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Parke Dickey has pointed out that there is a nearly linear increase in total dissolved solids (TDS) content with depth in oil field waters in areas of northern Louisiana and southern Arkansas. Similar relations exist in parts of east-central Louisiana and central Mississippi. A reexamination of brine analyses in the region shows that these linear increases in TDS are primarily the result of linear increases in sodium and chloride concentrations with depth. Other dissolved components, such as Mg, Ca, Sr, Ba, HCO3, SO4, and trace constituents can show more complex variations in concentration with depth.
It is proposed here that the linear gradients for sodium and chloride reflect the ongoing, steady-state mass transport of these constituents upward from thousands of feet of depth to the near surface. Sodium and chloride concentrations at depths exceeding 8,000-10,000 ft (2.5-3 km) are maintained at constant high levels by the subsurface dissolution of halite. Low dissolved salt concentrations are maintained at the near surface by recharge of meteoric waters. The mass-transport processes that could produce such nearly linear profiles are limited to molecular diffusion, thermal diffusion (Soret effect), and eddy diffusion. Preliminary estimates of fluxes and mass-transport velocities that could result from these processes suggest sodium and chloride ions could be migrating upward at ve ocities approaching 1 cm/year.
Nonlinear but systematic variations in the concentrations of other components with depth may reflect the presence of local stratigraphic sources and sinks of material. Zones of depletion of dissolved calcium, for example, may represent areas of active precipitation of calcite. If the mass transport coefficients for these nonconservative components can be estimated, then limits can be put on the absolute rates of ongoing diagenetic processes.
Of considerable interest is the possibility that dissolved or entrained hydrocarbons are also involved in active vertical migration. The systematic study of brine chemistry promises to provide important clues to the understanding of processes and rates of fluid migration and hydrocarbon migration in the region.
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