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Evaluating Links Between Sequence Stratigraphy and Reflux Dolomitisation of the Permian San Andres Formation

Abstract

Understanding the process of early dolomitisation in the context of sequence stratigraphy allows better prediction of the distribution of associated pore network modification. Reflux dolomitisation is ultimately dependent upon relative sea-level in development of the restricted environments from which brines are sourced. To explore these ideas we develop a diagenetic sequence stratigraphic reconstruction of the Permian San Andres Formation of New Mexico using a reactive transport model (RTM). The complexity of modelling transient boundary conditions that drive flow during the accumulation of platform carbonate is a barrier to developing more realistic simulations of syn-sedimentary diagenesis. RTMs have typically used generic, static platform geometries and have taken little account of the spatial variations in depositional facies found in real platforms. We have thus developed a novel RTM workflow with a temporally evolving grid which allows us to examine the evolution of reflux dolomitisation in a sequence stratigraphic context. Outcrop and subsurface data were used to constrain the accumulation of the platform over 3 My and build 2D numerical reconstruction of the system that preserves heterogeneity in depositional fabrics and accounts for syn-sedimentary compaction. The presence, extent and duration of platform-top brines are constrained by outcrop interpretations of shoreline position. The sea-water and brine chemistry are derived from fluid inclusion data and geochemical modelling. We then use TOUGHREACT to simulate flow, heat and solute transport in response to evolving boundary conditions, and the resultant kinetically-controlled dolomitisation. The simulation reveals the complex interactions between refluxing brines generated episodically on the platform top and convection driven by the geothermal heat flux. Convective circulation of seawater drives partial dolomitisation at the platform margin. Short periods of active reflux alternate with more extended periods of “latent” reflux, when subsurface brine flow continues despite the absence of a brine pool. The Mg/Ca ratio and distribution of brine flux are critical controls on the emergent patterns of dolomitisation. The simulation explains the distribution of dolomite as observed on outcrop as the product of amalgamation of dolomite bodies formed by episodic, short-lived reflux events and long-lived geothermal convection of seawater.