AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Reaction Transport Modeling of Dolomitization Reveals the Emergence of Self-Organizing Patterns
(1) Geological Sciences, University of Colorado, Boulder, CO.
(2) Sienna Geodynamics Consulting, Bloomington, IN.
Carbonate petrologists have studied modern and ancient dolomites for decades, with many important concepts established from those efforts. Now reaction-transport modeling (RTM) is providing opportunities to test those concepts and explore dolomitization as a complex non-linear system. Published RTM studies focus on basin- and platform scale dolomitization, examining the distribution of dolomite bodies, the duration of dolomitization, and the effects of platform geometry, fluid composition, and temperature. RTMs have yet to fully consider porosity and textural heterogeneity, or dolomitization at the scale of individual beds.
We employed Sym.8, a continuum-based water-rock interaction simulator that uses conservation of elemental mass, diffusive and advective mass-transfer, equilibrium reactions among solutes, and kinetic reactions between aqueous solutes and minerals. We modeled a 2 m-high by 18 m-long bed with 10 x 30 cm cells. Sym.8 is unique in that it includes a textural component that tracks textural evolution throughout the model domain, and links evolving grain size, permeability, and kinetic growth rate. Calcite grains and dolomite crystals were modeled as spheres that dissolve or grow through the simulation. Reconstructed Mississippian seawater was used as the dolomitizing fluid. Models tested the effect of variations in fluid-flow rates, bed height, and extent of initial porosity and textural heterogeneity.
The lateral patterns of dolomite abundance and porosity formed in the RTM are identical to those observed in lateral transect through many ancient dolomites. There is a near-random component (≤40% of the total variance), short-range autocorrelation, and a long-range periodic trend. Length scales of the patterning vary depending on geologic parameters with thin beds (≤ 1 m), high flow rates (≥ 1 m/yr) and initial porosity heterogeneity (range 20% or greater) associated with the greatest autocorrelation range and periodic patterns.
The emergence and development of pattern in the amount of dolomite is the result of geochemical self-organization. It is triggered by random heterogeneities in the initial porosity domain. It develops due to a combination of non-equilibrium kinetic reactions (calcite dissolution & dolomite precipitation), positive feedbacks between dolomite growth and fluid flow around low porosity/high dolomite patches, and the diffusion of carbon from sites of high calcite dissolution to sites of high dolomite precipitation.