Page 227 - Geology of Carbonate Reservoirs

208 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

making geological interpretations. At the end of the day, however, it is full - diameter
cores that are the best sources of information about carbonate rock and pore
characteristics.
Wireline logging tools and the seismograph are limited in what they can detect
at reservoir scale. Wireline logging tools are the primary source of data for strati-
graphic correlation, subsurface geological mapping, and petrophysical calculations

when core analyses are absent. In fields where lithological logs and rock samples
are limited or unavailable, even vintage wireline logs can be digitized and processed
with powerful new computers and software that can calculate estimates of lithology,
porosity, and saturation. Although these old logs provide useful information about
reservoirs, they are not reliable substitutes for direct observation of rocks because
the logging tools do not make direct measurements on minerals, porosity, or satura-
tion. They measure other petrophysical properties that are used as proxies for min-
eralogy, porosity, and saturation. Powerful software algorithms are also limited by
the number of minerals that can be distinguished in complex, multimineral rocks

(typically three in older software and up to five in new - generation versions using
modern, digital log data).
Some field scale reservoir zones may be thick enough to register on seismic pro-

fi les, but most are not. Some fractured zones can be detected by their seismic attri-
butes, but many fractured horizons are too thin to be detected with refl ection

seismology. Impedance contrasts are necessary for a reflection of the seismic impulse
to occur. In many carbonate reservoirs, impedance contrast is not great enough for
the seismograph to detect differences between reservoir and nonreservoir rock. It
is necessary in such cases to have as much information as possible about the reser-
voir rocks. Cores, cuttings, or lithofacies extrapolations from nearby outcrops can
provide helpful information. In short, the most accurate results on field scale reser-

voir characterization studies derive from observation of borehole cores or cuttings
worked in concert with log and seismic data. Of course, after drilling, production

and well test data are very useful for reservoir definition and flow unit evaluation.

8.2.3 Quality Ranking of Flow Units

Flow units, baffles, and barriers are field scale features. Information required for

identifying, describing, and quality ranking flow units includes lithological, petro-
graphic, and petrophysical data. Ideally, these kinds data should be evaluated
together to detect and interpret correspondences between them. Lithological data
from borehole cores provides detailed descriptions of fundamental rock properties,
diagenetic alterations, visible porosity, oil staining, and marker fossils for correlation,
and even small interbedded “ tight ” zones that may be baffles or barriers to fl ow.

Recognizing these small interbeds sometimes aids in depth shifting between cores
and logs and may provide markers for interwell correlation within fi elds. Petro-
graphic data from thin section descriptions is used to identify constituent composi-
tion, rock texture, pore types, pore geometry, and diagenetic alterations. Petrographic
data can be compared with core descriptions and petrophysical data, along with
production or flow tests, to identify and rank the quality (combined values of poros-

ity, permeability, and capillary pressure characteristics) of flow units. Wireline log

signatures sometimes correspond with rock properties in flow units so that ranked
fl ow units can be mapped from log character, petrophysical, and petrographic data