--> The Role of Fluid Flow in Dolomitization: Constraints from Numerical Models, by J. Kaufman; #90986 (1994).

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Abstract: The Role of Fluid Flow in Dolomitization: Constraints from Numerical Models

Jonathan Kaufman

Numerical simulations of groundwater flow in carbonate platforms are presented to evaluate the hydrologic setting of dolomitization during (1) burial compaction, (2) seepage reflux, (3) thermal and Kohout convection and (4) high-frequency eustasy. Groundwater models are generated from a two-dimensional, finite-difference code that simulates single-phase fluid flow during basin evolution.

Dolomitization during burial compaction is modeled with a reef-rimmed carbonate platform encased by basinal shales. Regional dolomitization by compaction fluids is considered unlikely because basin-derived fluids are focused into the reef from a limited distance of 10 km, while most pore fluids in the shale migrate to the surface at velocities below 0.1 mm/yr. Porous and permeable basinal limestones and sandstones may act as conduits to move pore fluids updip into the reef.

Surficial evaporation of seawater results in downward and seaward fluid migration; the magnitude of flow is a function of fluid density. Even fluids at salinities marginally elevated above normal marine values are capable of penetration several hundreds of meters into the underlying sediments, albeit at rates below 1 cm/yr. Calculations of rates of Mg mass flux indicate that dolomitization of large carbonate platforms (>10 km3) by fluids below halite saturation requires millions of years of reflux-driven flow.

Models for thermal convection of seawater through carbonate platforms result in convective flow with velocities of 7.5 cm/yr. The potential for dolomitization is high because a large reservoir of Mg (seawater) is available, and elevated temperatures produce favorable thermodynamic and kinetic conditions. The low Mg mass flux rate calculated for Kohout convection suggests that dolomitization is possible only when slope sediments are exposed to open oceanic circulation for millions of years. Glacio-eustatic sea-level changes produce a hydrologic regime characterized by buoyant flow of marine pore fluids at the coastline. High rates of Mg mass flux result from this flow regime, although observations of modem dolomite occurrences in this setting are rare.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994