Chemical evolution of pore fluids during diagenesis is important to oil and gas origin and migration and mineral deposits. Authigenic sphalerite, a potential geochemical indicator, has been found in reservoir rocks that include the following: (1) Mississippian Osage formation, (2) the Pennsylvanian Strawn Formation, (3) the Jurassic "Gray" sandstones (Smackover), (4) the Jurassic Cotton Valley Formation, and (5) the Cretaceous Hosston Formation. These and two previously reported occurrences in the Smackover suggest a widespread occurrence of sphalerite in subsurface pre-Tertiary sedimentary rocks of north Texas and the Gulf Coast.

Preliminary examinations reveal the following two modes of sphalerite occurrence: (1) as finely crystalline patches of cement in sandstones or limestones, and (2) as individual crystals in ooids or in the cement of oolitic limestones. In some sections, formation of sphalerite appears to be early, with zinc and reduced sulfur derived from nearby shales. In others, late formation and distant sources seem to be most likely. Further studies of the chemistries of sphalerite and fluid inclusions, associated host rock alterations, and temporal and spatial distribution of sphalerite may lead to improved understanding of pore-fluid chemistry during hydrocarbon origin and migration.

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