The Meramec and Osage formations in the STACK region of Oklahoma contain varying mixtures of detrital quartz sand and silt, biogenic silica, and authigenic chert phases. Operators in the STACK have experienced drilling delays due to the variability in both the species and spatial location of quartz phases. These delays can involve frequent bit changes, cave-ins, costly side-tracks, and uncertain lateral continuity within the horizontal wellbore.
Thin section petrography is the primary method for characterizing cryptocrystalline quartz, however this method may not be viable in projects with certain time or sample volume constraints. Alternatively, X-ray diffraction (XRD) can be used to provide an estimate for relative proportions of quartz phases. Methods for the study of disorder in quartz crystal structures, by using peak breadths in XRD patterns have existed since the 1970’s. Detrital, monocrystalline quartz grains have well-ordered crystal structures and narrow peaks, whereas chert and sponge spicules have varying degrees of disorder in their crystal structures and consequently have broad peaks. Peak breadths of quartz can be measured to model the average “crystallinity” of quartz phases in a sample, and trends in “crystallinity” can be observed over the length of a well.
Cores of both Meramec and Osage formations were studied to find relationships between depositional environment, lithology, and quartz speciation. The Quartz Crystallinity Index (QCI) calculated from XRD was compared against thin section petrography for validation. Additionally, lateral cuttings from an Osage sidetrack were studied to relate drilling delays to spatial trends in quartz phases.
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