Abstract

The Bantry Pekisko G Pool is a classic example of an unconformity trap. The porous crinoidal grainstone reservoir is located on a pronounced structural high where it is trapped against tight shales and sands of the Mannville Group. Boundaries of the pool are predominantly controlled by erosion at the Sub-Cretaceous Unconformity.

The crinoidal grainstones which form the reservoir in this pool were deposited in a series of crinoidal banks which exhibit a shoaling upward sequence. These crinoidal banks show significant vertical and horizontal heterogeneities. The reservoir has been modified post-depositionally by various actions which further create heterogeneities in the reservoir. These include leaching providing secondary porosity, re-cementation, fracturing, karstification and solution cavity infilling.

Evidence of the nature of these heterogeneities was dramatically demonstrated during the mapping of the pore volume of the reservoir. The pool was originally mapped for the purposes of unitization and these maps were used in a modified form for a reservoir simulation model for waterflooding. Subsequently two infill wells were drilled to fully develop the reservoir prior to waterflooding and the results from these wells necessitated a major revision to the maps of the pool. This exercise emphasized the complexities of the pool, the pitfalls which can occur in trying to predict the reservoir geometry and the difficulties encountered in trying to explore for this type of reservoir.

The original oil in place has been calculated from material balances to be 620,000 m3 (3.9 million barrels) of oil. Under the primary solution gas drive mechanism, recovery was expected to be approximately 10% or 62,000 m3 (390,000 barrels). Waterflooding is predicted to add a recovery of an additional 10% for a total of 124,000 m3 (780,000 barrels). Only time will tell if this prediction proves to be correct.