• journal_title：AAPG Bulletin
• Contributor：Andrew G. Hunt ; Thomas H. Darrah ; Robert J. Poreda
• Publisher：American Association of Petroleum Geologists (AAPG)
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.1306/03161211093
• journal_abbrev：AAPG Bulletin
• issn：0149-1423
• volume：96
• issue：10
• firstpage：1785
• section：Articles

Silurian and Devonian natural gas reservoirs present within New York state represent an example of unconventional gas accumulations within the northern Appalachian Basin. These unconventional energy resources, previously thought to be noneconomically viable, have come into play following advances in drilling (i.e., horizontal drilling) and extraction (i.e., hydraulic fracturing) capabilities. Therefore, efforts to understand these and other domestic and global natural gas reserves have recently increased. The suspicion of fugitive mass migration issues within current Appalachian production fields has catalyzed the need to develop a greater understanding of the genetic grouping (source) and migrational history of natural gases in this area. We introduce new noble gas data in the context of published hydrocarbon carbon (C₁,C₂₊) (δ¹³C) data to explore the genesis of thermogenic gases in the Appalachian Basin. This study includes natural gases from two distinct genetic groups: group 1, Upper Devonian (Marcellus shale and Canadaway Group) gases generated in situ, characterized by early mature (Δ¹³C[_{C1 − C2}][δ¹³C₁ − δ¹³C₂]: <–9‰), isotopically light methane, with low (⁴He) (average, 1 × 10⁻³ cc/cc) elevated ⁴He/⁴⁰Ar* and ²¹Ne*/⁴⁰Ar* (where the asterisk denotes excess radiogenic or nucleogenic production beyond the atmospheric ratio), and a variable, atmospherically (air-saturated–water) derived noble gas component; and group 2, a migratory natural gas that emanated from Lower Ordovician source rocks (i.e., most likely, Middle Ordovician Trenton or Black River group) that is currently hosted primarily in Lower Silurian sands (i.e., Medina or Clinton group) characterized by isotopically heavy, mature methane (Δ¹³C_{[C1 – C2]} [δ¹³C₁ − δ¹³C₂]: >3‰), with high (⁴He) (average, 1.85 × 10⁻³ cc/cc) ⁴He/⁴⁰Ar* and ²¹Ne*/⁴⁰Ar* near crustal production levels and elevated crustal noble gas content (enriched ⁴He, ²¹Ne*, ⁴⁰Ar*). Because the release of each crustal noble gas (i.e., He, Ne, Ar) from mineral grains in the shale matrix is regulated by temperature, natural gases obtain and retain a record of the thermal conditions of the source rock. Therefore, noble gases constitute a valuable technique for distinguishing the genetic source and post-genetic processes of natural gases.