A highly explored area is considered to be one in which the regional structure has been defined, the regional stratigraphy is well understood, and a representative number of demonstrably closed traps have been drilled and reliably evaluated, regardless of well density or productivity.

The approach to further exploration can follow 3

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routes: (1) a theoretical application of scientific knowledge to the already known geological details to determine where prospects should be; (2) working by "analogy"--finding areas where geological details resemble those of known producing areas; and (3) a "bird dog" search for details that have been overlooked or improperly interpreted that point up prospects. These approaches are interrelated.

The most important exploration methods can be summarized as surface geology, subsurface well geology, seismic survey, gravity survey, and magnetic survey. In a highly explored area surface geology, if feasible at all, is detailed and fairly complete. Gravity and magnetic surveys do not ordinarily refine an anomaly sufficiently to provide drilling prospects. Thus, the subsurface picture, growing more complete with each new well, becomes the most effective source of new prospects. Seismic survey commonly is useful in bridging the information gap between wells, especially when programmed to test a specific prospect lead.

How do discoveries result in highly explored areas?

1. Refinement of, or finding, new structural details.
2. Reinterpretation of structure.
3. Refinement of, or finding, new stratigraphic details.
4. Reinterpretation of stratigraphy.
5. Improvement of producing techniques.
6. Improvements in equipment.
7. Change in economics or management attitude.
8. "Stumbling into" an unanticipated productive situation.

Interesting examples of the foregoing include Dutch Slough; Little Sand Draw, Wyoming; Castaic Junction; Asphalto; Poso Creek; Brentwood; Lathrop; Beverly Hills; Anaheim; and Grimes.

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