Exploring a Mature Field—Offshore Gabon

Theunissen, R. J., T. Regan, S. Johansen, Perenco, London, United Kingdom

Faced with an anomalous well on a developed field Perenco uses an integrated approach to re-define the structure and ultimately prove new reserves.

Turnix is a Perenco operated field offshore Gabon. The accumulation is a salt piercement and overhang structure with oil found in steeply dipping flanks of Upper Cretaceous turbidite sandstones. Initially developed with four production wells and a gas injector anomalous behaviour was observed in the southernmost well. Here, in contrast to the rest of the field good production rates, low decline, low water cut and steady GOR were observed. Material balance indicated that increased volume and significant aquifer support were necessary to maintain this behaviour. Geochemical analysis of the oils showed different composition and maturity between the south and east of the field and suggested a separate, previously unde­fined block.

The field was re-mapped and a southern compartment extending beneath the salt over­hang was defined. Structural uncertainty was significant with poor seismic imaging beneath the salt and illumination studies indicated that there was little to no signal from beneath the salt wing. Despite this 3D modelling revealed that significant extra volume was possible beneath the salt. Various structural scenarios were used to plan optimal well trajectories.

A deviated well was recently drilled to appraise the southern compartment beneath the salt overhang. This encountered oil bearing sands at two target intervals beneath the salt and has given us a fresh insight into potential undiscovered volumes in salt flank and salt over­hang settings.