Sandstone-Shale Interaction of Gulf Coast: Calcite Mobilization in Response to Varying Concentrations of CO2(g) and Acetates, and Feldspar Buffering Capacity

Anthony Park¹ and Kitty Milliken^2
1 Sienna Geodynamics & Consulting, Inc, Bloomington, IN
² University of Texas, Austin, TX

Detrital calcite makes up an important fraction of shale in Gulf Coast sediments. The progressive disappearance of this calcite with depth suggests it is mobilized during diagenesis. To mobilize calcite requires a two-step process. First, an acidic condition is required to enhance calcite dissolution; second, dissolved solutes must be transported out to sandstone, where further interaction with feldspars may or may not result in calcite precipitation. Thus, important controls on calcite solubility in both sandstone and shale are acetates, CO2(g), and pH-buffering capacity of feldspars. Indirectly associated with these components are reactions of clays, which may or may not contribute H+ ions.

To predict the response of sandstone-shale coupled system to varying conditions of the above-mentioned controls, the Balance diagenesis simulator was used to produce a series of behavior diagrams, which are useful for predicting the trend of calcite cement formation in moderately-buried to deeper sediments. Balance addresses diffusive and advective mass-transfer, conservation of mass through empirical and kinetic reactions, and associated sediment compositional and textural changes, which makes it an ideal tool for this study.

First, the effect of introducing CO2(g) into the sandstone-shale couple, with varying initial acetate pore water concentrations, was carried out in the absence of feldspar reactions; subsequently, feldspars of varying plagioclase and K-feldspar mixes were added. Published values of acetates and CO2(g) concentrations for Cenozoic sediments were used, along with an estimated empirical reaction rates of feldspars. A fully kinetic calcite reaction rate law was used to assess calcite dissolution and precipitation behavior.