Effects of Physical and Chemical Diagenesis on Low-Porosity, Low-Permeability Sandstones, Mesaverde Group, Uinta Basin, Utah

ABSTRACT Examination of sandstones from core samples spanning a 110-m interval of Upper Cretaceous Mesaverde Group indicates that physical compaction of labile rock fragments and chemical diagenesis greatly modified the original lithology. These modifications reduced porosity (to less than 10%) and permeability (generally to less than 0.5 md at surface conditions). Local silty lamina further reduce vertical permeability. The sandstones are predominantly feldspathic litharenites composed of monocrystalline and polycrystalline quartz, igneous, metamorphic, and fine-grained sedimentary rock fragments, and small amounts of potassium and plagioclase feldspars. Cementing agents include quartz overgrowths, authigenic low-albite on plagioclase, intergranular calcite and dolomite, and authigenic clays (kaolinite, illite, and minor chlorite). Quartz overgrowths are more common than in the Mesaverde Group sandstones previously examined, and undoubtedly reduce porosity. The overgrowths are sometimes separated from the detrital-quartz host by an iron oxide-stained dust ring or by a thin (± 2 µ) film of chlorite. Locally, early formed intergranular calcite occupies as much as 40% of volume and, in addition to virtually eliminating porosity, has largely prevented compaction of labile rock fragments. Chert and fine-grained sedimentary rock fragments are commonly compacted between more competent framework grains. Following compaction and deformation, leaching of feldspars and rock fragments produced secondary porosity, which includes most measured porosity in these sandstones.