CLASSIFICATION SYSTEM TARGETS UNRECOVERED U.S. OIL RESERVES
R. Michael Ray
U.S. Department of Energy
Bartlesville, Okla.
Jerry P. Brashear, Khosrow Biglarbigi
ICF Resources Inc.
Fairfax, Va.
A geologic classification, developed by the U.S. Department of Energy, will provide a sound basis from which to set priorities for joint public-private cooperative research, development, and demonstration efforts for producing currently unrecoverable domestic light-oil resources.
The objective is to increase domestic oil production, realize the significant potential of the remaining reservoirs, and slow the accelerating pace of abandonment of U.S. light-oil reservoirs.
The classes in summary form describe the geologic processes that determine reservoir heterogeneities.
Reservoirs expected to have similar heterogeneities have now been grouped into classes and subclasses. These classes were evaluated for their future reserve potential and risk of abandonment.
DOE PROGRAM
Because two thirds of the known U.S. oil resources cannot be recovered by present production techniques, the objective of the DOE's Oil Research Program Implementation Plan' is to encourage technological improvements that could yield increased recovery of these reserves.
A major constraint to additional production from U.S. reservoirs has been the reservoirs' internal heterogeneity--variations in permeability and porosity that control the flow of fluids-especially at the interwell scale. Such heterogeneities are the result of the geological processes of deposition, diagenesis, and structural deformation.
DOE has hypothesized that grouping reservoirs into classes with common geologic processes could help isolate the distinctive types of heterogeneities that recovery process designs must overcome to become effective.
To implement this approach, DOE commissioned an effort to develop a suitable classification system based on geologic processes, and apply it to a nationwide sample of reservoirs for analysis and listing by priority.
CLASSIFICATION PROCESS
DOE requested the Interstate Oil & Gas Compact Commission (logcc) to assist in the classification of U.S. reservoirs as part of an ongoing effort to estimate the domestic oil recovery potential. The logcc enlisted the assistance of the Geoscience Institute for Oil & Gas Recovery Research.
A general classification scheme, or classifier, developed by the institute was expanded into application procedures by a team of senior geologists with extensive experience in the major U.S. oil-producing regions. 2
In all, 2,816 different geologic descriptions were determined to be possible through combinations of lithology, deposition, diagenesis, and structural deformation.
The actual distribution of U.S. reservoirs was expected to utilize far fewer categories, but the specific categories were not known at the project's onset.
The reservoirs contained in DOE's tertiary-oil-recovery information system (Toris) were used as a nationwide sample for the project. Toris contains detailed descriptions of more than 2,500 generally large reservoirs in 25 states that collectively account for 69% of the nation's original oil-in-place (Fig. 1).
For the present study, 2,300 light-oil reservoirs (greater than 20 API), collectively containing 308 billion bbl of original oil-in-place (OOIP), were described using the classifier.
The regional geologic experts examined the data in Toris concerning reservoir rock and fluid properties and the literature on each reservoir to assign the descriptors required by the classifier. Where more than one classification was applicable (e.g., multiple depositional or diagenetic processes), the descriptor deemed by the geologist to be dominant was recorded.
Throughout the effort, Geoscience Institute project leaders monitored the reliability and validity of the description process. Critiques of the classifier by the participating geologists at the end of their work indicated no difficulty with its use.
RESERVOIR CLASS SYSTEM
The 2,300 geologically classified reservoirs fell within 173 combinations defined by their lithology, depositional environment, structural deformation, and diagenetic overprint. While this number was an order of magnitude smaller than the number of possible combinations, it was still too large to provide a manageable number of groupings to guide the organization of the research and development program.
These groups were then collapsed into a smaller number of classes while maintaining meaningful descriptions of the processes that produce reservoir heterogeneities at the interwell scale.
The experts concluded that a basic classification system relying on lithology and depositional environment, with subclasses reflecting postdepositional processes, would suffice. As a result, 22 geologic classes, 16 clastic, and 6 carbonate, with structural subclasses for clastics and diagenetic subclasses for carbonates, were defined.
CLASTIC RESERVOIRS
Sixteen clastic classes (Fig. 2) were derived from 28 clastic depositional systems described in the classifier. These classes contain siliciclastic rocks deposited in the paleoenvironmental settings 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-dominated deltas), but for some, the unavailability of data or the complexity of the depositional processes required broader, undifferentiated classes (e.g., fluvial, strandplain, delta).
Heterogeneities due to the postdepositional 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 heterogeneities which 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.
