Page 258 - Geology of Carbonate Reservoirs

FRACTURED RESERVOIRS 239

the best quality reservoir rock as is usually expected. Instead, the partly muddy rocks
ultimately made the best reservoir.
This study offers several take - home lessons: (1) distal segments on carbonate
platforms can be the sites of shallow - water deposition during sea - level low stands;

(2) significant buildups may occur on insignificant paleostructural features like the

original (pre - slump) terrace at Happy Field; (3) multiple episodes of diagenesis may
obscure relationships between poroperm values and fundamental rock properties;

(4) multiple diagenetic overprints that are difficult to tie to specific events or envi-

ronments make predicting the spatial distribution and quality rank of fl ow units
very difficult; and (5) stratigraphic slice maps to “ take apart ” the depositional and

diagenetic facies in carbonate reservoirs help unravel complex reservoir attributes
to aid in constructing 3D flow unit models.

8.5 FRACTURED RESERVOIRS

Nearly all carbonate reservoirs have been fractured to some extent. That is the
essential point: they are fractured to some extent. Whether fractures have a great
influence on reservoir characteristics is a separate matter. Nelson ’ s ( 2001 ) classifi ca-

tion of fractured reservoirs points out that Type IV fractures have a negative infl u-

ence on reservoir characteristics. Nelson ’ s classification is a prerequisite for working
with fractured reservoirs because until the presence of fractures has been confi rmed

and the reservoir has been tested, it is difficult, if even possible, to evaluate the
extent to which fractures contribute to reservoir performance.

8.5.1 Finding and Interpreting Fractured Reservoirs
Exploration for fractured reservoirs usually focuses on tectonic features or regional
flexures where fractures are known or suspected to be present. Fractures that gener-

ally accompany folds, faults, structural hinge lines, or monoclinal flexures were dis-

cussed in Chapter 7 . Structural features are relatively easy to identify with modern
seismic methods. Knowing the kinds of fractures that commonly occur on specifi c
parts of structures, it is possible to target them. For example, the hanging wall of
normal faults usually has the highest fracture intensity, and the types and orienta-
tions of fractures that typically occur in folds can be predicted if the orientation of
the principal stresses is known. In cases where the quality of seismic data is particu-
larly good, it may be possible to directly identify fractures from seismic refl ection
character or seismic attributes. Unless fractures are already known to occur in a
given area, or they are suspected to be present in buried structures, or some hint of
them can be identified in seismic data, finding them in the subsurface is more a

matter of chance than of skill.
Once fractured rocks are found, it is necessary to test the reservoir to assess the
extent to which fractures influence reservoir performance. It is possible to evaluate

the importance of fractures to reservoir performance by well testing, the use of
imaging logs, drilling time and caliper logs, various porosity tools, and methodical
study of borehole cores or cuttings. Nelson ( 2001 ) designed his classifi cation
of fractured reservoirs directly with this in mind. Recall his classifi cation from
Chapter 7 :