43
Classification
Energy’s tertiary-oil-recovery information system (Toris). This resulted in the classification and
description of 2300 light-oil reservoirs (greater that 20°API), collectively containing 308 billion bbl
of original oil-in-place (OOIP) (Ray et al., 1991).
The reservoirs fell within 173 combinations defined by their lithology, depositional environment,
structural deformation, and diagenetic overprint. These groups were then collapsed into a smaller
number of classes while maintaining meaningful descriptions of the processes that produce reservoir heterogeneity at the interwell scale. Using a basic classification system relying on lithology and
depositional environment, with subclasses reflecting post-deposition processes, would suffice. As
a result, 22 geologic classes, 16 clastic, and 6 carbonate, with structural subclasses for clastic and
diagenetic subclasses for carbonates, were defined.
Sixteen clastic classes (Figure 2.2) were derived from 28 clastic depositional systems described
in the classifier. These classes contain siliciclastic rocks deposited in the paleo-environmental setting indicated by their names. While some are fairly uniform environments, such as those of the
eolian class, others are complex, as in the various deltaic environments.
For most reservoirs, relatively refined description of the depositional processes was possible
(e.g., fluvial-dominate deltas), but for some, the unavailability of data or the complexity of the depositional processes required broader, undifferentiated classes (e.g., fluvial, strand plain, and delta).
Heterogeneity due to the post-deposition structural and diagenetic history of a reservoir can have
an overriding influence on the flow of oil and other fluids. These descriptive reservoir modifiers
provided the basis for defining subclasses.
In clastic reservoirs, variations in types of structural controls on heterogeneity relative to diagenesis suggested that clastic reservoirs could be classified by combining depositional environment
with structural, rather than diagenetic, elements. Compaction and cementation was indicated as
the principal diagenetic event in 89% of clastic reservoirs analyzed. Structural modifiers include
fracturing, faulting, and folding, all of which can greatly affect reservoir heterogeneity. The
term structured was adopted to describe interwell heterogeneity that result from these structural
overprints on the reservoir. In combination, one-third of the clastic reservoirs have some sort of
structural overprint. The resulting structured and unstructured subclassifications were useful in
describing this lithology.
Six carbonate classes were derived from 20 individually described carbonate depositional systems defined in the classifier. There carbonate reservoirs classified well deposited in marine or
near-marine settings. The class names are descriptive of the location or conditions under which
deposition occurred. However, the shallow shelf/restricted carbonate class contains reservoir rock
deposited in the near-shore subtidal as well as the shallow shelf environment. In carbonate reservoirs, diagenetic factors have significant effects on heterogeneity. Therefore, diagenetic factors
were used as the basis of subclasses. Structural features of the described carbonate reservoirs were
not found to vary substantially (88% of the carbonate reservoirs are described as unstructured).
Therefore, structural features were not used to define carbonate subclasses.
The occurrence and variability of the diagenetic descriptors, however, justified the differentiation of carbonated reservoirs into three carbonate diagenetic subclass. These subclasses are dolomitization, massive dissolution, and the other. The subclass other combines compaction/cementation,
grain enhancement, and silicification. Five of the six carbonate classes are divided into three subclasses. The sixth, slope-basin, contains only reservoirs described by the other diagenetic processes,
the single subclass in the class.
The grouping of the reservoirs into classes created a smaller number of research targets, yet the
distinctness of the reservoir is preserved. The results of the classification effort present a focus for
specific studies.
Reservoirs within these classes are expected to manifest distinct types of reservoir heterogeneity
as a consequence of their similar lithology and depositional histories. The creation of subclasses
assists in the analysis of the impact of post-deposition events on reservoir heterogeneity.
Classification
Energy’s tertiary-oil-recovery information system (Toris). This resulted in the classification and
description of 2300 light-oil reservoirs (greater that 20°API), collectively containing 308 billion bbl
of original oil-in-place (OOIP) (Ray et al., 1991).
The reservoirs fell within 173 combinations defined by their lithology, depositional environment,
structural deformation, and diagenetic overprint. These groups were then collapsed into a smaller
number of classes while maintaining meaningful descriptions of the processes that produce reservoir heterogeneity at the interwell scale. Using a basic classification system relying on lithology and
depositional environment, with subclasses reflecting post-deposition processes, would suffice. As
a result, 22 geologic classes, 16 clastic, and 6 carbonate, with structural subclasses for clastic and
diagenetic subclasses for carbonates, were defined.
Sixteen clastic classes (Figure 2.2) were derived from 28 clastic depositional systems described
in the classifier. These classes contain siliciclastic rocks deposited in the paleo-environmental setting indicated by their names. While some are fairly uniform environments, such as those of the
eolian class, others are complex, as in the various deltaic environments.
For most reservoirs, relatively refined description of the depositional processes was possible
(e.g., fluvial-dominate deltas), but for some, the unavailability of data or the complexity of the depositional processes required broader, undifferentiated classes (e.g., fluvial, strand plain, and delta).
Heterogeneity due to the post-deposition structural and diagenetic history of a reservoir can have
an overriding influence on the flow of oil and other fluids. These descriptive reservoir modifiers
provided the basis for defining subclasses.
In clastic reservoirs, variations in types of structural controls on heterogeneity relative to diagenesis suggested that clastic reservoirs could be classified by combining depositional environment
with structural, rather than diagenetic, elements. Compaction and cementation was indicated as
the principal diagenetic event in 89% of clastic reservoirs analyzed. Structural modifiers include
fracturing, faulting, and folding, all of which can greatly affect reservoir heterogeneity. The
term structured was adopted to describe interwell heterogeneity that result from these structural
overprints on the reservoir. In combination, one-third of the clastic reservoirs have some sort of
structural overprint. The resulting structured and unstructured subclassifications were useful in
describing this lithology.
Six carbonate classes were derived from 20 individually described carbonate depositional systems defined in the classifier. There carbonate reservoirs classified well deposited in marine or
near-marine settings. The class names are descriptive of the location or conditions under which
deposition occurred. However, the shallow shelf/restricted carbonate class contains reservoir rock
deposited in the near-shore subtidal as well as the shallow shelf environment. In carbonate reservoirs, diagenetic factors have significant effects on heterogeneity. Therefore, diagenetic factors
were used as the basis of subclasses. Structural features of the described carbonate reservoirs were
not found to vary substantially (88% of the carbonate reservoirs are described as unstructured).
Therefore, structural features were not used to define carbonate subclasses.
The occurrence and variability of the diagenetic descriptors, however, justified the differentiation of carbonated reservoirs into three carbonate diagenetic subclass. These subclasses are dolomitization, massive dissolution, and the other. The subclass other combines compaction/cementation,
grain enhancement, and silicification. Five of the six carbonate classes are divided into three subclasses. The sixth, slope-basin, contains only reservoirs described by the other diagenetic processes,
the single subclass in the class.
The grouping of the reservoirs into classes created a smaller number of research targets, yet the
distinctness of the reservoir is preserved. The results of the classification effort present a focus for
specific studies.
Reservoirs within these classes are expected to manifest distinct types of reservoir heterogeneity
as a consequence of their similar lithology and depositional histories. The creation of subclasses
assists in the analysis of the impact of post-deposition events on reservoir heterogeneity.