Abstract

Sediment compaction provides a limited source of fluids for diagenesis and drives fluid flow at average rates of only ∼1 mm/yr. Nevertheless, many geochemical and petrographic studies of diagenesis provide evidence of significant mass transport in systems where fluid flow appears to be compaction driven. This apparent discrepancy could arise because diagenetic studies generally are concerned with relatively permeable petroleum reservoirs. Focused fluid flow through permeable zones could increase local flow rates and allow mixing of fluids from different sources. This study uses the Stevens Sandstone of the San Joaquin basin to explore the potential diagenetic effects of fluid focusing during compaction-driven flow over a 5 m.y. period.

Reactive-transport simulations incorporate a new kinetic expression for plagioclase dissolution and suggest that rate-limited plagioclase dissolution drove calcium enrichment of pore fluids and caused precipitation of calcite, kaolinite, and albite. In spite of the importance of this rate-limited reaction, simulations show that influx of fluids from compacting shale could limit the distribution of calcite and kaolinite in adjacent sandstones. Although calcium mass transport is predicted on a scale of kilometers, upward flow of calcium-rich fluids from deep beds does not significantly increase calcite volume relative to closed-system predictions. Increased transverse dispersivity increases mixing, which further limits precipitation of calcite and kaolinite. Results are consistent with field observations of fluid chemistry, although simulations account for <0.7% of the 1%–1.5% bulk volume that is observed in cores. Ample calcium is available, but additional reactions may have occurred, especially at cool temperatures.

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