Computer Science
Scientific paper
Jan 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55..309s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 1, pp.309-325
Computer Science
8
Scientific paper
A suite of formation-water samples from Silurian-Devonian reservoirs in the Illinois basin has been analyzed for major, minor and trace element concentrations and for H, O and Sr isotopic compositions in order to interpret origin of salinity and geochemical evolution of brine compositions in this evaporite- and shale-poor cratonic basin. Although chloride concentrations range from 2000 to 137,000 mg/L, Cl / Br ratios (291 ± 18) are consistent with those of seawater or seawater evaporated short of halite saturation ( Cl / Br = 292). Thus, during Silurian-Devonian time, subaerially evaporated, penesaline brine entered the subsurface where it was chemically modified through brine-rock interactions. Cation / Br ratios and mineralogy of associated strata indicate that Na and K were depleted through interaction with clay minerals, Ca was enriched and Mg depleted by dolomitization and Sr was enriched as a result of CaCO 3 recrystallization and dolomitization. Although significant dilution of the modified brine with meteoric water is supported by D - 18 O covariance, original marine waters have not been completely expelled from Silurian-Devonian strata. Hydrogen and oxygen isotopes exhibit covariant relations with cation and anion concentrations, implying that isotopic exchange between H 2 O and minerals has not greatly influenced the D - 18 O trend. Brine 87 Sr / 86 Sr ratios range from 0.7092 to 0.7108; when these ratios are plotted versus 1/Sr, a two-component mixing trend is suggested, although Sr concentrations have experienced local diagenetic modification. A 87 Sr-enriched fluid may have accompanied petroleum migration from New Albany shales into adjacent Silurian-Devonian carbonates where it mixed with remnant evaporated seawater. This event probably preceded the influx of meteoric water, as /gdD and 18 O are not correlated with Sr isotopic compositions of formation waters.
Stueber Alan M.
Walter Lynn M.
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