Regional Diagenetic Trends in the Lower Cretaceous Muddy Sandstone, Powder River Basin

Abstract Lower Cretaceous sandstones of the Muddy Formation have produced a number of significant hydrocarbon reservoirs in the Powder River Basin. These reservoir sandstones accumulated in a variety of fluvial, deltaic, and shallow marine environments. The porosity and permeability of Muddy sandstones are affected significantly by the amount and composition of diagenetic clay minerals. Diagenetic clay mineral assemblages within the Muddy are time controlled. Oldest Muddy sandstones (sedimentologic zones 5 and 6) are characterized by a Kaolinite-Chlorite-Illite-Quartz assemblage with some smectite. Intermediate Muddy sandstones (sedimentologic zones 3 and 4) are characterized by an Illite-Smectite assemblage with sporadic Chlorite, Quartz and Kaolinite. Youngest Muddy sandstones (sedimentologic zones 1 and 2) are characterized by a Kaolinite-Quartz assemblage. This diagenetic time-trend is matched by a change in the composition of detrital components. Older Muddy sandstones are more feldspathic, contain more rock fragments and less quartz than the younger sandstones. Well stimulation and completion treatments should take into account the diagenetic assemblages present in the reservoir sandstones. In the oldest Muddy sandstones, the rocks will tend to be acid sensitive, occasionally fresh water sensitive, and there will be migration of fines. In the younger Muddy sandstones, the principal problem will be the migration of fines and the stabilizing of the kaolinite.