Page 40 - Geology of Carbonate Reservoirs

CLASSIFICATION OF CARBONATE ROCKS 21

B

A

C D

Figure 2.4 (a) Photomicrograph of a lime grainstone with ooids, intraclasts, and pisoids.

Note the “ dogtooth spar ” isopachous rim cement on grain surfaces. Porosity in this rock is
mainly intergranular but the grains have been altered and intragranular microporosity is also

present. The horizontal width of the photo is 2.5 mm. (b) Photomicrograph of an intraclastic
(nonskeletal grains) conglomerate from the Cambrian of Central Texas. Note that the clasts
are aligned in a fabric created by currents at the time of deposition. The width of the photo

is 4 mm. (c) Photomicrograph of an intraclastic, peloidal grainstone from the Cambrian of
Central Texas. The width of the photo is 2.5 mm. The most common peloids are probably

microbial in origin and, of those, most are found as cavity - fillings in reefs and mounds. (d)

Photomicrograph of ovoid, polychaete worm fecal pellets from the Pleistocene Campeche

Calcilutite, Yucatan ramp, Mexico. The long axis of each pellet is about 2 mm.
(Grabau, 1960 ). Popular, modern classifi cations for detrital carbonates were devel-
oped by Folk (1959, 1962) (Figure 2.6 ) and Dunham (1962) (Figure 2.7 ). Classifi ca-
tions for reef rocks were developed by Embry and Klovan (1971) (Figure 2.8 )
and Riding (2002) . A scheme to include depositional, diagenetic, and biological
aspects of carbonates in one classification system was proposed by Wright (1992) .

There are two main purposes for classifi cation systems: (1) to make descriptions of