Page 56 - Geology of Carbonate Reservoirs

DEPENDENT OR DERIVED ROCK PROPERTIES 37

Figure 2.11 The essential elements of the Choquette − Pray (1970) porosity classifi cation for
carbonate rocks. The essence of the classification is whether or not porosity conforms to rock

“ fabric. ” Note that rock fabric in this sense includes both depositional and diagenetic fabrics.
(Adapted from an illustration by Choquette and Pray (1970) in Scholle (1978) .)
diagenetic in origin and to have occurred in the “ eogenetic, ” “ telogenetic, ” or “ meso-
genetic ” domains. Eogenetic changes are those affecting newly buried rocks still
within the influence of a depositional surface or an intraformational erosional

surface. Telogenetic changes affect long buried rocks that later became “ connected ”
to an erosional surface. Mesogenetic changes are those unaffected by surface condi-
tions or processes. The direction of change in pore characteristics is indicated by the

terms enlarged, reduced, and filled. These changes result from solution, cementation,
or internal sediment (infi lling).

Pore abundance in the Choquette – Pray classification is given as a percentage,
expressed as a ratio of pore categories, or as a ratio of pore types plus a percent
figure. Pore size categories are termed “ megapore, mesopore, and micropore, ” with

size limits closely paralleling the Wentworth (1922) grain size scale. Megapore sizes

are equivalent to gravel (4 – 256 mm), mesopore sizes are equivalent to sand -
plus - granules (4 to 1/16 mm), and micropores are equivalent to mud ( < 1/16 mm).

Letter codes are assigned to pore types, size modifiers, and genetic modifi ers, and