Geology topics

Thomas H. Darrah

Publications and source records attributed to Thomas H. Darrah.

3 recordsLinked to original sources

USGS2022

Integrated geochemical approach to determine the source of methane in gas hydrate from Green Canyon Block 955 in the Gulf of Mexico

Massive volumes of gas are sequestered within gas hydrate in subsurface marine sediments in the Gulf of Mexico. Methane associated with gas hydrate is a potentially important economic resource and a significant reservoir of carbon within the global carbon cycle. Nevertheless, uncertainties remain about the genetic source (e.g., microbial, thermogenic) and possible migration history of natural gas incorporated into hydrate. Previous studies have primarily used the hydrocarbon molecular (CH 4 /C 2 H 6 +) and isotopic (δ 13 C-CH 4 , δ 2 H-CH 4 ) compositions of natural gas to address these uncertainties. However, hydrocarbon tracers are altered by mixing, oxidation, secondary methanogenesis, or fluid migration, which presents challenges when deciphering the mechanisms responsible for methane formation and accumulation. To evaluate the genetic source of natural gases from Green Canyon Block 955 (GC 955), east of the Sigsbee escarpment, we collected and analyzed samples from the first pressurized hydrate-bearing sediment cores collected from a coarse-grained hydrate reservoir in the Gulf of Mexico. Gas samples were analyzed for hydrocarbon gas (C 1 –C 5 ), major gas (e.g., N 2 , CO 2 ), and noble gas (He-Xe) abundance and isotopic (e.g., δ 13 C-CH 4 , δ 2 H-CH 4 , δ 13 C-CO 2 , δ 15 N-N 2 , 3 He/ 4 He, 4 He/ 20 Ne) compositions. We determined that natural gas in hydrates from this location are predominantly of primary microbial origin (conservatively at least 76%) and are formed by the hydrogenotrophic (CO 2 reduction) methanogenesis pathway. We also note increased thermogenic proportions (∼6%) in a hydrate-bearing layer below the main hydrate-bearing interval (separated by a 5-m water-bearing layer). Our results suggest that microbial methane may be abundant below the base of gas hydrate stability at GC 955.

Louisiana↗

USGS2016

A gas-tracer injection for evaluating the fate of methane in a coastal plain stream: Degassing versus in-stream oxidation

Methane emissions from streams and rivers have recently been recognized as an important component of global greenhouse budgets. Stream methane is lost as evasion to the atmosphere or in-stream methane oxidation. Previous studies have quantified evasion and oxidation with point-scale measurements. In this study, dissolved gases (methane, krypton) were injected into a coastal plain stream in North Carolina to quantify stream CH 4 losses at the watershed scale. Stream-reach modeling yielded gas transfer and oxidation rate constants of 3.2 ± 0.5 and 0.5 ± 1.5 d –1 , respectively, indicating a ratio of about 6:1. The resulting evasion and oxidation rates of 2.9 mmol m –2 d –1 and 1,140 nmol L –1 d –1 , respectively, lie within ranges of published values. Similarly, the gas transfer velocity ( K 600 ) of 2.1 m d –1 is consistent with other gas tracer studies. This study illustrates the utility of dissolved-gas tracers for evaluating stream methane fluxes. In contrast to point measurements, this approach provides a larger watershed-scale perspective. Further work is needed to quantify the magnitude of these fluxes under varying conditions (e.g., stream temperature, nutrient load, gradient, flow rate) at regional and global scales before reliable bottom-up estimates of methane evasion can be determined at global scales.

Environmental Science & Technology↗

USGS2012

Determining the source and genetic fingerprint of natural gases using noble gas geochemistry: a northern Appalachian Basin case study

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 (C 1 ,C 2+ ) ( 13 C) 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 ( 13 C[ C1 C2 ][ 13 C 1 13 C 2 ]: –9 ), isotopically light methane, with low ( 4 He) (average, 1 10 3 cc/cc) elevated 4 He/ 40 Ar and 21 Ne / 40 Ar (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 ( 13 C [C1 – C2] [ 13 C 1 13 C 2 ]: 3 ), with high ( 4 He) (average, 1.85 10 3 cc/cc) 4 He/ 40 Ar and 21 Ne / 40 Ar near crustal production levels and elevated crustal noble gas content (enriched 4 He, 21 Ne , 40 Ar ). 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.

New York↗