CARBONATE RESERVOIRS
Six carbonate classes (Fig. 3) were derived from 20 individually described carbonate depositional systems defined in the classifier. The carbonate reservoirs classified were 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.
The diagenetic processes that impact fluid flow on the interwell scale extend from simple compaction and cementation through dolomitization and silicification to massive dissolution.
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 carbonate reservoirs into three carbonate diagenetic subclasses. These subclasses are: dolomitization, massive dissolution, and 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 that class.
The grouping of the reservoirs into classes creates a smaller number of research targets, yet the distinctness of the reservoirs 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 heterogeneities as a consequence of their similar lithologies and depositional histories. The creation of subclasses assists in the analysis of the impact of postdepositional events on reservoir heterogeneity.
THE RESOURCE
The reservoir classes were ranked by total original oil-in-place (Fig. 4a) to determine the relative volumes of oil associated with the classes in the sample analyzed.
The shallow shelf/open geologic class contains over 46 billion bbl, which represents nearly 15% of the total light oil in the sample, thus making it the largest geologic class.
The Meramec in the Sooner trend of Oklahoma and many Grayburg-San Andres reservoirs of the Permian basin in Texas and New Mexico are examples of carbonate shallow shelf/open reservoirs.
The delta/fluvial-dominated reservoirs comprise the second largest class and contain 44 billion bbl of OOIP, or approximately 14% of light oil in the sample. This class includes, among others, the deltaic sands of West Hackberry field in South Louisiana and several Bartlesville sands in northern Oklahoma and southern Kansas.
Slope-basin reservoirs such as the Spraberry of Texas or the Stevens sand of California, fluvial/braided stream reservoirs like the Sadlerochit sand in Prudhoe Bay or the Cut Bank sand of Montana, and shallow shelf/restricted reservoirs such as the Jay/Smackover of Florida or the Lansing-Kansas City of Kansas, round out the top five classes ranked by OOIP. These classes represent 32 billion, 27 billion, and 25 billion bbl of OOIP, respectively.
In all, the top five geologic classes represent 174 billion bbl of OOIP, accounting for 56% of the resource in the data base.
The top ten classes, which also include delta/wave-dominated, clastic shelf, strandplain/barrier core and shore-face, and the carbonate reef and peritidal classes, contain 86% of the studied resource.
Analysis of the oil remaining after conventional production suggests a similar concentration of the light-oil resource in the top five classes. The remaining oil-in-place (ROIP) is displayed in the lower bar of each bar cluster in Fig. 4a.
The shallow shelf/open and delta/fluvial-dominated classes continue to be the top two classes, with ROIP estimated at 31 billion and 29 billion bbl, respectively. The slope-basin class is third with over 22 billion bbl of ROIP. The shallow shelf/restricted class contains nearly 17 billion bbl and the fluvial/braided stream class contains about 15 billion bbl.
Collectively, the top five geologic classes represent nearly 60% of the estimated 194 billion bbl of light-oil ROIP in the sample reservoirs.
The top ten classes based on ROIP contain a total of 86% of the remaining light-oil resource in the studied data base.
POTENTIAL
In addition to its extensive reservoir data base, Toris contains detailed engineering and economic models that permit reservoir-by-reservoir examination of potential future reserves and abandonments.
The economics models permit after-tax discounted cash flow evaluation of future technologies as well as estimation of the economic limits of current technologies.
Economic limits and abandonments were estimated from reservoir-specific decline curves and operating costs estimated by region, depth, and fluid volumes.
The engineering models were, like the data, validated during the National Petroleum Council's 1984 study. The models estimate future production rates as a function of reservoir and fluid properties.
These Toris models, however, address all techniques for stimulating additional production beyond conventional technology: The NPC analyzed only enhanced oil recovery (EOR) technologies, such as chemical, thermal, and carbon dioxide injection. Subsequent modifications have expanded the models to address advanced secondary recovery (ASR) methods, such as infill drilling and polymer treatments to reduce permeability contrast.
In the present study, the recovery potential was estimated for two levels of technology performance. Implemented technology assumes the more extensive application of currently available technology. Advanced technology assumes that the scope and the application of existing technology is extended through R&D to reduce current technical and economic limitations.
Examples of technological improvements include:
- Increased injectant sweep efficiency
- Increased injectant tolerance to severe reservoir conditions (temperature, salinity, etc.)
- Decreased chemical retention
- Improved process displacement efficiency
- Reduced injectant costs.
A more detailed description of advanced technology assumptions are given in the NPC report.
At oil prices up to $20/bbl, utilizing EOR and ASR implemented technologies, the incremental potential reserves and the portion of those reserves that could be lost to abandonment are significant for each of the respective geologic classes (Fig. 4b).
