Page 200 - Geology of Carbonate Reservoirs

FRACTURES AND FRACTURED RESERVOIRS 181

Figure 7.5 Photo of a microbial bioherm in the Wilberns Formation (Cambrian) of Central
Texas showing differential compaction that has caused fracturing in bedded rocks immedi-
ately above and below the bioherm. This type of fracturing is commonly associated with large

reef and mound buildups. (Photo by Richard Rezak in Ahr (1971) .)
Surface - related fractures are created by unloading stresses. Quarrying and weath-
ering, for example, may remove stabilizing masses and create instability that leads
to collapse of quarry walls. Surface - related fractures are not considered to be impor-
tant in hydrocarbon reservoirs.
Fractures can also be produced by differential compaction at the local scale, such
as compaction of strata above buried topography. Antecedent features such as reefs
and mounds, shelf edges, erosional outliers, or horst blocks could cause overlying
beds to drape, extend, and fracture in patterns related to the size and shape of the
antecedent topographic feature. For example, brittle fractures have been docu-
mented by this author in the basal 5 – 20 meters of Mississippian “ mud mounds ” in
the Williston Basin of North Dakota, where thin (1 – 15 cm thick) mudstone and

cementstone beds appear to have been fractured by the overburden load of the
mature mound (up to 150 meters thick). Differential compaction of carbonate strata
above and below resistant mounds and reefs is common. An excellent example is
illustrated by a Cambrian microbialite mound in Central Texas (Figure 7.5 ), where
the beds above have draped (extension) over the mound and the beds below have
been compressed (compaction) by the concentrated overburden. Larger - scale dif-
ferential compaction fractures can occur along buried shelf edges, large reefs, ero-
sional remnants, and fault blocks.

7.1.4 Fracture Morphology

In fractured reservoirs, total porosity and permeability values consist of both matrix
and fracture components. In order to assess the importance of fracture porosity and
permeability to total reservoir performance, it is necessary to determine the relative
contributions of matrix porosity and permeability as compared to fracture porosity
and permeability. Nelson (2001) ranks the four most useful petrophysical determina-

tions to make on fractured reservoirs (in order of increasing difficulty of calculation)