Calcium mass transport and sandstone diagenesis during compaction-driven flow: Stevens Sandstone, San Joaquin basin, California

Sediment compaction provides a limited source of fluids for diagenesis and drives fluid flow at average rates of only similar to 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 com paction driven. This apparent discrepancy could arise because diagenetic st udies generally are concerned with relatively permeable petroleum reservoir s. Focused fluid flow through permeable zones could increase local flow rat es and allow mixing of fluids from different sources. This study uses the S tevens Sandstone of the San Joaquin basin to explore the potential diagenet ic effects of fluid focusing during compaction-driven flow over a 5 m.y. pe riod. Reactive-transport simulations incorporate a new kinetic expression for pla gioclase 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 re action, simulations show that influx of fluids from compacting shale could limit the distribution of calcite and kaolinite in adjacent sandstones. Alt hough 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 calcit e and kaolinite, Results are consistent with field observations of fluid ch emistry, although simulations account for <0.7% of the 1%-1.5% bulk volume that is observed in cores, Ample calcium is available, but additional react ions may have occurred, especially at cool temperatures.