--> Abstract: Geochemical Constraints on the Origins and Timing of Emplacement of Allochthonous Formation Waters, Frio Formation (Oligocene), South Texas, by T. N. Diggs and L. S. Land; #90987 (1993).

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DIGGS, T. N., Shell Western Exploration and Production Inc., Houston, TX; and L. S. LAND, Department of Geological Sciences, The University of Texas at Austin, Austin, TX

ABSTRACT: Geochemical Constraints on the Origins and Timing of Emplacement of Allochthonous Formation Waters, Frio Formation (Oligocene), South Texas

Stable and Sr isotopic compositions of three generations of calcite cement, coupled with highly variable major and trace element chemistries, {87}Sr/{86}Sr ratios, and Br/Cl ratios of modem formation waters from the Frio Formation (Kenedy and Kleberg counties, south Texas), provide evidence for invasion of allochthonous formation water into the Cenozoic section at burial depths =>1.8 km. {87}Sr/{86}Sr ratios of two generations of early calcite, interpreted to have precipitated between 25 degrees and 75 degrees C based on (isotope){18}O data, range from 0.7069 to 0.7072. Pore water {87}Sr/{86}Sr evolved rapidly from a Frio seawater signature, buffered by skeletal carbonates (0.7079), to unradiogenic values as unstable volcanic silicates (approx. 0.7066) began to dissolve during earl burial. Late, Fe-rich calcite post-dated partial dissolution of pre-existing calcites, and based on 6180 data, precipitated at temperatures exceeding 100 degrees C. Late Fe-rich calcite is characterized by {87}Sr/{86}Sr ratios ranging from 0.7072 to 0.7082, requiring introduction of radiogenic Sr for which an intraformational source appears to be lacking.

Local introduction of varying quantities of pore fluids from Mesozoic strata can account for observed variations in {87}Sr/{86}Sr ratios in different generations of calcite (0.7069 to 0.7082) and in modem formation waters (0.7069 to 0.7090). Radiogenic formation waters today are locally very saline, and saline water is characterized by high Br/Cl ratios. Saline, Br- and {87}Sr-rich water must have evolved deep in the Gulf of Mexico sedimentary basin in contact with evaporites and radiogenic crustal silicates, where these Ca- and CO<2>-rich, acidic waters are stable. Based on paragenetic relations, present geothermal gradients, and {87}Sr/{86}Sr ratios and (isotope){18}O compositions of calcite cements, saline, acidic, allochthonous water is interpreted to have migrated into the ertiary section at depths =>1.8 km between approx. 80 degrees and 100 degrees C, causing partial dissolution of early generations of calcite cement. Extensive water/rock reactions, including plagioclase albitization and calcite dissolution, continued after allochthonous waters invaded the Tertiary section, accounting for modifications to major and minor element formation water chemistry relative to compositions characteristic of Mesozoic reservoirs today.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.