Page 222 - Geology of Carbonate Reservoirs

ROCK PROPERTIES AND DIAGNOSTIC METHODS 203

ties of the rock – fluid system and shales tend to have less deflection from the log

baseline than coarser grained sections that have bigger fluid - filled pores. Mineralogi-

cal composition is used to classify sandstones but not carbonates. Carbonate rock

classification is based on grain type and depositional texture. Mineralogy may be
strongly correlated with porosity in carbonates but it has much less infl uence on
sandstone porosity. Sedimentary structures and biota can only be determined with
complete certainty by observing borehole cores. Sedimentary structures provide

clues to the hydrodynamics and directions of flow in ancient environments in both
terrigenous sandstones and carbonates. In some cases, image logs and sensitive
dipmeters can detect larger sedimentary structures such as large - scale crossbedding
in dunes. Fossil content is arguably more important for interpreting depositional
environment in carbonates than in terrigenous sandstones probably because most
carbonates form in marine environments where fossil assemblages can reveal subtle
differences in depositional settings. Diverse assemblages of fossils indicate favorable
environment for life. Low diversity indicates a stress environment such as a hyper -
or hyposaline lagoon, low oxygen content, or some other limiting factor on life. Low
diversity is rarely associated with grain - supported or reef rocks; therefore low diver-
sity can be a negative indicator for depositional porosity in reservoir rocks. There
are exceptions. Low diversity but very high abundance of a few tolerant species can
result in rocks composed of huge quantities of only one or two fossil species. One
example of this is the salt - tolerant bivalve Fragum hamelini of Shark Bay, Australia
(Logan et al., 1970 ). It is present in vast numbers in an environment that few other
organisms can tolerate.

8.1.2 Reservoir Morphology

Anatomy of the depositional unit — reservoir architecture — is a fundamental, depo-
sitional characteristic that is so important that it deserves a separate heading. In
depositional reservoirs, it represents the spatial distribution of both depositional
facies and their attendant porosity. The 3D morphology, the size, shape, and orienta-
tion with respect to depositional dip of siliciclastic reservoirs, can be predicted by
using idealized depositional models such as those described by LeBlanc ( 1972 ).
These standard models range from alluvial fans to deep - sea turbidites and they have
been refined in most recent literature to include vertical profiles of the typical tex-

tural, compositional, bedform, and petrophysical characteristics for each facies. The
method assumes that reservoirs consist exclusively of intergranular, depositional

porosity and that depositional architecture can be defined by choosing the best
look - alike (analog) from the catalog of known examples. Adjustments for differ-
ences between the actual reservoir and the look - alike are usually adjustments in
scale of the depositional unit rather than in its morphology. Validation of the look -
alike as a model for the real example is commonly done by comparing gamma ray
and resistivity log patterns to standard shape templates (generating electrofacies
maps), and this method can be quite accurate when used in conjunction with geo-
physical information about basin architecture and sequence stratigraphy.
The seven standard depositional successions for carbonate ramps and shelves
described in Chapter 4 can be used as aids in constructing depositional models for
carbonate sequences. Lithologic logs, especially those based on direct observation
and description of borehole cores, can reveal which of the seven depositional