The biggest challenge for geologists, geophysicists, and petroleum engineers now and in the decades ahead is to significantly improve hydrocarbon recovery from all new and previously discovered reservoirs. Keystone of the methodology required to improve oil and gas production, as well as to evaluate and delineate new reserves, is a detailed reservoir description. This is a characterization of the reservoir and nonreservoir rock-fluid system that is appropriate in content and

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detail for the particular stage of exploration and production. The type and amount of data required for a proper reservoir description are diverse, from several disciplines, and depend upon the stage of the reservoir's exploration and production cycle. The cycle is viewed as a continuous series of overlapping stages from discovery, through appraisal, planning, development, and reservoir management. The concepts and data needed to define and exploit reservoirs become more complex and quantitative as the production becomes more mature. Concepts, data, and models developed during the production phases, when reapplied to exploration, provide important guides to the explorationists for evaluating trapping elements, seals, reservoir quality, and risks in basin and wildcat evaluation.

When one looks at the question "When is a reservoir description needed?" the answer is simple. The need starts once a discovery is made and the discovery is being appraised as to the best estimates of hydrocarbon in place, recoverable reserves, and production rates. As a field or reservoir goes through its typical cycle of discovery, appraisal, planning, development, and reservoir management, a more complete description is both necessary and possible.

A critical first step in the reservoir description process is recognizing any correlative reservoir subzones or layers and any intervening dense, impermeable, or low-permeability strata. Knowledge of the depositional/diagenetic processes controlling reservoir and nonreservoir rock is essential to determine one's ability and degree of confidence in correlating these units. Seismic sequence, lithologic, and fluid analyses, and well-documented outcrop studies can add significantly in establishing interwell correlations. Recognizing and mapping all vertical or horizontal fluid-flow barriers, as well as theif zones or zones of unusual permeability contrast and faults, are critically important to all recovery processes. Flow-test data dovetailed with knowledge of the reservoir and nonreserv ir framework based on geology/geophysics provide the best reservoir description of continuity/discontinuity.

Structural and stratigraphic maps, cross sections, and fence-and-block diagrams convey the three-dimensional geometry, distribution, and continuity of the reservoir, nonreservoir, and aquifer. A variety of computer programs aid in preparing these illustrations. Isopach maps without the accompanying detail correlation sections have been the pitfall of many projects. Net-pay isopach maps drawn to provide the basis for determining hydrocarbons in place have tricked many petroleum engineers into believing a reservoir is more continuous, more homogeneous, and less stratified than it actually is.

The importance of discontinuous shale barriers of limited areal extent on coning and the drainage of oil from a gas-invaded area illustrate the need to include shale dimensions in many types of recovery calculations and predictions.

The recognition, selection, and description of reservoir units or layers, and the communication of this picture to the petroleum engineers are fundamental contributions and responsibilities of the geologists/geophysicists. A coordinated data-acquisition program can greatly improve the probabilities of correct assessments in discovery, appraisal, planning, development, and reservoir management.

A good reservoir description designed to answer key reservoir performance questions is a fundamental tool. The incremental well costs to obtain adequate data for a reservoir description are very small compared with its value in improved recovery. The time to complete a reservoir description is before significant expenditures are planned and spent. Mathematical models and simulation of reservoir performance that do not have a realistic reservoir rock-fluid description are interesting, but are expensive exercises that potentially lead to inappropriate or incorrect management decisions.

In exploration ventures, detailed reservoir-description studies made during the production stages provide the critical data needed by the explorationist to estimate reservoir and seal quality from seismic, well logs, and samples.

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