Petrologic Assessment of Diagenetic Evolution in Temporally Equivalent Carbonates of the Marcellus “Shale” From West Virginia, Pennsylvania, and New York

Abstract

Thin, laterally continuous carbonates that separate the upper and lower “shale” units of the Marcellus subgroup are texturally and compositionally dissimilar due to divergent diagenetic histories. Stratigraphic control of these carbonate units, formally described in New York State as the Cherry Valley Member of the Oatka Creek Formation, is inconsistent across state boundaries and often confusing due to its range in lithology. The Cherry Valley Member contrasts lithologically and petrophysically with bounding organic-rich mudstones, regionally the East Berne Member of the Oatka Creek Formation (overlying) and Bakoven Member of the Union Springs Formation (underlying), and is compositionally dominated by diagenetic carbonates that are crystallized due to preferential fluid flow. Petrologic evaluation of these units indicates spatial relationships associated with various burial, thermal, and exhumation histories in West Virginia, Pennsylvania, and New York. Aggrading neomorphism, or recrystallization in which the crystal size of the same or replaced carbonate mineral has increased, has overprinted many depositional fabrics, though the presence of pelagic fauna, including dacryoconarids and goniatites, suggest a deeper water depositional environment. Diagenetic processes include eogenetic nodule formation prior to lithification and growth of granular pore-filling calcite cement or bladed-to-equant calcite fossil fill, mesogenetic recrystallization and structural deformation associated with Appalachian burial and thermal maturation, and telegenetic interaction with meteoric waters during exhumation. Calcite is the predominant diagenetic mineral, with other phases that include ferroan calcite, dolomite, ankerite, siderite, barite, and sphalerite.

Eight cores from West Virginia (3), Pennsylvania (2), and New York (3) are qualitatively and quantitatively described by petrologic methods, including thin section petrography and scanning electron microscopy, to integrate spatial relationships between the diagenetic framework of these rocks at a range of scales. Compositional analyses describe the distribution of mineral phases and textures, as determined by petrological observation, and include reflectance spectroscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy. Stable carbon and oxygen isotope compositions of CO₂ gas, which result from the acidification of carbonate minerals, are measured using off-axis integrated cavity output laser spectroscopy.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018