Making Sense of Carbonate Pore Systems: Implication for Porosity Cut-Off and STOOIP

Arve Lønøy
Norsk Hydro ASA, P.O. Box 7190, N-5020 Bergen, Norway

The most widely used pore type systems are limited by the fact that the relation between porosity and permeability is poorly defined. Furthermore, these systems do not optimally integrate sedimentology, diagenesis and flow-related properties. In many carbonate reservoirs it is therefore difficult to generate predictive models for reservoir quality distribution, giving significant uncertainty in STOOIP calculations.

Based on empirical data, mostly from Europe and the Middle East, a new system for carbonate pore types has been developed. The new system uses elements from the Choquette & Pray (1970) and Lucia (1983) pore type classification systems, but also introduces many new elements. The new pore type system includes 14 pore type classes, which show a predictable relationship between porosity and permeability. It combines sedimentological/diagenetic features and flow-related properties, and reservoir critical parameters can thus to some extent be predicted using sedimentological and diagenetic models.

The new classification system significantly improves porosity to permeability relationship for individual pore types, with coefficient of determination (R²) mostly in the range of 0.7-0.9. Porosity cut-offs derived from the cross-plots show significant variability among the different pore types, ranging from 5% to 30% porosity cut-off at 1 mD permeability. This has a major impact on STOOIP calculation as it strongly affects net/gross. Simple models run on four different oil reservoirs and prospects indicate that STOOIP calculations may vary with up to several hundred percent based on which pore type or pore type combinations that have been used in the model.