Page 259 - Geology of Carbonate Reservoirs

240 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

Type I. Fractures provide essential reservoir porosity and permeability.
Type II. Fractures provide essential permeability.
Type III. Fractures assist permeability in an already producible reservoir.
Type IV. Fractures provide no additional porosity or permeability but they do

impose significant anisotropies such as barriers to flow.
Fracture types I and II are probably the categories that are commonly thought of
as true fractured reservoirs. In those cases fractures may provide most of the perme-
ability or porosity — or both — to the reservoir. Types III and IV are also common

but do not provide significant amounts of porosity and permeability. Instead, they

may offer a moderate assist, as in type III, or even have a negative influence if they
act as baffl es or barriers to fl ow, as in type IV reservoirs. Type III and IV fractures
include healed fractures such as those with gouge, mineral cements, stylolitized
fractures, and discontinuous, small fractures that do not improve permeability. Addi-
tional consideration must be given to the relationship between fractures and the in
situ stress in the borehole. Rocks may undergo several periods of fracturing such
that older fracture sets are cut by younger ones. In those cases, the fracture set that
is parallel to the present - day, maximum principal stress is the one most likely to be
open and contributing to porosity and/or permeability. Finding this fracture set
depends on measuring the in situ stresses within the borehole and identifying the
fracture set that is aligned parallel to the principal stress in that regime.

8.5.2 Field Examples of Fractured Reservoirs

Fractured reservoirs make up about one - fifth of the fields listed by Scott et al. ( 1993 )

and Roehl and Choquette ( 1985 ). North ( 1985 ) has an extensive list of examples of
fractured reservoirs that include the Asmari Limestone (Tertiary) fields in Iran,

Mesozoic carbonates in the Lake Maricaibo area of Venezuela and the Yucat á n area
of Mexico, Paleozoic carbonates of West Texas, and Mesozoic and Tertiary chalks
of the North Sea and North America, among others. Lorenz et al. ( 1997 ) describe
fractured carbonate reservoirs in mildly deformed, moderately deformed, and
severely deformed settings. The two examples of fractured reservoirs selected for
inclusion here were chosen because this author worked on both of them and obtained

first - hand information about them. These examples are not ideal, however, because
in both cases the original geological concept was not built around a search for frac-
tured reservoirs. Also, the degree to which natural fractures infl uenced reservoir
performance was not known until after the discovery wells were completed, but that
is the usual case in early exploration drilling. The exploration wells in both of these
examples were not targeted at fractured reservoirs, but rather at “ mounds ” or
“ reefs ” in Carboniferous strata in Texas and the Williston Basin of North Dakota.
Such carbonate buildups are usually rather easy to recognize on seismic profi les.
Only after studying cores, cuttings, logs, and other data such as well tests was the
extent and significance of fracturing fully realized. The number of successful explo-

ration wells that have been targeted — by design — at naturally fractured rocks, as
compared with fractured reservoirs discovered by accident, is an interesting
unknown. In the examples presented next, both cases exhibit mixtures of deposi-
tional, diagenetic, and fracture porosity, but it is fractures that have the dominant
influence on reservoir performance.