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ABSTRACT: Relative Importance of Cementation vs. Compaction to Porosity Evolution in Quartzose Sandstone: Tensleep and Quadrant Formations (Pennsylvanian), Northern Rocky Mountains

W. C. James

Very fine- to fine-grained, very well-sorted quartzarenites in the Tensleep (average sandstone unit thickness is 4 m) and Quadrant (average unit thickness is 2 m) formations have average porosity values of 10 and 5%, respectively (range 0-23%, Tensleep; range 0-21%, Quadrant). Porosity evolution in sandstones was strongly influenced by proximity to contacts with interbedded carbonates and the timing of cement precipitation. Early anhydrite (A) and dolomite (D) cements are preferentially located near upper and lower contacts of sandstones. The average minus-cement porosity for these types of cemented zones is 37%. A later quartz (Q) cement is concentrated toward the middle of sandstone bodies, with average minus-cement porosity values near 30%. High porosity zones (>= 1 %) are associated with D + A/D + A + Q ratios of <= 0.5 and preferentially developed in the middle 50% of individual sandstone units.

Estimates of initial porosity and measurements of volumes associated with cements, intergranular pressure solution, and present porosity allow assessment of the relative contributions of cementation and compaction to pore space reduction. On average in Tensleep sandstone, about 74% (lower 25% of individual sandstones) and 77% (upper 25%) of porosity reduction was the result of cementation. However, in the middle 50% of sandstones, compaction (47%) was subequal in importance with cementation (53%) as the cause of pore space loss. In the Quadrant, porosity was primarily destroyed by cementation regardless of location within a sandstone body (75% lower, 78% middle, 82% upper). The difference in behavior of the middle of Quadrant vs. Tensleep sandstones bodies may be related to the absolu e timing and abundance of quartz cementation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990