GEOCHEMISTRY, MINERALOGY AND LEACHATE CHEMISTRY OF BASAL MARCELLUS FORMATION WELL CUTTINGS

Horizontal well development in the hydrocarbon-rich basal Marcellus Formation (e.g. Union Springs Member and equivalents) produces relatively large volumes of well cuttings. Sulfide minerals, including pyrite and sphalerite, are common in the basal Marcellus cuttings. This has led to concerns regarding handling and disposal of cuttings due to the potential for acidification of surface waters and mobility of dissolved metals, including lead, copper and zinc. Samples for this study include basal Marcellus well cuttings and fresh outcrop materials from quarry exposures. X-ray diffraction and X-ray fluorescence analyses demonstrate that basal Marcellus samples are generally enriched in sulfides and trace metals, including Cu and Zn, compared to the NASC shale standard. SEM-EDS petrographic examination provides evidence for a primary phase of sulfide mineralization associated with deposition and early diagenetic sulfate reduction; continued burial and early hydrocarbon maturation involved some additional pyrite and sphalerite mineralization. Sulfide mobility was surprisingly limited during late hydrocarbon maturation, and carbonate vein development associated with regional décollement structures.

Leachate systems were constructed to mimic soil burial of Marcellus well cuttings in an open field setting. Leachate collection has been carried out at short period intervals for approximately six months. Initial results demonstrate that pH varies from moderately basic to moderately acidic in samples from the early intervals (a few days to a few weeks), but becomes near neutral within a few weeks, and is sustained over the remaining sampling period. This is consistent with the presence of calcite in the basal Marcellus cuttings samples, as revealed by X-ray diffraction and SEM-EDS analyses. These observations are consistent with previous work with other Paleozoic units, including the Marcellus and Utica Formations (Baughman, et al 2011). Dissolved metals analyses of leachate are currently underway.