Page 49 - Geology of Carbonate Reservoirs

30 CARBONATE RESERVOIR ROCK PROPERTIES

and evoke mental images of framework/detritus ratio, which translates into type
and spatial distribution of pore categories, assuming that diagenesis has not radically
altered them. It is reasonable to infer that depositional porosity and permeability
are highest in frame reefs and lowest in micrite mounds and cement reefs. Reser-
voirs exist in those reef categories but usually owe their existence to enhanced
porosity and permeability formed by diagenesis or fracturing. Predicting reservoir
connectivity is diffi cult in all reef categories but is especially hard to predict in dia-
genetically altered, complex mixtures of frame and detritus. Diagenetic porosity
may be strongly bimodal in size. Microporosity is common in lime mud portions of
reef rocks, for example.

2.3.3 Wright’s Genetic Classifi cation

The Folk and Dunham classifications for detrital carbonates were introduced nearly
a half - century ago when our understanding of diagenetic processes and their prod-
ucts was in its infancy and before much effort was made to develop classifi cations

for reef rocks. Recognizing these deficiencies in descriptive terminology, Wright
(1992) proposed an integrated scheme that links the depositional classifi cation of

Dunham (1962) and the biological classification of Embry and Klovan (1971) with

a new classification for diagenetic rocks. In concept, this classification is logical and
more utilitarian for the reservoir geoscientist than any existing single classifi cation.

This is a genetic classification system in which carbonate rocks are grouped by mode
of origin — depositional, biological, and diagenetic. Each category has subheadings
to distinguish the various rock properties that typify each mode. The terms intro-
duced for diagenetic carbonates draw attention to whether or not the diagenetic
process has obliterated the original texture and fabric. This distinction requires
examination and interpretation of thin sections under the polarizing microscope,
however. In addition, the terms for compacted rocks with microstylolitic grain con-
tacts should include packstone along with grainstone. It is difficult to have a perfect

classification for all applications, but this genetic scheme represents an advance.

Later in this chapter we will discuss classifications of porosity and we will see that

purely descriptive classifications of porosity, like nonintegrated classifi cations for
carbonate rocks, may be less useful in analyzing carbonate reservoirs than inte-

grated, genetic classifications.
2.4 DEPENDENT OR DERIVED ROCK PROPERTIES

Porosity, permeability, and bulk density depend on fundamental properties such as
texture, mineralogical composition, and fabric. Dependent properties, especially
porosity and permeability, are among the most important variables that determine
reservoir quality. While rocks are classified according to their fundamental proper-

ties and inferences are made from rock classifications about depositional environ-

ments, porosity is classified according to physical attributes that may not be related

to mode of origin. But unless mode of origin is included in porosity classifi cations,
it is not possible to deduce the environment in which the porosity was formed, when
it was modified, and which genetic pore types correspond to highest permeability.

Comprehensive reservoir description depends on identification and description of