Page 33 - Geology of Carbonate Reservoirs

14 CARBONATE RESERVOIR ROCK PROPERTIES

of calcified organisms. Secondary properties traditionally refer to features produced

by diagenesis. This oversimplified language creates more problems than it solves
because diagenesis is a continuous process that may affect a reservoir rock many
times in its burial history. If diagenetic and fracture porosity are “ secondary, ” then
how do we distinguish between multiple episodes of diagenesis or fracturing that
had major impacts on reservoir characteristics? How do we develop accurate res-
ervoir models that take into account the different times and modes of change? Some
texts imply that primary rock properties such as texture and fabric are depositional
only. However, texture and fabric can also be used to describe diagenetic properties
of carbonate reservoir rocks. Because diagenetic characteristics represent multicycle
changes, not just one - time, secondary changes, it can be confusing to classify rock
and reservoir properties simply as primary and secondary. It is important to distin-
guish between timing and mode of origin of the various diagenetic events to deter-
mine the history of porosity development. Similarly, it is necessary to distinguish
between timing and mode of origin of different fracture sets in fractured reservoirs.
The problem of dealing with descriptive terms is made less troublesome if carbonate

rock properties and pore categories are classified genetically as products of deposi-
tional, diagenetic, and fracture - related processes. Those are end - member processes.
Further subdivision of the classifications can be made to identify detrital, chemical,

and biogenic deposition. Diagenetic attributes can be linked to time and mechanism
of change. Mechanically produced fractures, cataclastic textures, mylonites (gouge),
ductile folding, and plastic deformation can be related to different times, stress
conditions, and material properties. Hybrid properties such as those produced by
depositional facies - selective (texture plus - or - minus fabric) diagenesis, or strain
recrystallization, or stylolitization, can also be described in terms of time and mode
of origin. Time and mode of origin of depositional, diagenetic, and fracture rock
properties are, as we will demonstrate throughout this book, critical to understand-
ing the architecture of carbonate reservoirs.
Depositional, diagenetic, and tectonic rock properties, although they are genetic,
still represent basic descriptive characteristics of carbonate reservoirs. They can
even be thought of as fundamental properties in the sense that other reservoir prop-
erties are dependent on them. Properties such as porosity, permeability, and bulk
density are dependent or derived properties. Yet another set of properties is encoun-
tered in the study of carbonate reservoirs: third order or tertiary properties . Tertiary
properties include electrical resistivity and conductivity, acoustic transmissivity,
natural radioactivity, and the various attributes measured by most wireline logs,
gravity meters, magnetometers, and the seismograph. Those characteristics depend
on porosity, fl uid content, radioactive element content, rock density, magnetic sus-
ceptibility, and acoustic characteristics, none of which are measures of fundamental
rock properties. In describing rock properties, the words primary, secondary, and

tertiary have no time significance and they may not be related to deposition, dia-
genesis, or mechanical fracturing. It is preferable to describe rock properties as
fundamental, dependent, and tertiary.

2.2 FUNDAMENTAL ROCK PROPERTIES

Fundamental properties of carbonate rocks include texture, fabric, grain type, min-
eralogical composition, and sedimentary structures. Note that texture and fabric are