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CHAPTER TWO
CARBONATE RESERVOIR
ROCK PROPERTIES
2.1 DEFINITIONS
This chapter focuses on the definitions of fundamental rock properties, how the prop-
erties are used to classify both rocks and porosity, and how fundamental rock proper-
ties are related to reservoir properties. It is traditional in geology to use purely
descriptive terms for rock properties because objective descriptions are less likely to
contain subjective interpretations or biases. This philosophy has merit in most cases,
but in the end, the task of the reservoir geologist is to formulate interpretive models
for use in exploration and development. In carbonate rocks, reservoir porosity and
permeability can be formed by a variety of processes. These processes create the rock
properties we describe with rigidly objective terms. Some of the formative processes
may have affected reservoir rocks more than once; therefore an accurate reservoir
description should incorporate terminology that classifies the altered properties, the
processes that created them, and at least an estimate of the number of times the rock
properties underwent change. To produce such a classification requires the use of
genetic terms along with descriptive ones. For example, subjective interpretations are
required to determine which processes caused diagenetic changes at which times
during the burial history of the rocks. Diagenetic porosity can be classified in a totally
objective manner, but without interpretations of how, when, and where different dia-
genetic events changed preexisting pore characteristics it is hardly possible to predict
the spatial distribution of the ultimate porosity.
Traditional geological literature includes the terms primary and secondary to
describe rock properties. This is not helpful because those terms are ambiguous.
Primary may be used in a temporal sense to indicate the depositional origin of rock
properties such as grain size, grain composition, or skeletal morphology in the case
Geology of Carbonate Reservoirs: The Identification, Description, and Characterization of Hydrocarbon
Reservoirs in Carbonate Rocks
By Wayne M. Ahr Copyright © 2008 John Wiley & Sons, Inc.
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