THE ROLE OF GEOLOGY IN RESERVOIR ENGINEERING

Abstract

Every petroleum reservoir is the result of its own peculiar origin, as determined by provenance, depositional environment, and post-depositional history. These geologic factors control all properties generally thought of as reservoir properties—porosity, permeability (specific, relative, and directional), irreducible water saturation, water sensitivity, as well as continuity and homogeneity.

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

Once a reservoir has been discovered, the actual petroleum container can be cored, logged, and studied first-hand. Such a study should then guide field development through primary and secondary production. It can also aid materially in shaping an exploration approach for similar reservoirs and in influencing both wildcat and development drilling programmes.

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

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

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

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 U.S. oilfields, 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.