Page 110 - Geology of Carbonate Reservoirs

ANATOMY OF DEPOSITIONAL UNITS 91

depositional anatomy. In this case, both depositional anatomy and diagenetically

produced microporosity were influenced by antecedent topography. The present

structural configuration of the area has been modified by syn - to - post - diagenetic salt

doming. Oolite sand bodies on crests of antecedent structural highs around the
perimeter of a buried basement feature are altered and productive; those not on
that paleo - high are unaltered and tight. Although older oolite strata in one of the
wells are presently high on a salt structure, they are nonporous and were not dia-
genetically altered to become microporous. This apparent paradox is resolved by
understanding that the microporosity in the productive, younger oolites was formed
shortly after deposition above the relatively static, basement high and before salt
doming lifted the older oolites to their present - day position. These subtle differences
in sand body anatomy associated with antecedent topography can be mapped to

facilitate optimum field development. The cause – effect relationships between topog-
raphy, localized deposition of oolites, diagenesis, and reservoir porosity provide the
foundation of a geological concept to use in exploration for additional prospects.

4.4.1 Facies, Successions, and Sequences

The word facies derives from the Latin facia , meaning face, countenance, or exterior
appearance. In geology, facies is mainly used to describe the fundamental rock
properties that characterize depositional units smaller than member rank. It is
purely a rock - stratigraphic term with no time value. Metamorphic rocks may be

classified into different metamorphic facies on the basis of the mineral assemblages
they contain. Biofacies are determined on the basis of fossil content. In sedimentol-
ogy, facies are determined on depositional rock properties such as texture, constitu-
ent composition, and sedimentary structures. Sometimes the differences between
sedimentary facies are not obvious to the unaided eye. Thin section petrography
may be used in such cases to identify differences in microscopic rock properties
such as constituent percentages, microscopic pore characteristics, or biological

microstructures. Facies defined on petrographic data are called microfacies . Exten-
sive discussions of microfacies analysis can be found in Flugel (1982) , and some
pioneering applications of thin section petrography to establish facies boundaries
in modern carbonate sediments include Ginsburg (1956) , Purdy (1963a) , and Logan
et al. (1969) . In his study of modern Bahamian carbonates, Purdy (1963a) deter-
mined microfacies boundaries by applying a statistical procedure called factor anal-
ysis to constituent point - count data. The resulting “ reaction groups ” provided
objective, but rather abstract, microfacies boundaries. In general practice, facies
boundaries are determined on estimated percentages of constituents, general
textural trends, and sedimentary structures.
Facies, or more precisely lithofacies, form in depositional environments where
hydrological and biological processes create genetically distinct associations of
texture, constituent composition, and sedimentary structures, or lithogenetic units .
In carbonate reservoirs, those rock properties are generally not distinguishable with
wireline logs; consequently, the reservoir geologist must examine cuttings or cores
to accurately identify carbonate facies. If the process – response mechanisms that
produced the depositional facies are known, then it is possible to work backward
from data on rock properties to interpret both depositional environment and
anatomy of the depositional body. Sometimes facies are named for the environment