Strontium Isotopes in Brines and Associated Rocks From Crectaceous Strata in the Mississippi Salt Dome Basin (Southeastern Mississippi, U.S.A.)

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Geography and Geology


Sr concentrations and 87Sr86Sr ratios have been determined for brines from formations spanning most of the Cretaceous in two oilfields in the Mississippi Salt Dome Basin and from local salt domes, chalk, sands and shales. The Selma Group is predominantly a fossiliferous chalk whereas the remainder of the stratigraphic sequence consists of sands and shales with only minor carbonate phases. The sampled interval ranges from 2000 to just over 4000 m below land surface.

Sr concentrations range from 443 to 1718 mg kg−1 for brines from Baxterville field and from 528 to 1762 mg kg−1 for brines from Soso field. Brine 87Sr86Sr ratios range from 0.7071 to 0.7116 at Baxterville field and from 0.7083 to 0.7119 at Soso field. With the exception of brines from the Selma Chalk, all 87Sr86Sr are more radiogenic than Cretaceous seawater.

On plots of 87Sr86Sr vs. Sr concentration, brines from individual formations within a producing field occupy distinct non-overlapping areas with the exception of a single sample. In some cases, individual reservoirs within formations can be distinguished. Brines from formations that are present in both fields, which are ∼ 85 km apart, do not have the same 87Sr86Sr ratio.

In each field, brine Sr concentration increases with depth. 87Sr86Sr ratios increase with depth to ∼ 3000 m and then appear to level off. These trends are interpreted to be caused by dissolution or recrystallization of carbonate minerals which contain significant amounts of non-radiogenic Sr, and decomposition of Rb-rich detrital micas and K-feldspar, which are sources of radiogenic 87Sr.

Salt and cap rock from domes in southeastern Mississippi have an average 87Sr86Sr ratio of 0.70707, consistent with a Middle Jurassic age for the Louann Salt. In Baxterville field, chalk from the Upper Cretaceous Selma Group has a Cretaceous seawater 87Sr86Sr ratio of 0.7076. 87Sr86Sr ratios range from 0.7091 to 0.7111 for Sr leached from sands and shales from both Baxterville and Soso fields. If these values reflect the 87Sr86Sr values of the easily dissolved interstitial carbonate phases, they indicate that the calcite cements precipitated from sedimentary fluids which were more radiogenic than the seawater of that time, but which were not as evolved as the brines which are now present. Alternatively, they may reflect leaching of radiogenic sources.

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Chemical Geology





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