Regional Diagenetic Trends in the Lower Cretaceous Muddy Sandstone, Powder River Basin
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.
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There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.