FRACTURE SIDEWALL CEMENTATION AND VEIN CARBONATE: TRACKING VERTICAL MIGRATION AND OXIDATION OF NATURAL GAS IN THE MARCELLUS FORMATION, CENTRAL NEW YORK

Exposures of the upper members of the Marcellus Formation in Madison County, central New York host joints with cm-scale bands of tightly cemented siltstone and shale symmetrically arranged parallel to vertical fractures. The thickness of cemented fracture sidewall bands decreases upward from fracture bifurcations, suggesting that the cementation fluids migrated upward along fractures. Quantitative x-ray diffraction analyses and scanning electron microscopy indicate that calcite is more abundant in the cemented bands compared to adjacent host rock. Fractures with sidewall cementation occasionally host thin (to 5 mm thick) low-magnesium calcite veins. The calcite forms 0.2-1.0 mm long blocky to prismatic crystals with random orientations relative to the vein walls; finer, scaly low-magnesium calcite microspar is also present in veins. Stable isotope signatures of calcite vein fill and sidewall cemented shale (δ¹⁸O = -7 to -8 PDB; δ¹³C = -8 to -11 PDB) are distinct from carbonate vein material in nearby structurally disturbed basal Marcellus (δ¹⁸O = -8 to -11; δ¹³C = +1 to +4). Biogenic calcite (δ¹⁸O = -5 to -6; δ¹³C = +1 to +2) from outcrops containing sidewall cementation is also isotopically distinct from vein calcite. The strongly negative carbon isotope signatures of the calcite from veins and sidewall cementation bands are most consistent with methane oxidation as a source of carbonate. The negative oxygen values are consistent with precipitation from local meteoric groundwater.

The stratigraphic units containing fracture sidewall cementation features overly the gas-rich basal Marcellus, and natural methane seeps exist within one kilometer of outcrop exposures with these features. Water wells tapping fractured shale aquifers in the area south of the outcrop exposures often yield water with dissolved natural gas sufficient to be flared at the faucet. Such wells are miles from the nearest natural gas production, and many such water wells were in existence prior to gas development in the region. Fracture sidewall cementation documents natural up-migration of methane from the basal Marcellus. These features should be present in geologically similar settings, and may be used to establish natural gas migration pathways in bedrock aquifer systems.