Petrographic Reservoir Characterization—Direct Measurement and Analysis of Pore and Grain Size Data from Thin Sections
Thin section petrography has traditionally offered only limited insights into the details of sedimentology and even less into reservoir characterization. Petrographic reservoir characterization, making comparisons to engineering and petrophysical data, has not been feasible. Utilization of CCD camera control software provides the means to gather grain and porosity data that can be directly compared to conventional core analysis and mercury injection capillary pressure (MICP) data. Samples of Dakota “J,” Nugget, and Mesaverde sandstones were analyzed and compared. Control samples were used to estimate the effects on grain size measurements in thin section. Data gathered consist of length and width measurements of large numbers of sand grains. Frequency distributions of these data provide a sensitive means of discriminating between samples. Interactive selection of pores impregnated by blue-stained epoxy allows gathering measurements of pore area, perimeter, and equivalent circular diameter (ECD). Frequency distributions of pore area and total pore area show the relative contributions to porosity of pores of different sizes. Plotting frequency distributions of pore ECD with grain size shows the relative contribution of intergranular and moldic porosity. Plotting ECD with the pore area and total pore area frequency distributions reveals the contribution of each pore size class to total porosity. Image analysis shows that the interactive pore selection process captures over 90% of the blue-impregnated porosity, down to a few microns. Using this calculation, the pore area data can be plotted by size class, simulating an MICP curve.