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Abstract

Anomalously high porosities (20 -29%) in deeply buried (4.9-5.2 krn) Miocene sandstones at Picaroon field, offshore Texas are largely a result of porosity enhancement by dissolution of calcite cement , Dissolution is not important in equivalent strata nearby, including Doubloon field, where reservoir porosities are generally less than 18%. Doubloon reservoir sands contain moderately saline (TDS = 63-74 gil) ,"NaCl-typc" water characterized by low concentrations of Ca and other cations , enrichment in δ18O (+7.8%0 SMOW) and radiogenic Sr ("SrI86Sr = 0.71109). Formation waters of this type are common throughout the Gulf Coast Terti ary section . In contrast, Picaroon waters have salinities of 151 to 243 gil TDS , Ca concentrations of 13 to 22 gil, heavier in δ18O (+ 8.0 to + 9.3%0 SMOW) and less radiogenic Sr ("Sr! .6sr= 0.70992 to0.71023). In addition, Picaroon water samples contain unusually high concentrations of other cations such as Sr, Ba, Fe, Pb and Zn. Picaroon sandstones contain late diagenetic fracture-filling ankerite, barite and sphalerite. Ankerite is in oxygen isotopic equilibrium with formation water at temperatures indicated by fluid inclusions (> 147°C) and has a similar Sr isotopic composition ("Sr!,6S r = 0.70970).

Picaroon brine s are interpreted to be allochthonous to the Miocene section, because they have elemental and isotopic compositions similar to waters produced from Mesozoic reservoirs in south Texas and central Mississippi. The association of these waters with high quality reservoirs at Picaroon suggests a potential link between deep sources of fluids and carbonate dissolution . A model is proposed in which hOI , acidic water s from the underlying Mesozoic section are injected along major faults into Picaroon sands, resulting in significant porosit y enhancement. Fluid flow is likely episodic, driven by periodic buildup and release of geopressures.

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