Page 125 - Geology of Carbonate Reservoirs

CHAPTER FIVE

DEPOSITIONAL CARBONATE

RESERVOIRS

This chapter focuses on depositional porosity, depositional environments, and depo-
sitional rock properties. Different depositional environments that exist across ramps
and shelves produce distinctive depositional successions. The fundamental rock
properties that characterize each of these depositional successions can be synthe-
sized to create seven ideal depositional successions . For ease in visualization, each
ideal depositional succession is described and illustrated to resemble borehole cores.
Most successions represent shallowing - upward deposition and most of them can be

divided into several parts (the microfacies defined earlier) to enhance their indi-

vidual character and make it easier to identify them in the field or in borehole cores.
Review questions on volumetric calculations, on exploration strategies, and on the

best methods for field development in different facies are included at the end of the
chapter and selected case histories of depositional reservoirs are presented in
Chapter 8 .
Depositional reservoirs are those in which reservoir porosity is the result of
depositional processes only. Older literature might classify this kind of porosity as
“ primary. ” Recognizing depositional reservoirs depends on the ability to distinguish
depositional porosity from diagenetic and fracture porosity, along with the various
hybrids of end - member pore types that are usually present. Purely depositional
porosity is rare but depositional texture, fabric, or mineralogy usually have so much

influence on diagenesis that altered reservoirs can be exploited as if they were gov-
erned by depositional porosity only. For example, it is common to fi nd reservoirs
with fabric - selective, texture - selective, or facies - selective porosity and permeability
even though the rocks have been altered by diagenesis. These are hybrid reservoirs
because the pore characteristics reflect both depositional and diagenetic infl uences.

In order to confirm that hybrid reservoirs have both depositional and diagenetic

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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