δ7Li of Saline Water: Northern Appalachian Basin and Gulf Coast Sedimentary Basin, USA

Macpherson, Gwendolyn L.; Capo, Rosemary C.; Stewart, Brian W.; Phan, Thai; Schroder, Karl; Hammack, Richard W.

Hydraulic fracturing of shale to stimulate natural gas, natural gas liquids, and oil production liberates shale pore waters, and these "produced waters" accompany the target gas and/or liquid products. Formation water from conventional oil and gas production is usually from sandstone or carbonate. Produced waters from shale hydraulic fracturing provide a new opportunity for understanding water-rock interaction in shale reservoir rocks. We used multi-collector ICP-MS to determine the stable isotope ratio of lithium in formation waters from conventional oil and gas reservoirs in the Gulf Coast Sedimentary Basin (GCSB), and in produced waters from hydraulically fractured shales of the Middle and Upper Devonian Marcellus Formation in the northern Appalachian Basin in Pennsylvania. Marcellus produced waters and some GCSB formation waters contain elevated Li/Cl*10,000 (~10 to 15 vs. <1 to ~12 for other oil-and-gas-associated waters and seawater) as well as elevated Li/Mg and Br/Cl. For most fluids analyzed, δ7Li varies approximately inversely with concentration, similar to other published data for formation waters. The δ7Li of the shale fluids analyzed ranges from ~8 to 13‰, much lighter than average seawater (~31‰), and cluster at the lowest δ7Li and highest Li concentrations of formation waters published so far. One sample of GCSB formation water has an anomalously low concentration of Li for its δ7Li value, possibly due to contribution of clay dehydration water. The deep-basin settings of the Marcellus produced-water and GCSB formation water suggest strong influence on Li from clay minerals because clays are the probably the largest reservoir of Li. Previously published experiments on smectite-to-illite transformation, an important diagenetic reaction in sedimentary basins, suggest that fluids should be isotopically heavier than the transformed illite and that Li should partition into the clays. If seawater or evaporated seawater are buried with the sediments, then produced waters should have δ7Li heavier than seawater, not lighter, and total Li concentration should be lower. Our data suggest that either the experimental data do not represent in situ processes, or that there is another source of isotopically light Li being added to pore water in sedimentary basins.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013