Page 195 - Geology of Carbonate Reservoirs

CHAPTER SEVEN

FRACTURED RESERVOIRS

7.1 FRACTURES AND FRACTURED RESERVOIRS

This chapter covers fractured carbonate reservoirs — how they form, where and why
they form, what characteristics make them different from depositional and diage-

netic reservoirs, and finally the basic methods for identifying, mapping, and exploit-
ing them.
Working with fractured reservoirs is very different from working with deposi-
tional and diagenetic reservoirs and it requires an adjustment in ways of thinking.
Fractures are generally unrelated to depositional or diagenetic rock properties
because fractures are produced by mechanical stresses after the rocks have been

lithified. Depositional or diagenetic processes are long - since over by the time the
fractures form, although the fractures could be affected by later dissolution or
cementation. It is also possible for fractures to be linked with depositional or dia-
genetic rock properties to form hybrid pore types. Diagenetic - fracture hybrids can
be confused with purely fractured reservoirs unless cores or image logs are exam-
ined. Vuggy, cavernous, channeled, or otherwise connected dissolution - diagenetic

porosity may exhibit petrophysical and flow characteristics that mimic fracture
porosity and permeability, making it more difficult to separate hybrids from purely

fractured reservoirs without direct observation of borehole cores or image logs.

Above all, finding fractures requires knowledge of how they formed so workers can
narrow the search to areas where fracture - forming processes are known or sus-
pected to exist. Fractures may be associated with tectonic features such as folds,

faults, regional flexures, or localized zones of differential compaction around buried
shelf margins, large reefs and mounds, or other prominent and resistant bodies. More
than one generation of fractures may be present in a single reservoir, although most

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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