Under these conditions, an overall potential incremental recovery of more than 8.0 billion bbl is estimated for the sample reservoirs nationwide. Of this potential recovery, slightly over 3.1 billion bbl are in danger of abandonment by 1995.
Abandoned reservoirs could possibly be developed later, but redrilling all the wells would result in significantly higher costs with a corresponding loss in total potential reserves due to these higher costs.
The delta/fluvial-dominated reservoir class holds the greatest potential under these conditions, with more than 1.5 billion bbl recoverable being estimated. However, close to one third of this potential reserve is at risk of abandonment by 1995.
The shallow shelf/open class also has more than 1.5 billion bbl recoverable, but less of this, approximately 370 million bbl, are at risk of imminent abandonment.
The greatest potential at risk of abandonment is in the strandplain/barrier cores and shorefaces class, which ranks third in potential recovery, but could forfeit over 770 million of its nearly 920-million-bbl recovery potential (84%) unless current technologies are implemented at a rapid pace.
The slope-basin and shallow shelf/restricted reservoir classes complete the top five classes and have recovery potentials of approximately 850 million bbl each with potential abandonments being slightly in excess of 300 million and 200 million bbl, respectively.
In all, the top five classes account for more than 5.6 billion bbl of the 8.0 billion bbl recoverable (70%) through available EOR and ASR implemented technologies at prices to $20/bbl. This also represents two thirds of the resource at risk of abandonment by 1995.
The top ten reservoir classes account for 93% of all incremental recovery potential utilizing implemented EOR and ASR technologies at oil prices up to $20/bbl.
By utilizing advanced EOR and ASR technologies, as improved through research and development, at oil prices up to $32/bbl, a total of nearly 24 billion bbl potential incremental recovery is estimated to be possible (Fig. 4c). However, 8.1 billion bbl (33%) are at risk of abandonment by 1995.
The shallow shelf/open and delta/fluvial-dominated reservoir classes have the greatest incremental recovery potentials, of 3.8 billion and 3.5 billion bbl, respectively. Yet, potential abandonments within the next few years could eliminate access to approximately 1 billion bbl of this potential incremental increase in each class.
The slope-basin class ranks third with 2.6 billion bbl of incremental recovery potential with close to 1 billion of this potential at risk of abandonment.
The strandplain/barrier cores and shorefaces class is fourth with 2.2 billion bbl of potential and more than 1.4 billion bbl at risk of abandonment.
The clastic shelf geologic class holds the fifth largest potential in this technology/price scenario with an incremental recovery potential of close to 1.8 billion bbl and a potential abandonment of 0.4 billion bbl.
In all, the top five classes account for close to 14 billion bbl of the nearly 24 billion bbl potential recovery, or almost 60% of the total.
Potential abandonments by 1995 amount to 4.8 billion bbl out of a total potential abandonment of 8.1 billion bbl. The top ten reservoir classes account for nearly 90% of all estimated incremental recovery potential utilizing advanced EOR and ASR technologies at oil prices up to $32/bbl.
ACKNOWLEDGMENTS
The geologic classification system was developed under a grant from the U.S. Department of Energy, Bartlesville project office, to the Interstate Oil & Gas Compact Commission (logcc). The logcc effort was coordinated by W. Timothy Dowd, executive director. ICF Resources Inc. served as the principal contractor, assisted under subcontract by a multidisciplinary task force, headed by the Geoscience Institute for Oil & Gas Recovery Research. The institute's original classification project and the project reported in this article were directed by Marcus Milling, with assistance of Jerry Lucia.
Five other geologists coordinated the regional data collection and geologic assessments of each reservoir: John Echols, director, LSU Basin Research Institute; Lee Gerhard, director, Kansas Geological Survey; Donald Oltz, Illinois State Geological Survey; Gary Glass, director, Geological Survey of Wyoming; and Donald Zieglar, a West Coast consultant.
Charles Mankin, director, Oklahoma Geological Survey, coordinated efforts and provided geologic information on Oklahoma reservoirs.
REFERENCES
- U.S. Department of Energy, The Oil Research Program Implementation Plan, April 1990.
- Geoscience Institute for Oil & Gas Recovery Research, Reservoir Heterogeneity Classification System for Characterization and Analysis of Oil Resource Base in Known Reservoirs, prepared for U.S. Department of Energy/Office of Fossil Energy, Bartlesville project office, 1990.
- National Petroleum Council, Enhanced Oil Recovery, 1984.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.