--> ABSTRACT: Comparative Petrology of Arkosic Sandstone Lithofacies of Cowlitz, Spencer, and Yamhill Formations, Northwest Oregon, by Robert O. Van Atta; #91040 (2010)

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Comparative Petrology of Arkosic Sandstone Lithofacies of Cowlitz, Spencer, and Yamhill Formations, Northwest Oregon

Robert O. Van Atta

Recent discovery (1979) and commercial production of natural gas in arkosic sandstone ("Clark and Wilson sand," informal) of the Cowlitz Formation near Mist, Oregon, has stimulated interest in the sedimentology and stratigraphy of Narizian sandstones of northwestern Oregon and the Willamette lowlands. Petrographic study of arkosic sandstone in the lower part of the Cowlitz Formation in surface outcrops in Clatsop, Tillamook, and Columbia Counties, Oregon, and in the Texaco 1 Clatskanie well reveals that the composition is more lithic than that of arkosic sandstone ("Clark and Wilson sand") in the upper part of the Cowlitz Formation. The petrography of highly permeable, arkosic sandstone in the Spencer Formation in the western Tualatin Valley south of the Mist gas field is identical in framework grain composition, texture, matrix composition, and heavy mineral content to that of the gas producing "Clark and Wilson sand." The petrography of sandstone in the Yamhill Formation in the western Tualatin Valley is very like that of sandstone in the lower part of the Cowlitz Formation of northern northwest Oregon.

Diagenetic alteration is more pronounced in arkosic sandstone in the lower part of the Cowlitz Formation and in the Yamhill Formation compared to diagenesis, which is almost nil in arkosic sandstone in the upper part of the Cowlitz and in the Spencer Formations.

Natural gas exploration will be aided by a clearer and more certain understanding of these petrologic differences between Eocene sandstones in northwestern Oregon.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.