Several common factors are functionally relevant to the occurrence of typical as well as atypical oil and gas fields. Attention is focused on the more clearly atypical fields.

We may observe an apparent relation between the presence of oil or gas and certain geologic or geochemical factors without determining the true causality of that relation (which could involved other vital factors unperceived). Thus, our conclusions can be founded on mere coincidence and, once reached, those conclusions may carry a lot of momentum.

Some pertinent criteria of effective entrapment which can be examined in both typical and atypical fields are: upward reservoir convergence; differential compaction; stratigraphic shunting; deep-water discharge; structural coherence; minimum potential energy; local cover weakness; hydrothermal chimneys; near-vertical faulting; and hydrochemical plumes.

At this stage in our knowledge about petroleum occurrence we can probably learn more from the "atypical" than from the "typical," because some of our tacit assumptions are challenged. The atypical situation forces us to answer new questions. The new answers then may enable us to fine-tune the search for more dependable oil and gas prospects--typical or otherwise.

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