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Abstract: Controls of Reservoir Quality in the Muddy Formation: Rozet Field, Campbell County, Wyoming, USA


Reservoir quality of Muddy Sandstones at Rozet Field is principally controlled by the following: (1) distribution of authigenic clays, (2) dissolution of feldspar grains and rock fragments, (3) occurrence of thin zones of intense pressure solution facilitated by higher mica content, and (4) the presence of detrital clay in lower energy facies. Post Muddy erosion allowed fresh water flushing of the paralic sand system which dissolved unstable rock fragments and feldspar grains. The dissolved constituents reprecipitated as kaolinite, chlorite, and illite-smectite cements. Closely spaced minipermeameter measurements show the presence of thin zones (less than 1 ft.) of intense kaolinite cementation. Kaolinite is the most abundant clay in the upper portion of the reservoir where fresh water leaching was most pronounced. Most kaolinite is a scattered pore filling cement that does not block pore throats. With increasing distance from the erosion surface, the intensity of rock fragment and feldspar dissolution decreases. Concurrently, mixed layer illite-smectite and chlorite increase in relative abundance. The illite-smectite partially reduces pore throats lowering permeability relative to the kaolinite dominant sandstones. The occurrence of authigenic clays in high energy facies Muddy sandstones is a consequence of the presence of reactive feldspars and rock fragments in the depositional system. Beds containing 5 to 7 percent mica show pressure solution compaction that destroys intergranular porosity. Adjacent beds that lack mica or shale laminae show no pressure solution and retain intergranular porosity.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado