Geology Reference
In-Depth Information
Plate 30 Porosity Evaluation in Thin Sections
The impregnation of carbonate rocks with blue-dye stained resin before sectioning makes the evaluation of
porosity straightforward and facilitates the recognition of pore distribution.
Questions
which should be answered in evaluating thin sections with regard to porosity are:
A Are the pores completely or partially occluded by cement or sediment or are they still open?
B Which of the pore types shown in Fig. 7.5 occur and which types dominate?
C Classify the sample according to the Choquette and Pray scheme (Fig. 7.5)!
D Do primary depositional pores still exhibit their original shape or are they enlarged and enhanced by solution,
thus grading into irregular vugs?
E Does porosity postdate cementation and compaction (e.g. fracture porosity)?
F Estimate the percentage of open (and/or closed) rock porosity!
1
Bioclastic limestone
with terrestrial gastropods (
Hydrobia
, H). A blue color reveals open pores. The snails were deposited
near the coast and covered by sediment. The dark particles (bottom left) are micritized algal stems. The primary aragonitic
shells were dissolved and replaced by calcite under meteoric conditions. The interior of the shells was filled with peloids
prior to the final deposition of the shells. The peloids are fecal pellets exhibiting different preservation; light peloids are
calcitized aragonite pellets. The fine-grained sediment exhibits a distinct geopetal fabric consisting of dark micrite at the
bottom and lighter, more coarser sediment at the top. The remaining interparticle voids between the shells are lined by thin
isopachous cement rims of bladed calcite crystals and blocky spar. Porosity:
Moldic porosity
is indicated by white arrows.
Some pellets show open
intraparticle porosity
(top right). Black arrows point to
vuggy porosity
. Brackish-lagoonal envi-
ronment. Late Tertiary (Early Miocene): Mainz Basin, Germany.
Answers: (A) Most pores filled with sediment and cement, some pores open. (B) Moldic, interparticle, vuggy, rare inter-
crystalline. Moldic porosity now dominates. (C) Primary depositional interparticle porosity mainly filled and reduced,
open secondary moldic porosity. (D) Primary pores with original shape. (E) No compaction. (F) Original porosity about
50%, open porosity < 5%.
2
Detail of the sample shown in -> 1. Gastropod shell (
Hydrobia
) filled with peloids and calcite cement. Note the difference
in crystal size and shape of cements within the shell and in the void at the top of the geopetal structure. Many of the
peloids and the complete gastropod shell are dissolved. The shape of the gastropod is preserved because of thin protecting
organic coatings.
3
Predominantly vuggy porosity
(blue) caused partly by solution-enlargement of interparticle pores. Solution enhancement
has changed the original fabric-selective porosity to non-fabric-selective porosity. Some intraskeletal porosity is pre-
served in the whorls of foraminiferal tests. In terms of the Choquette and Pray classification the open porosity is classified
as
solution enlarged secondary vuggy porosity characterized by mesopores
(see Fig. 7.5). This bioclastic grainstone
consists of skeletal grains and some micritic intraclasts. Bioclasts are predominantly miliolid foraminifera and a few
bryozoans (B). The chambers of the miliolid foraminifera were partly occluded by cement. Non-miliolid foraminifera
have been almost completely replaced by calcite (arrows). Tertiary (Miliolid limestone): Carrière Saint-Pierre-Aigle,
Paris Basin, France.
4
Fenestral porosity
in a travertine formed by spring waters in shallow ponds (see Pl. 2/1-3). Small open fenestral pores
occur within a laminated fabric. Note the different cementation within large and small pores: Larger pores are bordered by
large scalenohedral calcite crystals, small pores by small bladed crystals. Some scalenohedral crystals are zoned (black
arrow). The micritic layers border larger voids that are incompletely filled with blocky calcite crystals. Fenestral porosity
occurs in caliche deposits, freshwater tufa, and travertines, but is most common in carbonate mud- and bindstones of tidal
flats. Travertine. Terme di Rapolano, north of Rome, Italy.
5
Fenestral porosity
in travertine. Isopachous calcite cements lining open pores in places. The crystals are more or less
equidimensional, indicating precipitation in a meteoric phreatic environment. Remains of organic-rich laminae are shown
as a brown color. Same sample as -> 4.
6
Moldic porosity
(blue). Because of the metastability of aragonite the gastropod shell was dissolved, creating moldic
porosity. Same locality as -> 4.
