Page 24 - Geology of Carbonate Reservoirs

DEFINITION OF CARBONATE RESERVOIRS 5

optimum extraction of the resource, (4) determine the practicality of drilling addi-

tional (infill) wells to achieve the optimum spacing between field wells during

development, and (5) predict the path that will be taken by injected fluids as they
“ sweep ” remaining hydrocarbons during secondary and enhanced recovery. In the
broad sense, reservoir studies include reservoir geology, reservoir characterization,
and reservoir engineering. To avoid confusion in terminology about carbonate res-
ervoirs, some common terms are discussed in the following paragraphs.
Reservoir geology deals with the origin, spatial distribution, and petrological
characteristics of reservoirs. The reservoir geologist utilizes information from sedi-
mentology, stratigraphy, structural geology, sedimentary petrology, petrography, and
geochemistry to prepare reservoir descriptions . Those descriptions are based on both
the fundamental properties of the reservoir rocks and the sequence of geological
events that formed the pore network. Data for these descriptions comes from direct
examination of rock samples such as borehole cores and drill cuttings. Borehole
logs and other geophysical devices provide useful information, but they are indirect
measurements of derived and tertiary rock properties. They are not direct observa-
tions. Direct observations of depositional textures, constituent composition, princi-
pal and accessory minerals, sedimentary structures, diagenetic alterations, and
pore characteristics provide the foundation for reservoir descriptions. The geologi-
cal history of reservoir formation can be traced by interpreting depositional, diage-
netic, and tectonic attributes. The goal of such interpretations is to formulate
geological concepts to guide in predicting reservoir size, shape, and performance
characteristics. In the absence of direct lithological data from wells, as in the case
of frontier exploration and wildcat drilling, geologists commonly study nearby out-
crops of the same age and geological formation as the expected reservoir. A measure
of care is given to interpreting reservoir geology from distant outcrops because

depositional and diagenetic characteristics may vary significantly from place to
place and from outcrops that have been altered by surface weathering to subsurface
reservoirs that have never been exposed to weathering.
Reservoir characterization , like reservoir geology, deals with physical character-
istics of the reservoir. It differs from geological description in that data on petro-

physics and fluid properties are included. In addition to data from direct examination
of reservoir rocks, reservoir characterization involves interpretation of borehole
logs, porosity – permeability measurements, capillary pressure measurements, reser-

voir fluid saturations, and reservoir drive mechanisms.
Reservoir engineering deals with field development after discovery. The main goal

of the reservoir engineer is to optimize hydrocarbon recovery as part of an overall
economic policy. Reservoirs are studied throughout their economic lives to derive
the information required for optimal production. In addition to geological data and
borehole log characteristics, reservoir engineering deals with reservoir pressures,
oil – water saturation, and gas – oil ratio in order to provide estimates of in - place
hydrocarbon volumes, recoverable reserves, and production potential for each well
in a field (Coss é , 1993 ).

Petroleum geoscientists not only study reservoirs, but they also study traps, seals ,
and source rocks that make up most of the petroleum system described by Magoon
and Dow (1994) . Traps are bodies of rock where hydrocarbons accumulate after
migrating from their source and are restricted from further movement. It is conve-
nient to think of traps as large - scale geometrical features that form boundaries