Modeling Pore Fluid Interactions in Sedimentary Basins: Some New Approaches to Some Old Problems

Jeffrey S. Hanor

Recent work by a number of different investigators has demonstrated that the major solute composition of saline waters in sedimentary basins is far more systematic than previously recognized. The simplest explanation for the observed broad pattern of increases in dissolved group Ia and IIa cations, such as H, Na, K, Mg, Ca, and Sr, and the heavy metals Pb, Zn, and Cu, with increasing total anionic charge, A-, is that the approach toward thermodynamic buffering by multiphase and metastable silicate - carbonate ± halide ± sulfide mineral assemblages is a first-order control on subsurface fluid compositions, even at temperatures well below 100°C. Variations in the concentrations of buffered cations with A^- are non-linear, and variations in A^{-</ UP> in time and space thus have the capability of inducing simultaneous mass transfer between fluid and rock of all buffered cations plus dissolved silica and carbonate. One mechanism for changing A- is by dispersive mixing of fluids of diverse salinity.}

It is of interest in terms of diagenetic modeling to establish the time and length scales required to achieve this apparent local metastable equilibrium (LMEQ). In Gulf Coast brines, the unsupported dissolved radioactive cation, ²²⁸Ra, with a half-life of only 5.8 years, appears to be buffered, indicating that under some conditions LMEQ time scales can be relatively short. The large number of individual mineral phases needed to buffer multicomponent systems and the heterogeneous distribution of these phases in typical sedimentary sequences may require LMEQ length scales much larger than the scale of observation of a thin section, core sample, or even the thickness of a single sedimentary unit.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995