--> ABSTRACT: How Effective Are Shales in Retarding Fluid Flow and Solute Transport in the Gulf of Mexico Basin? Modeling Thermohaline Convective and Diffusional Transport, by C. T. Simmons, J. M. Sharp, Jr., and T. E. McKenna; #91021 (2010)
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How Previous HitEffectiveTop Are Shales in Retarding Fluid Flow and Solute Transport in the Gulf of Mexico Basin? Modeling Thermohaline Convective and Diffusional Transport

SIMMONS, CRAIG T.,  JOHN M. SHARP, JR., and THOMAS E. MCKENNA

Pressure, temperature, and salinity data in the South Texas portion of the Gulf of Mexico Basin reveal some interesting trends, including the presence of pore-fluid salinity inversions. We infer that some of these are caused by pulses of deeper saline fluids from out of the geopressured zone. Our models of these systems indicate that salinity inversions are largely dissipated by cross-formational diffusion over time scales on the order of >10{4} years. If thermohaline (free) convection induced by the density gradients can occur, then these inversions can dissipate even more rapidly (approx. 10{3} years). Free convection can create enhanced fluid flow and chemical mass transport over significantly larger spatial scales than can be expected by diffusion alone. We quantify the relative importance of free thermohaline convection and cross-formational diffusion with numerical models using parameter ranges compatible with Gulf Coast conditions. Of critical importance is the heterogeneity of the shaly units, particularly the variation permeability created by sedimentary facies or fractures. Nonuniform vertical permeabilities, preferential flow paths, and lateral variations in fluid density are important factors creating density-induced flow. Our analyses indicate that traditional Rayleigh criteria, based upon average parameter values, may under estimate the potential instability of these systems. Average values do not consider the heterogeneities that trigger the instabilities and cause thermohaline convection. If so, this may resolve the paradox of how an overpressured system can still permit significant fluxes of water and chemical species.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.