Pore system characterization in diagenetically complex Mississippian-aged carbonate reservoirs (Kansas, USA)

Abstract

Advanced petrophysical methods and detailed statistical analysis help to quantitatively and qualitatively characterize pore structure variations in carbonate rocks. Utilizing a core from the Mississippian limestone play in south-central Kansas, we investigated geological constraints on reservoir properties using observations of the variability in pore architecture and measured petrophysical properties, i.e., porosity, permeability, and nuclear magnetic resonance (NMR) response. The sample-set includes facies that retain their original depositional texture and rocks that have been subject to dolomitization, silicification, and dissolution to varying extents. Dominant macropore types include intercrystalline, moldic, and dissolution-enhanced vuggy porosity, whereas the dominant microporosity types are micritic and dolomitic intercrystalline micropores and nano-intercrystalline within silicified samples. The porosity-permeability relationships and T2 modal distribution of the carbonates we investigated correlate with dominant pore types depending on the extent and type of diagenetic modification. The fractal nature of pore systems in dolograinstones is not apparent in partially dolomitized samples, and pore complexity significantly decreases with increasing dolomitization. T2cutoff and bound fluid volume (BFV) estimates indicate that independent of geologic origin, small and intricate pore systems with microporosity are likely to host higher amounts of capillary-bound fluids. This relationship holds, especially among samples with nano-intercrystalline porosity. Correlation coefficients from predicting NMR-derived parameters with porosity improved from 0.1 to 0.78 by including pore architecture data and pore types in the multiple linear regression model. The results and observations presented here improve the current understanding and predictability of petrophysical parameters in carbonate rocks with complex pore systems.