Geology Reference
In-Depth Information
Plate 29 Distinguishing Carbonate Pore Types
The plate displays different pore types, using the Choquette and Pray classification. See Fig. 7.5 and Box 7.4.
1
Primary (fabric-selective)
and
secondary (non-fabric-selective)
porosity
. Poorly cemented bryozoan rudstone with differ-
ent types of bryozoans. Primary pores are pores between particles (interparticle or intergranular pores, IP), and within
particles (intraparticle pores, black arrows). Secondary, still open pores appear as vugs (VP) originating from solution
enlargement of interparticle pores forming vugs that grade into elongate channel pores. Note that outlines of vuggy pores
intersect and destroy grain outlines (white arrows). Primary pores have been partly filled with isopachous equant calcite
and calcite blocky cement, and partly reopened by solution. Large idiomorphic calcite crystals (top right) were formed
subsequent to equigranular cement. The preservation of open interparticle porosity was favored by the stable mineralogy
of the skeletal grains (Low-Mg calcite) and burial solution-enhancement of interparticle pores. The dominance of bryozo-
ans, grain association, and weak primary cementation are common criteria in bryozoan-dominated temperate and cool-
water carbonates (see Sect. 16.4.1). Tertiary (Middle Miocene): Albacete near Tobara, Alicante, SE Spain.
Use this picture as an exercise! You should find the following answers to the questions summarized on Pl. 30: (A) Primary
interparticle pores occluded by cement. Open secondary vuggy and intraskeletal pores. (B) Vuggy pores. (C) Primary
reduced porosity. Secondary vuggy solution-enlarged porosity. (D) Primary pores enlarged and enhanced by solution,
grading into irregular vugs. (E) Vuggy porosity postdates cementation, no compaction. (F) Open porosity about 20%.
2
Open intraparticle
(
intraskeletal) porosity
(P) within a rugose coral. The oblique section shows septa (S) and horizontal
tabulae (T). These skeletal elements consist of recrystallized calcite crystals growing perpendicular to the median line of
the original skeletal elements. The coral microstructure is still preserved because of the stability of Low-Mg calcite. Some
intraparticle spaces were filled with cements, but most intraskeletal pores are still open. Middle Devonian: Belgium.
3
Fabric-selective oomoldic porosity.
Ooid grainstone. The primary aragonitic ooid cortices have been leached (arrows) by
vadose meteoric solution. Leaching of cortices can create secondary intraparticle porosity, which is either occluded by
cement or leads to oomolds. No evidence for strong compaction. Interparticle pores were filled with thin isopachous
cement rims and fine-grained drusy mosaic cement. Early Cretaceous (Purbeck facies, Berriasian): Subsurface, Bavaria,
Germany.
4
Fabric-selective oomoldic porosity
(OM). Ooid grainstones/packstones are important hydrocarbon reservoirs. Principal
effective porosities of carbonate reservoirs in the Four Corners Area of the Colorado Plateau include interparticle, inter-
and intraskeletal (algal blade), oomoldic and intercrystalline dolomite types (Weber et al. 1995). Oomoldic porosity on
paleodepositional highs is characterized by high porosity values (average 11.3%, McComas 1963) associated with low
permeability (average 3.4 millidarcys). High porosity is caused by meteoric leaching of originally aragonitic ooids. Low
permeability is due to the insufficient pore interconnection system because of the scarceness of leached mold walls. Note
that some ooids have not been leached. This may indicate differences in the primary mineralogy of the ooids. Middle
Pennsylvanian (Paradox Formation): Goosenecks of San Juan, Four Corners Area, Utah, U.S.A.
5
Intercrystalline and interparticle
nannofossil
porosity
. Soft Chalk is a very fine-grained, light-colored and friable carbon-
ate sediment, composed largely of minute Low-Mg calcite skeletons of planktonic coccolithophorids (
Cribrosphaerella
ehrenbergi
(Arkhangelsky); see Pl. 7/3-5). Porosity occurs between fragments of nannofossils and between calcite crys-
tals. Small cement crystals form overgrowth on coccolith elements. Intraparticle pores are partly filled with coarse blocky
cement. High-porosity chalks are characterized by well-preserved nannofossils indicating minimum fragmentation and
dissolution, in contrast to lowest-porosity chalks containing poorly preserved nannofossils. The hydrocarbon production
of many chalk reservoirs (e.g. in the North Sea) is due to the preservation of high primary porosity (caused by a unique
combination of pore fluid overpressuring and exclusion of water from pores by migrating oil) and permeability enhance-
ment by post-lithification fracturing (Feazel 1985). SEM. Late Cretaceous (Maastrichtian) chalk: Logster, Denmark.
6
Interparticle porosity
(IP). Grains are superficial ooids. Note open pores between the grains. Thin white rims around
grains are isopachous meteoric cements. Subrecent grainstone: San Salvador, Bahamas.
7
Open
interparticle (IP) porosity
between skeletal grains (predominantly corallinacean red algae, CA) and
intraskeletal
porosity
within foraminifera (arrows). Corallinacean algal grains are covered by thin isopachous Mg-calcite cement rims.
Pleistocene calcarenite: El Haouira, Cap Bon, northern Tunisia. Crossed nicols.
8
Moldic porosity
formed by selective dissolution of ooids and bioclasts. Reservoir rock characteristics of dolomitized
oolitic grainstones in the Fore-Sudetic area of western Poland are controlled by vadose dissolution of aragonite and Mg-
calcite grains. Late Permian (Zechstein Main Dolomite): Subsurface, Laba, Poland.
9
Keystone vug porosity.
Keystone vugs (KV) are mm-sized fenestral pores occurring in tidally influenced carbonate beach
sands. They form when tides rise and air or gas bubbles enclosed within the fine-grained sediment try to escape from the
sediment. The air bubbles push grains aside, thus creating voids. The grains at the top of the vug form a keystone arch that
prevents collapse. Preservation of keystone vugs points to rapid cementation. Keystone vugs are generally larger than
birdseyes (see Pl. 20/7, 8). Example: Particles are predominantly peloids and micritized ooids as well as a few aggregate
grains. Tidal sand bars. Early Cretaceous (Purbeck facies, Berriasian): Subsurface, Kinsau, Bavaria, Germany.
