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The depositional system within which the Upper Cretaceous "Parkman Formation" (= Parkman Sandstone Member of the Mesaverde Formation) of the Wyoming part of the Powder River basin was formed, was delineated using a combination of outcrop data, subsurface core data, and geophysical well logs. The post-depositional history of these rocks was determined, semi-quantitative compositional analyses of authigenic components were made, and paragenetic sequences were established with the aid of scanning electron and transmission electron microscopy.
The Parkman Formation in the study area is composed of a progradational deltaic complex of interstratified sandstone, siltstone, and shale that overlies the extensive marine shelf deposits of the Steele Shale. Four distinct units within the Parkman have been delineated: (1) interbedded shale, siltstone, and very fine grained, well-sorted sandstone, which were deposited in prodeltaic environments; (2) coarsening-upward sequences of sandstone with lenticular siltstone beds and some shale interbeds, which were deposited in distributary mouth-delta front environments; (3) fine-grained, moderately sorted, horizontal to crossbedded sandstones deposited in beach environments; and (4) dark lignitic shales, carbonaceous siltstones, and fining-upward sandstone sequences deposited in floodplain, swamp, and distributary channel environments.
The timing and intensities of diagenetic alterations have profound effects upon reservoir properties of the Parkman Sandstone in the Wyoming part of the Powder River basin. In the subsurface samples, major authigenic minerals observed in the Parkman Sandstone are clay minerals (chlorite, illite, montmorillonite, kaolinite), quartz, feldspar, calcite, dolomite, and iron-oxides, whereas in the surface samples there was no authigenic chlorite and only rare quartz overgrowths, but rather more kaolinite, calcite, and iron-oxide.
The generalized diagenetic sequence can be summarized as follows: (1) ductile grain deformation and original porosity reduction due to settling and mechanical compaction; (2) authigenic chlorite formation as grain coatings or pore linings; (3) authigenic quartz overgrowths from dissolution of silica grains, and clay diagenesis, as well as pressure solution; (4) authigenic feldspar overgrowths; (5) minor authigenic mineral deformation due to continued mechanical compaction; (6) authigenic feldspar alterations to clay minerals; (7) calcite cementation; (8) dolomitization; (9) calcite replacement of siliciclastic grains; and (10) iron-oxide development.
All of the authigenic minerals in the Parkman Sandstone, where they are abundant, reduce the effective primary porosity and permeability of potential reservoir sandstones prior to petroleum generation, migration, and accumulation. In the subsurface samples where authigenic chlorite occurs, the pore space was reduced by 5 to 10 µ. However, where chlorite is thin or absent, quartz overgrowths tend to grow larger to a maximum crystal diameter of 30 to 40 µ, filling most of the pore space. Therefore chlorite coatings actually prevent the porosity from being completely destroyed with quartz overgrowths. In surface samples, chlorite coatings have not been observed, but late-stage calcite cement replaced most of the siliciclastics and, along with iron-oxide cements, further reduced porosity and permeability.
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