Page 58 - Geology of Carbonate Reservoirs

DEPENDENT OR DERIVED ROCK PROPERTIES 39

depositional facies selectivity transforms those facies into proxies for porosity. Many

of the penetrative features classified as fabric selective or not (burrows, borings, and
desiccation cracks) may in fact be facies selective. As such, they can appear at certain
positions in stratigraphic cycles where the occurrences of facies - selective porosity
are more predictable. Fabric - selective porosity may not be mappable at reservoir
scale, especially if it does not conform to facies boundaries as is commonly the case
with diagenetic porosity. Diagenetic rock and pore properties must be identifi ed,
linked to the type of diagenesis, tied to its time of occurrence, and ultimately placed
in the stratigraphic framework.

2.4.1.4 The Lucia Classifi cation

Lucia ’ s (1983) classifi cation of carbonate porosity (Figure 2.12 ) evolved from work
done at Shell Oil and Shell Development Companies during the 1960s. Although

the influence of Archie ’ s work is evident in Lucia ’ s classification, Lucia ’ s division of
carbonate pore types into vuggy and interparticle categories distinguishes it. Like
Archie, the objective of Lucia ’ s classification is to provide a practical field and labo-

ratory method for visual description of porosity in carbonate rock samples. For rocks
with interparticle and separate vug porosity, Archie ’ s m factor can be estimated if
the particle size, amount of separate vug porosity, and total porosity are known.
Lucia ’ s emphasis on the relationship between porosity, permeability, inferred capil-
lary displacement pressure, and particle size, some of which relationships were also
recognized by Craze (1950) and Bagrintseva (1977) , is an early method for “ rock
typing ” or ranking reservoir zones on the basis of petrophysical characteristics.
A particularly important attribute of Lucia ’ s classification is its emphasis on the

petrophysical significance of separate and touching vugs. Vugs are pores larger than
surrounding framework grains. They may have originally been moldic, interparticle,

Interparticle Vuggy
(P) (V)

Particle size Connection
Through Through
interparticle other
Fine Medium Large Pores VUG5
(F) (M) (5) separate touching
< 20 m 20–100 m > 100 m (5) (T)

Porosity No Yes (n) (y) > 70 psia 70–15 psia < 15 psia Porosity (%)
P d
P d
P d
Figure 2.12 Classification of carbonate porosity by Lucia (1983) . This scheme is especially

important because it emphasizes that interparticle (grains or crystals) porosity and separate
or touching vuggy porosity have profound effects on such reservoir petrophysical character-
istics as Archie ’ s m (cementation exponent), porosity − permeability relationships, and capil-

lary pressure behavior. The latter influence is reflected by the “ P d ” values in psia, which

indicate the mercury displacement pressure required to enter the pore systems corresponding
to small, intermediate, and large particle sizes.