Petrography and Reservoir Physics II: Relating Thin Section Porosity to Capillary Pressure, the Association Between Pore Types and Throat Size

Porosity in reservoir rocks is configured into a few types of pores whose size and shape are controlled by depositional fabric and postdepositional processes. The size, shape, and abundance of each pore type can be objectively determined from thin section using image analysis and pattern recognition procedures. Each pore type tends to be associated with a limited range of throat sizes. The association between pore type and throat size can be determined using regression procedures linking pore type data obtained from thin section with capillary pressure data. To do so, a set of samples is required wherein the association between pore type and throat size is fixed, but where pore type proportions vary between samples. This condition is met by a sample suite representing reservoir facies from a single core or, in many cases, from a single field. The relationship between pore type and throat size is an effective means to relate reservoirs in terms of the efficiency of the porous microstructure to multiphase flow. Parameters derived from the relationship can be used to construct accurate physical models that subdivide physical response in terms of the contributions of each pore type.