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Abstract

Parrylands is a multiple pay oil field with production from unconsolidated sand reservoirs of late Tertiary age in the Southern basin of Trinidad.

Stratigraphy and sedimentology of the formations penetrated indicate three cycles of delta progradation resulting in the deposition of the Cruse, Forest, and Morne L'Enfer formations. Primary production is obtained from the deeper Cruse Formation whereas the overlying Forest and Lower Morne L'Enfer formations contain heavy oil and tar sands.

The Forest 'A' Reservoir represents a delta fringe/barrier bar system containing approximately 100 million barrels of heavy oil in place. Within the Forest 'A' Reservoir, the oil occurs in north-northwest-trending barrier bars on the crest and flanks of the Lot 1 anticline. Geochemical evidence suggests that the oil was generated from a Cretaceous source and migrated along major faults into upper Tertiary reservoirs, where it occurs as a heavy degraded oil. Gravity segregation occurred resulting in 16-19°API crude in the crestal area of the anticline and 10-12°API crude on the lower flanks of the anticline. The higher API gravity crude is produced by primary methods while that of lower gravity can only be produced commercially by thermal methods.

This observed phenomenon of segregation within the same reservoir is significant in identifying primary production in areas of heavy oil. This can be done by carefully mapping and correlating structure, API gravity, and other characteristics of the crude.

The economic considerations of this technique in exploration and exploitation of heavy oil cannot be over-stressed since a (favorable) cash flow can be generated prior to intensive capital investment for thermal enhanced oil recovery.

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