. 120/2; Pl. 134/7.
Intraparticle (intragranular) porosity:
Primary pore space corresponding to defined parts of skeletons (
intraskeletal
porosity
, e.g. chambers of foraminifera: Bachman 1984) or to open spaces created by the removal of less calcified
internal elements (e.g. central part of
Halimeda
-> Pl. 7). Pl. 29/2, 7; Pl. 67/1. Depends on the morphology and
microstructure of tests and skeletons of organisms as well as on the ultrastructure of the grains (e.g. ooids or aggre-
gate grains). Common pore sizes are <0.01 to 1 mm. Pl. 29/2, 7; Pl. 58/ 2, 5.
Growth framework porosity:
Primary porosity associated with the growth of reef-building organisms. Framework po-
rosity may be high (in modern coral reefs) or low (in reefs dominated by encrusting organisms). It tends to become
quickly reduced by infilling of sediment and by carbonate cements. Pl. 42/1; Pl. 48/7; Pl. 65/3.
Fenestral porosity:
Primary porosity bound to synsedimentary open-space structures (Sect. 5.1.5), and commonly asso-
ciated with supratidal and intertidal, algal- and microbial-related, mud-dominated sediments. Pl. 20/1, 2, 7; Pl. 29/9,
Pl. 30/5; Pl. 41/2; Pl. 46/2; Pl. 50/5; Pl. 123/2.
Shelter porosity:
A type of primary porosity created by the shelter and umbrella effect of relatively large grains which
prevent the sediment infilling of pore space underneath lying. Shelter porosity is favored by the existence of large
platelike fossils (e.g. larger foraminifera, platy algae). Pl. 17/2; Pl. 29/1, 4; Pl. 105/2.
Intercrystalline porosity:
Porosity between more or less equal-sized crystals, often related to early and late diagenetic
recrystallization and dolomitization processes. Common pore sizes are <1 to 10
m. Pl. 39/1-3.
Moldic porosity:
Results from the selective removal, commonly by solution, of grains, e.g. fossils or ooids. Requires a
distinctive mineralogical or microstructural difference between the solubility of grains and matrix or cements. Molds
form preferentially in rocks of mixed mineralogies in meteoric-phreatic, but also in burial settings. Common pore
sizes are 0.1 to 10 mm. Pl. 29/3, 8, Pl. 30/6.
Biomoldic porosity
refers to porosity caused by removal of fossils. Pore
sizes <0.5 to several millimeters. Pl. 30/1; Pl. 149/5. Dissolution of (mainly aragonitic) ooids results in the formation
of oomoldic porosity, particularly in meteoric vadose and meteoric phreatic environments. Common pore sizes <0.20
to >0.5 mm. Pl. 29/3, 4.
Nonfabric selective pores
Fracture porosity:
Results from the presence of openings produced by the syndepositional, depositional or post-deposi-
tional burial breaking of rocks (Sect. 5.3.2). Often caused by brittle fracture of shells as a result of increasing over-
burden before cementation, folding, faulting, salt solution, or fluid overpressing. Common in brittle homogeneous
carbonates, e.g. chalks. Fractures may be healed by late diagenetic calcite. Fracture porosity is the main porosity type
in many reservoir rocks. Pl. 25/2, 6.
Channel porosity:
A system of secondary pores in which the openings are markedly elongate and have developed
independently of texture or fabric.
Vuggy porosity:
Caused by irregularly distributed early and late diagenetic dissolution cutting across grains and/or
cement boundaries and creating millimeter- to meter-sized holes that must be studied on different scales (McNamara
et al. 1992, Dehghani et al. 1999). Dissolution may start from molds or interparticle pores. Decimeter-sized vugs of
different size (globular; vertically elongated; irregularly elongated) and corroded walls that are lined by marcasite
may be caused by synsedimentary to early diagenetic biogenic methane exhalation originating from the decay of
organic matter (Liu et al. 1988). Pl. 15/4; Pl. 29/1; Pl. 30/1, 3.
Cavern porosity:
Non-fabric selective porosity characterized by large caverns. The term cavern applies to man-sized or
larger openings of channel or vug shapes formed predominantly by karstic solution processes.
Fabricselective or not
Breccia porosity:
Depositional, solution and karst breccias may yield high porosities. Pl. 26/3; Pl. 114/3.
Boring porosity
:
Micro- and macroborers contribute to the formation of very small to centimeter-sized borings. Microborers
are effective in producing microporosity. Pl. 52/7, 8.
Burrow porosity
: Various organisms create organic burrows in relatively unconsolidated sediment. Pl. 19/2.
Shrinkage porosity:
Shrinkage of carbonate mud in tidal flats can result in the formation of characteristic pore systems.
