Page 215 - Geology of Carbonate Reservoirs

196 FRACTURED RESERVOIRS

monoclinal flexures over buried shelf edges or buried reef trends. Seismic

attributes may provide indications of regional fractures in low - dipping beds.
On a smaller scale, differential compaction fractures tend to be localized
around antecedent highs such as horsts, buried mounds, or erosional outliers.
2. After the presence of fractures has been confirmed, determine whether they

are related to present structure or paleostructure. Using image logs or cores,
if available, determine fracture orientation, spacing, and intensity with respect
to structural geometry. Determine whether fracture spacing and intensity have
any relationships to rock and stratigraphic properties such as mineralogical
composition, grain or crystal size, bed thickness, or bulk - rock porosity.
3. Evaluate well test and production data to determine the extent to which frac-

tures influence reservoir porosity and permeability so you can classify the
fracture system according to Nelson ’ s (2001) four categories. Develop strate-
gies that incorporate the positive and negative characteristics of each of the
four fractured reservoir categories. Examples of some positive attributes and
common problems associated with the different types of fractured reservoirs
are listed in Tables 7.1 and 7.2 .
4. Using data from at least one well, refine the geological concept. By this stage,

the types of fractures have been classified, their relationship to paleostructural

or present structural geometry is known, and the rock and stratigraphic char-
acteristics of the fractured zones are known. Utilize observations from cores
or image logs to refine estimates of fracture spacing, intensity, width, and ori-

entation. Incorporate these refined estimates in volumetric calculations and

future development planning.

TABLE 7.1 Positive Attributes of Fractured Reservoirs Types I – III
Type I Fractures: Type II Fractures: Type III Fractures:
Fractures Provide Essential Fractures Provide Fractures Provide
Porosity and Permeability Essential Permeability Permeability Assist
Large drainage areas per well Can develop in low - Reserves are dominated
permeability rocks by matrix properties
Few wells needed for fi eld May have unexpectedly Reserve distribution
development high well rates may be relatively
homogeneous
Good correlation between well Hydrocarbon charge is High sustained well
flow rates and well reservoirs commonly facilitated rates

by fractures
Best wells are commonly early Excellent reservoir
continuity
High initial potential (IP)
common
Can produce from nonreservoir
quality and nonstandard rocks
Source : Adapted from Nelson (2001) .