Formation of bed‐scale spatial patterns in dolomite abundance during early dolomitization: Part I. Mechanisms and feedbacks revealed by reaction–transport modelling

Abstract

Dolomites of varied ages exhibit metre-scale nested patterns of lateral periodic variation in permeability and porosity and, by inference, dolomite abundance as most examples are 100% dolomite. Two-dimensional reaction-transport modelling simulations of bed-scale dolomitization were used to assess whether those patterns in dolomite abundance could form during near-surface replacement dolomitization. Simulations used a 2 m high and 18 m long model domain, a low-Mg calcite grainstone precursor and an evaporated Mississippian seawater brine (430 parts per thousand salinity) as the dolomitizing fluid. The domain was initially populated with random variations in porosity and/or grain size. Results reveal that spatial patterns in dolomite abundance emerge when there is as little as 1% dolomite formed, with similarities between the modelled patterns and outcrop-documented patterns. The nested patterns include a nearrandom component that constitutes ≤40% of the total variance, short-range correlation ranging from 1Á5 to 3Á3 m and a longer-range periodic trend with a wavelength up to 6Á5 m. The emergence of pattern in dolomite abundance is the result of an autogenic self-organizing phenomenon. It is triggered by variation in initial calcite reactive surface area that occurs due to the random heterogeneities in initial porosity and/or grain sizes. The pattern develops due to a combination of kinetic disequilibrium reactions (dolomite precipitation and calcite dissolution) and positive feedbacks between dolomite growth, calcite dissolution and fluid flow. Flow is around loci of higher dolomite, lower porosity and higher reactive surface areas, but through loci of lower dolomite, higher porosity and lower reactive surface areas. The resulting less porous/more dolomite and more porous/less dolomite structures at the metre-scale arise from those localized interactions. This self-organizing mechanism for pattern formation constitutes a new model for geochemical self-organization during dolomitization and is the only self-organization model that is proven applicable to the formation of metre-scale patterns during early, near-surface dolomitization.

Process‐based Modelling of Syn‐depositional Diagenesis

“…In general, RTM simulations model the change in porosity due to mineral transformations, dissolution and precipitation, but permeability is inferred, most commonly using the Carmen--Kozeny relationship. Simulations of dolomitisation by Budd and Park (2017), have shown the effect of the positive feedbacks resulting from diagenetic porosity enhancement on replacement of calcite with dolomite. Heterogeneity in initial porosity and permeability fields, and consequent focusing of reactive fluids, results in the development of pronounced perturbations in the geometry of the dolomite front.…”

Bed‐scale spatial patterns in dolomite abundance: Part II. Effect of varied fluid chemistry, flow rate, precursor mineralogy, temperature, textural heterogeneity, nucleation density and bed geometry

“…Budd & Park () used bed‐scale reaction‐transport modelling (RTM) to assess the second hypothesis – formation of lateral pattern during dolomitization of a carbonate precursor. These authors found that pattern did emerge at the metres‐scale in the abundance of dolomite, with randomly distributed heterogeneities in the initial calcite reactive surface area (RSA) triggering pattern development.…”

“…Schematic illustrating the feedbacks between (A) high and (B) low reactive surface area (RSA), calcite dissolution dolomite precipitation, fluid flow, and development of pods of high and low dolomite abundance. Modified from Budd & Park ().…”

“…The Budd & Park () simulations of pattern development during dolomitization were limited in the potentially controlling factors considered. Although these authors explored the impact of varied initial porosity and grain sizes (i.e.…”

“…Based on all of the simulation results, the variables that generate the largest lateral patterns in dolomite abundance are identified, as well as the conditions that promote or suppress development of the patterns. The mechanisms by which a pattern develops is not revisited because that was the focus of Budd & Park ().…”