Sedimentary basin brines

Potential external sources of concentrated fluids can be found on and adjoining every crystalline rock mass on the planet. Seawater and the derivatives of seawater such as evaporite deposits and sedimentary basin brines are the primary candidates for the external sources of salinity. Dilute seawaters from the Yoldia and Litorina stages of the Baltic Sea (<10" yr) are recorded as entering crystalline rocks along coastal sections of the... 

Continental-scale hydrologic forces can control the flow of basinal brines and are another major potential external source of saline fluids that may enter crystalline rock environments. Studies by Bottomley et al. (1999) suggest that hydraulic gradients in northwestern Alberta are such that brines are forced from Devonian strata into the underlying Canadian Shield. Similarly, it appears that western Canadian sedimentary basin brines have entered the Canadian Shield in some parts of the Lac du Bonnet batholith, Manitoba (Gascoyne et al., 1987). 

Gascoyne M., Purdy A., Fritz P., Ross J. D., Frape S. K., Drimmie R. J., and Betcher R. N. (1989) Evidence for penetration of sedimentary basin brines into an Archean granite of the Canadian shield. In Proc. 6th Int. Symp. Water-Rock Interaction (ed. D. L. Miles). Malvern, UK Balkema, Rotterdam, The Netherlands, pp. 243 -245. 

It can be seen in Fig. 8 that the high concentrations of acetic acid in oilfield brines (activities typically from 10 to 10 ", see below) are preserved metastably with respect to the decarboxylation reaction. In other words, if stable equilibrium was reached in sedimentary basin brines, concentrations of acetic acid would be several orders of magnitude lower than they are. Similar results can be obtained for propanoic acid with the data in Table 3. This disequilibrium with respect to the decarboxylation reaction indicates that large kinetic barriers to decarboxylation of organic acids exist under sedimentary basin conditions. 

If we assert that oxidation states in sedimentary basin brines are likely to be intermediate between those set by MH and PPM, then it is possible to determine whether CO2 or CH4 may be in metastable equilibrium with acetic acid. Using an activity of 10 for CH3COOH (see below), the 100°C plots show that attainable fugacities of CO2 are less than the total pressure of 500 bar at all/H2 values above about 10. In contrast, the same activity of CH3COOH can only be in equilibrium with high fugacities of CH4 between values of /H2 set by MH and about which is a very narrow... 

Values of /H2 revealed by the relative concentrations of acetic and propanoic acid can be used to test explicitly whether the organic acids can be in metastable equilibrium with CO2, CH4, hydrocarbons, or other organic compounds. At present, little is known about the concentrations of hydrocarbons and other organic compounds in sedimentary basin brines. Little is also known about the mixing of hydrocarbons in petroleum and how to evaluate activities of individual components in such complex mixtures. Nevertheless, thermodynamic calculations allow the construction of a variety of plausibility arguments regarding the compositional extent of metastable... 

Although the pH at the temperatures of the experiments is never buffered and seldom considered in the experimental design, speciation calculations like those used to construct Fig. 5 indicate that organic acids are likely to be highly associated at the temperatures used in hydrous pyrolysis experiments. Therefore, it can be assumed that the molalities calculated from the yields reported for the experiments (see Appendix) can be equated with activities without taking account of acid dissociation (unlike the situtation described above for sedimentary basin brines). These activities, together with values of logK for reaction (28) at the experimental temperatures and pressures, allow estimates of log/H2 appropriate for each experiment. 

Table 2. Dissolved carboxylate ligands detected in sedimentary basin brines...

Figure 3. Histogram of 5 C1 in groundwater and formation water brines from sedimentary basins and oil fields relative to Cl/ Cl in SMOC (vertical dashed line at 0%o). N is the number of analyses, and bracketed numbers identify references as follows [1] Kaufmann et al. 1993 [2] Eggenkamp 1994 [3] Ziegleret al. 2001 [4] Eastoe et al. 2001 [5] Kaufmann 1984 [6] Kaufmann et al. 1984 [7] Kaufmann et al. 1988 [8] Eastoe and Guilbert 1992 [9] Eastoe et al. 1999 [10] Desauliniers et al. 1986 and [11] Eggenkamp et al. 1994.

Complexation of metal cations and transport in ore-forming solutions derived from sedimentary basins by organic acid anionic complexes present in oil field brines... 

