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High-Resolution Reservoir Characterization of the Mixed Carbonate-Siliciclastic Wolfcamp and 3rd Bone Spring Sand of the Delaware Basin, West Texas, Using Combined Chemostratigraphic and Mechanical Hardness Data


Detailed chemostratigraphic and mechanical stratigraphic characterization of heterolithic tight gas and liquid-rich source-rock reservoirs can play an important role in the exploration, development, and completion of these unconventional plays. Traditional formation evaluation has relied on the integration of a variety of core-based data sets, such as bulk mineralogy, clay content and type, organic matter richness (TOC), and brittleness. The time and cost associated with these analyses is reflected in widely spaced samples; typically on the order of five to ten feet. Additionally, well log resolution issues hamper detailed evaluation of these complex producing intervals. High-resolution formation evaluation can be achieved when widely spaced data sets are augmented with less expensive, very closely sampled elemental data, employing portable energy-dispersive X-ray fluorescence (ED-XRF) spectroscopy, and geomechanical hardness data, using the rebound-hammer. Here we present chemostratigraphic and geomechanical data from a ~ 200 ft. long cored section of the mixed carbonate-siliciclastic Upper and Middle Wolfcamp and the siliciclastic 3rd Bone Spring Sand from the southeastern Delaware Basin, West Texas. Elemental data were used to: (a) model bulk-mineralogy, (b) model TOC using redox-sensitive trace elements, and (c) compute a high-resolution gamma ray. Hardness values were converted to an unconfined compressive strength (UCS) using empirical correlations. Using the hardness and lab-measured geomechanical data, we build a high-resolution brittleness index profile of the cored interval. Our data show that hardness (HLD) values for limestones in the Middle and Upper Wolfcamp mostly range from 500-700 HLD, whereas thinly interbedded silty mudstones and siltstones show a broader range of values from 250-650 HLD. This suggests that bulk mineralogy is the key control on hardness and that reservoir facies (mudstones) vary widely in their mechanical properties. A correlation matrix linking hardness and elemental data from various lithofacies shows detrital-proxy elements (Al, K, Ti, Zr) and redox sensitive elements (Mo, U, Ni, Cu, Pb, Th) are mostly negatively correlated, while carbonate proxy elements (Ca, Mg, and Mn) are mostly positively correlated with the hardness values. The positive correlation between Si and HLD in organic rich-mudstones is attributed to the presence of biogenic silica.