Abstract

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 predominantly negative (−) carbon isotope values while Wolfcamp shales were source for crude with positive (+) values. Local paleoenvironments have caused minor differences in the oils. Late migration or remigration into structural traps, and vertical mixing with Pre-Pennsylvanian crudes in structural fields explains several distinct oil types. Unconformity traps contain another Pennsylvanian crude which results from indigenous 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 following groups are recognized: