Diagenesis of Upper Cretaceous Teapot Sandstone, Powder River Basin, Wyoming: ABSTRACT

The Upper Cretaceous Teapot sandstones of Well Draw field, Converse County, Wyoming, are turbidite fan deposits bounded stratigraphically by marine shales. They presently occur from 6,360 to 7,200 ft (1,920 to 2,195 m), dipping to the northwest. Cored samples selected from nonbioturbated A bedsets show that the sandstones are fine to very fine-grained feldspathic litharenites. Major authigenic minerals include carbonate cement, quartz overgrowths, and clay minerals. The clay minerals originated either as alteration rims on detrital silicates or as precipitates from pore fluids. Alteration rims typically consist of illite, smectite, mixed layer illite/smectite, and lesser chlorite. Feldspars are altered to kaolinite. Precipitated clays occur as (1) thin, unoriented, grain coating chlorite and kaolinite, (2) pore lining mixed layer illite/smectite and lesser chlorite oriented with (001) normal to the pore wall, and (3) unoriented, poorly crystalline, pore filling chlorite. The diagenetic sequence is (1) compaction and limited quartz overgrowth development, (2) complete calcite cementation and precipitation of grain-coating clays, (3) dissolution of carbonate cement, (4) precipitation of pore lining and later pore filling clays, and (5) development of second stage quartz overgrowths. Development of silicate alteration rims occurred throughout the diagenetic history. Dissolution of carbonate cement produced the majority of present-day porosity; however, this secondary porosity was reduced by precipitation of clay minerals. In the downdip sandstones, hydrodynamic flow and an increase in the abundance of detrital labile grains have caused an increased abundance of clay mineral precipitates, reducing the reservoir potential. The pore fluids which controlled sandstone diagenesis were likely provided by dewatering and diagenesis of enclosing shales. End_of_Article - Last_Page 465------------