Every petroleum reservoir has a distinctive origin, peculiar only to itself, as determined by provenance, depositional environment, and post-depositional history. The following geologic factors control all properties, generally thought of as reservoir properties: porosity, permeability (specific, relative, and directional),

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and irreducible water saturation.

The state of knowledge regarding reservoirs is such that, in some cases, rock and reservoir properties can be related directly; in other cases, they can be related empirically or only through speculation and surmise.

After a petroleum-bearing reservoir has been discovered, it can be cored, logged, and studied first-hand. Such a study then should guide field development through primary and secondary production. This type of study also can aid materially in shaping an exploration approach in search of similar reservoirs and in influencing a wildcat drilling program.

Each of the following examples of geologic reservoir studies was directed toward a different specific reservoir problem.

1. A large anticline in Wyoming produces from several reservoirs of different age and genesis; each reservoir has a separate set of production characteristics and problems.

2. A pair of structural-stratigraphic traps in Illinois, although similar in some properties, have different origins, internal geometry, heterogeneity, and recoverable reserves.

3. A gas-condensate reservoir in Oklahoma, where a combined petrographic-relative permeability study led to the installation of a dry-gas repressuring plant and a marked increase in recoverable reserves.

Although these examples are all from United States oil fields, the principles and methods of study are applicable in any petroleum province. Best conservation practices require the integration of geologic reservoir studies into drilling, logging, completion, stimulation, and primary or supplementary recovery operations.

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