Eighty-seven crude-oil samples were used in the Pennsylvanian-Wolfcamp study. Most of these were from carbonate reservoirs on the Eastern shelf and Horseshoe atoll.

Four geologic factors influence the Pennsylvanian-Wolfcamp crude-oil compositions: (1) the age of the shale envelope, (2) the paleogeography, (3) the migration history, and (4) the association with unconformities. Pennsylvanian shale envelopes yield crudes with low carbon isotopes. Local paleoenvironments have caused minor differences in the oil-generating character of the shales. Late migration or remigration into structural traps, and vertical mixing with pre-Pennsylvanian crudes in strato-structural fields, explain several distinct crude-oil types. Unconformity traps contain another type of Pennsylvanian crude which results from indigeneous generation and mixing with older crudes at the subcrop.

A classification of the Pennsylvanian-Wolfcamp crudes evolved from the use of carbon-isotope measurements in conjunction with the oil analyses. The classification appears also to be compatible with geological factors. The following groups are recognized:

Table

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