Page 226 - Geology of Carbonate Reservoirs

DATA REQUIREMENTS 207

reservoirs based on genetic pore type), to pore scale (associations of pore types, or
pore facies, and their corresponding petrophysical characteristics).

8.2.1 Regional Scale Investigations
Regional studies depend mainly on outcrop studies, syntheses of subsurface geo-

logical data, geophysical data such as reflection seismic profiles, seismic attribute
analyses, and gravity or magnetic surveys. Outcrop and subsurface geological data
are especially valuable if available, because they provide direct measurements and
observations on lithology, wireline log character, petrophysical attributes, and nature
(size and shape) of the reservoir trap – seal system. Seismic profiles are commonly

studied to determine platform geometry, to determine structural anomalies, and to
enable interpreters to generate seismic and sequence - stratigraphic models that aid
in generating geological concepts for drilling. Seismic attributes can be helpful to
identify fractured reservoirs and possibly other pore systems that have distinct
acoustic signatures detectible by the seismograph. Gravity and magnetic anomaly
maps are used mainly to locate structural anomalies or potential traps. Interpreted
regional seismic lines with other geophysical or geological information super imposed
can provide basin and platform models for field scale investigations.

8.2.2 Field Scale Studies

Successful drilling that finds commercial quantities of oil or gas leads to more wells
being drilled until the boundaries of the productive unit are established. The reser-

voir or reservoirs within these boundaries comprise a field. Field scale investigations
and reservoir characterization involve detailed stratigraphic correlations to outline
the anatomy of the depositional, diagenetic, or fracture system that makes up the

reservoir. They involve identification of structural or paleostructural characteristics

that may have influenced deposition, diagenesis, and fracturing. And they involve

identification and characterization of depositional and diagenetic facies or fracture
systems that determine reservoir quality. Most of this information must be obtained
from direct examination of borehole cores. Lithological logs from cuttings, when
used in conjunction with wireline log and seismic data, can sometimes provide

enough information for successful field development. However, even the best
description of cuttings cannot detect subtle changes in lithology in stratigraphic
units less than 10 feet thick because cuttings are collected at 10 - foot intervals during
drilling. Each sample of millimeter - sized (or smaller) cuttings represents a mixture
of every rock type penetrated in the 10 - foot interval. The geologist who prepares
lithological logs from cuttings must know the local stratigraphic section rather well
in order to detect changes in lithology from bag to bag of cuttings without confusing
contamination ( “ cave ” ) from higher in the borehole with in situ rock. In general,
lithological logs from cuttings provide useful information about stratigraphic “ tops ”
but they are usually less useful as sources of data on reservoir characteristics. In
cases where dolomite crystals, ooids, or fragments of distinctive reef organisms are
recognizable in cuttings, they are very useful. Cuttings sometimes contain crystalline
cements, exotic minerals, or bits of slickensides (grooves formed by differential slip

along polished, gouge - filled fracture surfaces). These features can be helpful in