Fossil fuel related water-rock interaction in the Appalachian Basin, Pennsylvania and New York: A geochemical and strontium isotope investigationChapman, Elizabeth C (2012) Fossil fuel related water-rock interaction in the Appalachian Basin, Pennsylvania and New York: A geochemical and strontium isotope investigation. Doctoral Dissertation, University of Pittsburgh. (Unpublished) This is the latest version of this item.
AbstractFossil fuel resources (coal/oil/natural gas) have been extracted from the Appalachian Basin for over a century. Related fluids, including outflows from inactive coal mines and produced water from oil and gas wells, pose environmental concern. Three studies are presented which integrate strontium (Sr) isotopes with geologic and geochemical analyses to investigate the movement of fossil fuel-related fluids through the environment and their interactions with natural waters and geologic materials. In the first study, abandoned oil and gas wells tapping Upper Devonian units in northwestern Pennsylvania serve as conduits for the movement of waters with high amounts of total dissolved solids (TDS). This work demonstrates that the waters originate as acid mine drainage (AMD), which infiltrates shallow, siderite-cemented sandstone aquifers. Dissolution of the siderite cement produces waters rich in dissolved iron, which flow to the surface via wells with compromised casings. The second study focuses upon high-TDS water produced during natural gas extraction from the Middle Devonian Marcellus Shale. Strontium isotopic ratios of produced waters collected from across Pennsylvania define a relatively narrow set of values (87Sr/86Sr = 0.71008-0.71212) that falls above that of Devonian seawater, and is distinct from most western Pennsylvania AMD and Upper Devonian Venango Formation oil and gas brines. The uniformity of the isotope ratios suggests a basin-wide source of dissolved solids. Mixing models indicate that Sr isotopes can potentially detect even small amounts (<0.01%) of Marcellus produced water interacting with natural waters. In the third study, a sequential extraction procedure was carried out on drill cuttings from a well in south-central New York that penetrates the Marcellus Shale and adjacent units. Sr isotopic values of water and ammonium acetate leachates, representing soluble salt and clay-exchangeable fractions, fall within or just below the range measured for Marcellus produced waters, indicating that the waters’ dissolved solids originate from within the shale. Acetic acid leaches targeting carbonate minerals yield 87Sr/86Sr ratios below those of the water and ammonium acetate leaches, but higher than those expected for Devonian seawater. Rb-Sr isotope systematics suggest mixing between a seawater-derived source and more radiogenic material, and multiple generations of carbonate mineral precipitation. Share
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