Page 268 - Geology of Carbonate Reservoirs

CONCLUSIONS 249

only after the failure of the first well to produce from a deeper objective zone. There
are lessons to be learned from this experience. Note that some of the reasons for
testing the Lodgepole zone included problems with lost circulation. If close study
had been made on drilling time, mud circulation, caliper and acoustic log character,
and especially on the presence of saddle dolomite in cuttings, the mound interval

would probably have been identified immediately as a fractured carbonate reser-

voir. Whole core and pressure tests confirmed that the reservoir is fractured. Image
logs could have been much more useful had they been studied more extensively in
conjunction with whole cores to identify fracture density, spacing, and spatial
orientation.

8.6 CONCLUSIONS

We have focused on fundamentals of geology related to carbonate reservoirs and
on the application of those fundamentals in exploration and development. Much of
the information in this book comes from the author ’ s more than forty years of
experience with carbonate rocks and reservoirs in both industry and academia. The
hope is that this book will be useful to geoscientists and engineers who work with
carbonate reservoirs and aquifers, and especially that it will open new vistas for
university students. A few main points about carbonate reservoirs bear repeating as
we conclude. Carbonate reservoirs are rock bodies but they do not necessarily

conform to stratigraphic boundaries because reservoirs are defined by porosity and
permeability. In strong contrast to sandstone reservoirs, porosity and permeability
in carbonates can be independent of depositional facies or formation boundaries,
as exemplified by diagenetic and fracture porosity that cut across depositional facies

boundaries. Many carbonate reservoirs have pore systems that formed long after
sedimentation. Removal or replacement of original rock texture and fabric can
create pore characteristics that did not exist at the time of deposition. In siliciclastic
sandstone reservoirs, diagenesis such as cementation or authigenic clay formation
may reduce porosity, but rarely does diagenesis increase porosity. If the number of
papers about fractured reservoirs in sandstones as compared to carbonates is any
indication, fractured reservoirs are less common in sandstones than in carbonates.
Because porosity in carbonate reservoirs and aquifers is not always the result of a
single geological process and because porosity in carbonates can undergo repeated
episodes of change during burial, we developed a simplified genetic classifi cation of

porosity to focus on the three end - member pore types; namely, depositional, diage-
netic, and fracture pores along with their respective hybrids.
Geoscientists and engineers must recognize pore types by origin so that they can
design exploration and development projects around geological concepts that take
pore origin into account as one of the methods for correlating at reservoir scale. It
is far too risky to plan a strategy simply around depositional facies, stratigraphic
units, or present - day structural anomalies. Recognition of genetic pore types enables
one to develop concepts to exploit diagenetic reservoirs where porosity and perme-
ability are related to ancient water tables or episodes of rock – water interaction
during later burial. Similarly, one can exploit fractured reservoirs that conform to
the geometry of tectonic faults and folds or in situ stresses rather than to stratal
geometries. Recognizing carbonate pore types by origin requires direct observation