This paper traces the evolution in models of the Lisburne carbonate reservoir from early wildcat days, through the decision to fund primary development, to waterflood assessment. This work is an integration of geologic and reservoir studies based on nearly ten years of extensive data collection (well test, core, log, pressure and field performance data). Early reservoir descriptions assumed uniform enhancement of continuous pay layers resulting from small-scale fractures. Additional recovery over solution gas drive was expected by gravity drainage and waterflood. Integration of performance and geological data after field start-up reveal greater heterogeneity than originally thought: faults and irregularly distributed fractures dominate performance; much of the porosity is ineffective; and the producing oil-water contact (OWC) varies by as much as 300 ft [90 m]. The evolution in simulation techniques reflects the evolution in geologic models toward increased recognition of heterogeneities.

The complexity of the Lisburne reservoir is a result of faulting, two periods of uplift and erosion, and three distinct phases of porosity development. Particularly important among these diagenetic events is the youngest phase of porosity development, which post-dates significant faulting. As a result, many faults are open and conductive. An additional complexity is a high permeability layer created by a laterally extensive set of small open fractures beneath an unconformity at the top of the reservoir.

Initial development was based on models with limited data which understated the impact of these heterogeneities. In retrospect, several important clues and their implications were not fully appreciated. Despite extensive data collection, the unpredictable influence of fractures still creates uncertainties in reservoir description (e.g. well performance cannot be accurately predicted pre-drill).

Recommendations for evaluating potentially complex carbonates include: give full weight to known uncertainties, conduct inter-well tests early in the evaluation, and minimize risk by proceeding gradually with field development.

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