--> Abstract: Diagenesis and Porosity Evolution of the Flathead Sandstone (Middle Cambrian), Wyoming and Montana; #90063 (2007)

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Diagenesis and Porosity Evolution of the Flathead Sandstone (Middle Cambrian), Wyoming and Montana

 

Ozkan, Aysen1, Earle F. McBride2 (1) University of Texas at Austin, Austin, TX (2) The University of Texas at Austin, Austin, TX

 

The Flathead is a shallow marine sandstone deposited during middle Cambrian transgression on the Precambrian basement rocks over much of Wyoming and southern Montana. It is composed of coarse to medium grained sandstones. Polycrystalline quartz types, feldspars, biotite, metamorphic and granitic rock fragments indicate that underlying crystalline rocks and similar rocks outside the basin of deposition were the major source rocks. Composition ranges from quartzarenite to arkose (average Q92 F7.4 R0.6). The Flathead has undergone physical and chemical compaction, recrystallization, dissolution, oxidation, reduction, grain fracturing, and generation of different authigenic cements and replacement minerals in the course of burial from 5 to 7 km. Authigenic phases include quartz, Fe-oxide, kaolinite, calcite, siderite, ankerite, dolomite, chalcedony, and opal, with trace amounts of K-feldspar, pyrite, Ti-oxide, barite, and tourmaline. According to fluid inclusion data from Teton Pass locality, minimum estimate of Tmax (maximum formation temperature) reached is 150-160 oC. Silica for the quartz cement was derived mainly from intergranular pressure solution, and stylolites, and also dissolution of silicate framework grains. S/I transformation within the shale layers and overlying formation were minor silica sources. Oxygen and carbon stable isotope data, microprobe analyses, textural, and CL studies indicate that calcite and dolomite cement formed during deep burial, but later were recrystallized in meteoric water. Total porosity ranges from absent to 11.5% and averages 3.6%. Calculations indicate that compaction was more important than cementation in reducing the porosity of the sands. Permeability ranges from 0.5 to 439 mD, with a geometric mean of 5.8 mD.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California