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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 68 (1984)

Issue: 4. (April)

First Page: 491

Last Page: 491

Title: Textural and Compositional Controls on Silica Diagenesis--A Case Study from Nugget Sandstone, Washakie Basin, Wyoming: ABSTRACT

Author(s): W. C. James

Abstract:

Grain-size laminated quartzose sandstones (Q93F5R2) illustrate well-defined relations among framework grain size (GS), quartz type, silica cement (SC), and pressure solution (PS). The percentage of monocrystalline quartz grains with well-developed overgrowths (greater than 50% of grain not in contact with adjacent grains containing overgrowth) is related to grain size (71% coarse, 20% medium, 8% fine sand). Regardless of grain size, less than 10% of the polycrystalline quartz population exhibits well-developed overgrowths.

For medium sand-size nonundulose, undulose, and polycrystalline quartz, the portion of grains with well-developed overgrowth is 29%, 8%, and 2%, respectively. All other factors being equal, first-cycle quartzose sandstones derived from metamorphic source rocks will lose porosity and permeability, due to silica cementation, at a slower rate than those sandstones (nonundulose quartz rich) derived from other sources.

The silica cement present in the Nugget Sandstone is largely the result of pressure solution. Over the average grain size range of 0.22-0.35 mm, the volume represented by pressure solution can be depicted by the equation PS = -54(GS) + 26 (r = -0.85). SC = 99(GS) - 19 (r = +0.80) defines the relation between grain size and silica cement abundance. Sandstone intervals with an average grain size less than about 0.29 mm are characterized by an excess of silica, whereas for an average grain size above 0.29 mm the opposite is the case. Pressure-solution-produced silica mainly originated in finer grained units and migrated to coarser grained zones. When all grain sizes are considered, the volume of silica generated by pressure solution exceeds the amount of silica precipitated by overgrowth .

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