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38949-AC9
Characterizing Heterogeneity of Low-Permeability Strata and Its Effect on Solute Transport
Research on thermohaline convection in sediments and sedimentary rocks proceeded on several lines- theoretical analyses, combined field/numerical analyses of dolomitization on Permian rocks of West Texas and New Mexico, analysis of free convection in tidal flats of Padre Island, and testing for free convection in drying pozas of El Zacatón, Mexico.
First, we completed our theoretical analyses using the USGS code SUTRA. Stochastic realizations of permeability distribution and modeling indicate that: 1) free convection can occur through heterogeneous low permeability media; 2) increases in the mean, standard deviation, and vertical correlation length of the permeability field promote free convection in systems of different geometries and boundary conditions. This finding is also confirmed by critical permeabilities determined from patch analysis; 3) the internal structure of the heterogeneous permeability field in low-permeability strata is critical in controlling the onset and development of the instabilities. For a given density gradient, a threshold permeability is required for convection to commence at a local scale; and 4) interbedding with high-permeability strata is important in initiating free convection and promote convective flux across heterogeneous low-permeability strata.
Second, we examined the roles fractures in free convection in these settings. Our study shows that there are 3 possible modes of convection in fractured reservoir rocks. One mode dominates. A paper on this topic has been submitted to Water Resources Research.1 These findings also indicate that analysis of free convection ion dual porosity media requires a detailed 3-D assessment of fracture geometries.
Third, the MS thesis by Stevens2 provided the first complete documentation of thermohaline free convection in the shallow subsurface. Data from 3-D earth resistivity methods, pressure transducers, and specific conductance demonstrate free convection. A paper is being prepared on this and preliminary results were presented by Al-Johar et al.3 and Stevens et al.4
Fourth, we examined the role of thermohaline convection (seepage reflux) in dolomitization; this process is often invoked as a mechanism for dolomitization of carbonate platforms. This conceptual model has been broadly accepted, but its hydraulics are poorly understood. We hypothesize the source of dolomitizing fluids are coastal carbonate depositional environments (tidal flats); we construct a hydrologic model on the basis of tidal-flat-related paleotopography. Our preliminary work suggests that reflux seepage takes place in repeated short-lived episodes, simultaneously with the deposition of the carbonate platform, perhaps related to tidal flooding events. We test this hypothesis with a numerical model of the carbonate sediment-brine interface, where the concentration of the brine increases evaporation. We combine numerical modeling of thermohaline convection with field mapping studies of an outcrops in Permian rocks of New Mexico and Texas where this mechanism of dolomitization has been proposed.5
Fifth, we investigated the deep pozas/shafts/cenotes of Sistema Zacatón in Tamaulipas, Mexico. During the recent drought, the shallow poza Tule was evaporating. The waters were shallow and very saline. We used electrical resistivity to analyze whether or not free convection might be occurring through the underlying travertines. We did not document free convection, but we discovered that the travertine poza floor was relatively thin (~2meters) and overlies a very deep water-filled cavern. The nearby poza Zacatón is over 350 meters deep and we may have documented a hitherto unknown but similarly deep shaft. Results were presented at the 2006 and 2007 annual meetings of the Geological Society of America.6,7
1 Simmons, C.T., Sharp, J.M., Jr., and Nield, D.A., submitted, Modes of free convection in fractured low-permeability media: Water Resources Research.
2 Stevens, Joel D. 2007, Variable-Density Groundwater Flow Beneath the Wind-Tidal Flats of Padre Island, M.S. thesis, Univ. Texas, Austin, TX, 175p.
3 Al-Johar, M.M., Davidson, S.C., Linhoff, B.S., Rempe, D.M., and Sharp, J.M., Jr., 2007, Variations in the freshwater/saltwater system on North Padre Island, Texas: Free convection or Ghyben-Herzberg lens? Geo. Soc. America, Abs. with Programs (Ann. Mtg.), v. 38, no. 6, p. 474.
4 Stevens, J.D., Shi, M., Garner, T.T., Gary, M.O., and Sharp, J.M., Jr., 2003, A Hydrogeologic Transect of North Padre Island: Laguna Madre to the Gulf of Mexico: Geol. Soc. America, Abs. with Programs (Ann. Mtg.), v. 35, p. 249
5Garcia-Fresca, B., Lucia, F.J., Sharp, J.M., Jr., and Stevens, J.D., 2007, Coastal evaporative environments, hydrology, and dolomitization [abs]: SEPM Annual Meeting, Carbonate Research Group, Long Beach CA.
6 Gary, M.O., Halihan, T., Sharp, J.M., Jr., Mouri, S., and Thorstad, J., 2006, Electrical resistivity imaging of travertine-capped sinkholes: Deep lakes with lids: Geo. Soc. America, Abs. with Programs (Ann. Mtg., v. 38, no. 7, p. 523.
7 Gary, M.O., Sharp, J.M., Jr., and Ramirez Fernandez, J.A., 2007, Sistema Zacatón: Hydrogeologic Discoveries from DEPTHX: Geol. Soc. America Special Session, DEPTHX — The DEep Phreatic THermal eXplorer: Robotic exploration and characterization of Sistema Zacatón on the mission path to Europa, Geological Society of America Annual Meeting, Denver, CO, October 2007.
