Page 203 - Geology of Carbonate Reservoirs

184 FRACTURED RESERVOIRS

fillings composed of soluble material like calcite may later be dissolved by under-

saturated fluids that migrate through the fracture system. Calcite healed fractures
are common in many rocks, not just carbonates. Fractured shale, mudrocks, and
other siliciclastics commonly exhibit calcite cement that partially or completely fi lls
fractures. A common mineral filling in fractured carbonate reservoirs is saddle

dolomite, which crystallizes at elevated temperature in burial fluids. In many cases,
the presence of saddle dolomite is so strongly correlated with fractures that its pres-
ence in well cuttings can be used as a fracture indicator (Ahr, 1982 ). It is also com-
monly associated with lead – zinc ore mineralization in veins that are essentially fi lled
fractures. Examples of this association occur in the Tri - State District of the United
States and in the lead mining district of the Irish Midlands. Fracture filling by sul-

fides, particularly lead minerals and fluorite, have been exploited since Roman times
in the Peak District of central England. Evaluating reservoirs with mineralized
fractures requires knowledge of both the origin and orientation of the fracture
system and the diagenetic system that precipitated or subsequently dissolved the
mineral fi llings.
Vuggy fractures are not a separate fracture category; instead, they are enlarge-

ments within and along natural fractures where undersaturated fluids have removed
the rock matrix, or matrix plus healing cements. This type of dissolution diagenesis
typically produces centimeter - sized (or larger) ellipsoidal to spherical or subspheri-
cal vugs that are elongate parallel to the fracture planes. The vugs greatly increase
porosity in comparison with ordinary, nonvuggy fractures. In addition, the vuggy
nature of the fractures is not easily compacted during pressure drawdown, so the
fractures tend to remain open. According to Nelson (2001) the supergiant Asmari
Field in the Middle East is one of a large number of giant fields that produce mainly

from vuggy fractures. Evaluating vuggy fracture porosity requires understanding of
the origin and orientation of the open fractures as well as understanding the diage-
netic processes that formed the vugs. Most vuggy fracture systems are enlarged by
water percolating from unconformities or shallow aquifers, although some expulsion

fluids migrating upward could also dissolve fracture walls. Other vuggy fractures are
related to karst systems. Once the presence of vuggy fractures is confirmed, it is

necessary to determine the relative timing of dissolution diagenesis with respect to

other forms of diagenesis that may influence reservoir performance. Timing is also
important if more than one fracture set is present. Once the relative timing of vug -
forming dissolution has been established, it is important to locate possible sources,
or entry points, for the dissolving fluids because that will be the zone with the highest

concentration of vugs. Unconformities and karst zones commonly show up as spikes
on gamma ray logs and they are typically associated with major breaks in the strati-
graphic record such as sequence boundaries or abrupt changes in bedding character.
Some unconformities and karst surfaces may also be visible on image logs.

7.1.5 Where Do Fractures Occur?
Natural fractures are present in carbonates in settings that vary from undeformed,
near horizontal beds to fold and thrust belts (Lorenz et al., 1997 ). Those writers
describe fractured carbonate reservoirs in mildly, moderately, and severely deformed
strata. Fractures in mildly deformed beds include regional fractures. There has been
controversy over the origin of regional fractures, but Rhett (2001) offers a convinc-