Page 26 - Geology of Carbonate Reservoirs

FINDING AND DEVELOPING CARBONATE RESERVOIRS 7

drill cuttings, logs, well tests, production data, and geophysical measurements.
Modern reservoir geological studies are much more than geological descriptions of
reservoir rocks. They include detailed research on the origin and spatial distribution
of effective porosity, how it is connected through pore throats, and how rock and

pore properties infl uence fluid storage and transmission.
In the following chapters, depositional, diagenetic, and fractured reservoirs are
described and interpreted to illustrate methods of critical analysis of reservoirs
rather than offering case histories that may be applied uncritically as analogs, or
“ look - alikes, ” for exploration and development programs. Petrophysical as well as
petrological and structural characteristics of carbonate reservoirs are described,
borrowing from both reservoir characterization and reservoir engineering. The
study of fl ow units that make up all or part of a reservoir is emphasized throughout

the book. Flow units are the rock bodies that transmit fluids, to a greater or lesser
degree, through connected porosity. Identification and mapping of flow units requires

knowledge of reservoirs in three or four dimensions (porosity in three dimensions

plus a fourth dimension that may include connectivity or fluid saturation, for
example). Such knowledge involves understanding the timing and mode of origin
of the pore systems. Mapping fl ow units also requires a basic knowledge of subsur-
face mapping techniques and methods for evaluating reservoir connectivity. In the
end, accurate and realistic reservoir models to provide the basis for fi nding new

reservoirs and developing existing ones can be constructed from flow unit maps.
I hope these discussions will stimulate analytical and synthetic thinking that
will enable you to evaluate each reservoir on its petrological and petrophysical
characteristics — not on its similarity to a “ look - alike. ” Finding and developing car-
bonate reservoirs requires critical analysis and interpretation of essential geological
data. Some of the kinds of data used in reservoir studies are mentioned in the fol-
lowing section.

1.2.1 Sources of Data on Reservoirs

Carbonate reservoirs may have mixed - origin pore systems, but an understanding of
the end - member categories, how and when they formed, and how they are con-
nected is necessary before a geological concept for exploration or development can
be created. The most important data for reservoir studies comes from direct exami-
nation of rock samples. Basic rock properties studied in cores and cuttings include
texture, mineral and grain composition, fossil content (taxonomic diversity), and
sedimentary structures. These properties provide data on depositional characteris-
tics. Petrographic study provides data about the diagenetic history of both the rock
matrix and the pore system. Microscopic descriptions of borehole cores also provide
data on the presence or absence of through - going, natural fractures. Porosity and
permeability are measured from cores and porosity can be calculated from appro-
priate borehole logs. The statistical relationship between porosity and permeability
is commonly used to distinguish between pore systems of different geological origins.
Fractured reservoirs may exhibit high permeability at comparatively low porosity,
for example. Another important kind of data is capillary pressure measurements.
These measurements on cores provide data on the behavior of fluids in the reservoir

pore system. Plots of capillary pressure at different fluid saturations within the

reservoir rock sample, or capillary pressure curves, are related to pore throat size