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Upper Miocene channel turbidite sandstones of Yowlumne and Rio Viejo fields, southern San Joaquin Valley, California, are clean arkoses buried to depths of 11,000 ft (3,550 m) or more. Petrographic analyses of cored intervals from seven wells show that these medium-grained sandstones have no detrital constituents other than quartz and feldspars (at an approximately 2:1 ratio). The relative abundance of K-feldspar to plagioclase, however, varies among the intervals studied. This variation has given rise to different paths in clay authigenesis, which then control reservoir-rock properties. Nearly all clays, which constitute from a few percent to more than 20% of the rocks, are derived from feldspar alteration.

Sandstones with high K-feldspar relative to plagioclase yield clay assemblages dominated by kaolinite, with minor amounts of illite and vermiculite. Those with high plagioclase content yield abundant expandable clays. Chlorite is practically absent in all intervals studied. Besides detrital composition, abundance and mineralogy of the clays are also affected by depth of burial of the rock.

Feldspar alteration has resulted in porosities in the range of 14 to 20% and permeabilities up to 200 md at depths of 11,000 to 15,000 ft (3,550 to 4,570 m). In contrast, other reservoirs of different composition in the same area have greatly reduced reservoir properties at the same depths. Knowledge of sandstone composition and diagenesis, therefore, is important to exploration.

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