Natural seepage of shale gas and the origin of “eternal flames” in the Northern Appalachian Basin, USA

Abstract

Natural hydrocarbon gas seeps are surface expressions of Petroleum Seepage Systems, whereby gas is ascending through faults from pressurized reservoirs that are typically associated with sandstones or limestones. A spectacular “eternal flame” in western New York State marks a gas macroseep of dominantly thermogenic origin emanating directly from deep shale source rocks, which makes this a rare case in contrast to most Petroleum Seepage Systems where gas derives from conventional reservoirs. The main flaming seep releases about 1 kg of methane per day and may feature the highest ethane and propane (C2 þ C3) concentration ever reported for a natural gas seep (w35 vol. %). The same gas is also released to the atmosphere through nearby invisible and diffuse seepages from the ground. The synopsis of our data with available gas-geochemical data of reservoir gases in the region and the stratigraphy of underlying shales suggests that the thermogenic gas originates from Upper Devonian shales without intermediation of a conventional reservoir. A similar investigation on a second “eternal flame” in Pennsylvania suggests that gas is migrating from a conventional sandstone pool and that the seep is probably not natural but results from an undocumented and abandoned gas or oil well. The large flux of the emitted shale gas in New York State implies the existence of a pressurized gas pool at depth. Tectonically fractured shales seem to express “naturally fracked” characteristics and may provide convenient targets for hydrocarbon exploration. Gas production from “tectonically fracked” systems might not require extensive artificial fracking.