In the bituminous coals of the US Illinois and Appalachian basins, arsenic primarily occurs in pyrite. The arsenian pyrite probably originated from subsurface fluids that existed about 270 million years ago during the formation of the Ouachita and Appalachian mountains (Goldhaber, Lee and Hatch, 2003). The arsenic-bearing fluids in the midcontinent Illinois Basin were primarily brines derived from surrounding sedimentary basins that were also responsible for the formation of the Mississippi Valley lead-zinc deposits. In contrast, the fluids that were responsible for the arsenian pyrites in the Appalachians (especially in the coals of the Warrior Basin of Alabama) were metamorphic and not as saline as those in the midcontinent (Goldhaber, Lee and Hatch, 2003). 

Figure 8.6. Ca Cl ratios in brines from sedimentary basins in North America. 1-California 2-West Virginia 3- Alberta 4- Illinois 5-Texas 6-Louisiana. (After White, 1965.)...

Figure 8.7. Representation of subsurface brine compositions on a ternary diagram for brines from U.S. sedimentary basins. 1- Texas 2- California 3- Kansas and Oklahoma 4- Appalachia 5- Arkansas (After Hanor, 1983, based on data from Desitter, 1947.)...

Land (1987) has reviewed and discussed theories for the formation of saline brines in sedimentary basins. We will summarize his major relevant conclusions here. He points out that theories for deriving most brines from connate seawater, by processes such as shale membrane filtration, or connate evaporitic brines are usually inadequate to explain their composition, volume and distribution, and that most brines must be related, at least in part, to the interaction of subsurface waters with evaporite beds (primarily halite). The commonly observed increase in dissolved solids with depth is probably largely the result of simple "thermo-haline" circulation and density stratification. Also many basins have basal sequences of evaporites in them. Cation concentrations are largely controlled by mineral solubilities, with carbonate and feldspar minerals dominating so that Ca2+ must exceed Mg2+, and Na+ must exceed K+ (Figures 8.8 and 8.9). Land (1987) hypothesizes that in deep basins devolatilization reactions associated with basement metamorphism may also provide an important source of dissolved components. 

Land L.S. (1987) The major ion chemistry of saline brines in sedimentary basins. 

Carpenter A. B. (1978) Origin and chemical evolution of brines in sedimentary basins. Okl. Geol. Surv. Circularly, 60-77. 

A number of descriptive terms, including oilfield brine, basinal brine, basinal water, and formation water, have been used in the literature to describe deep aqueous fluids in sedimentary basins. No satisfactory overall classification system exists, due to the fact that these waters can be assessed by several different criteria. These include the sahnity of the water, the concentration and origin of various dissolved constituents, and the origin of the H2O, which is commonly different from that of the solutes. The following terminology has been extracted mainly from Han or (1987) and from Kharaka and Thordsen (1992). The interested reader should also consult White et al. (1963) and Sheppard (1986). 

There is also field evidence that halite-derived brines can be transported over long distances in sedimentary basins. For example, the chemical compositions of waters from the Houston-Galveston area, Texas, and several other areas in the northern Gulf of Mexico basin indicate dissolution of halite (Kharaka et al., 1985 Macpherson, 1992). However, in a number of these areas, there are no known salt domes within 50 km of the sampled sites. Large-scale fluid advection is probably the main mechanism for the... 

The principal sources of dissolved chloride in the more saline fluids of sedimentary basins include dissolved chloride buried at the time of sediment deposition, chloride derived by refluxing of subaerially evaporated surface brines, chloride derived from subsurface mineral dissolution, principally halite, and marine aerosols. The Cl-Br systematics of sedimentary brines provide useful constraints on interpreting the origin of chloride in these waters (Carpenter, 1978 Kharaka et al., 1987 Kesler et al., 1996). 

Spencer R. J. (1987) Origins of Ca-Cl brines in Devonian formations, western Canada sedimentary basin. Appl. Geochem. 2, 373-384. 

Localized action of aggressive metal-bearing brines faults (e.g., Taylor and Soule, 1993) and around salt domes (Kyle and Saunders, 1997) attests to the likely presence of fluids with the imprint of acid-generating mineralogical reactions (Hemley and Jones, 1964) in deep sedimentary basins at, and perhaps in large measure, just beyond the range of our deepest sampling. 

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