Gas hydrate systems at Hydrate Ridge offshore Oregon inferred from molecular and isotopic properties of hydrate-bound and void gases

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doi: 10.1016/j.gca.2004.08.021
Author(s): Milkov, Alexei V.; Claypool, George E.; Lee, Young-Joo; Sassen, Roger
Author Affiliation(s): Primary:
BP America, Exploration and Production Technology Group, Houston, TX, United States
Other:
Korea Institute of Geoscience and Mineral Resources, South Korea
Texas A&M University, United States
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, 69(4), p.1007-1026. Publisher: Pergamon, Oxford, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. 85 refs.; illus., incl. 4 tables, sketch map
Summary: We report and discuss molecular and isotopic properties of hydrate-bound gases from 55 samples and void gases from 494 samples collected during Ocean Drilling Program (ODP) Leg 204 at Hydrate Ridge offshore Oregon. Gas hydrates appear to crystallize in sediments from two end-member gas sources (deep allochthonous and in situ) as mixtures of different proportions. In an area of high gas flux at the Southern Summit of the ridge (Sites 1248-1250), shallow (0-40 m below the seafloor [mbsf]) gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic C2- gases, which migrated vertically and laterally from as deep as 2- to 2.5-km depths. In contrast, deep (50-105 mbsf) gas hydrates at the Southern Summit (Sites 1248 and 1250) and on the flanks of the ridge (Sites 1244-1247) crystallize mainly from microbial methane and ethane generated dominantly in situ. A small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth (e.g., Sites 1244 and 1245). Non-hydrocarbon gases such as CO2 and H2S are not abundant in sampled hydrates. The new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at Hydrate Ridge formed from gas derived from decomposition of deeper and older gas hydrates. Gas hydrate formation at the Southern Summit is explained by a model in which gas migrated from deep sediments, and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone (GHSZ). Free gas migrated into the GHSZ when the overpressure in gas column exceeded sealing capacity of overlaying sediments, and precipitated as gas hydrate mainly within shallow sediments. The mushroom-like 3D shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit, the decrease of gas solubility in shallow sediment, and refocussing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure. Abstract Copyright (2005) Elsevier, B.V.
Year of Publication: 2005
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 29 Economic Geology, Energy Sources; C-13/C-12; Carbon; Carbon dioxide; Chemical composition; East Pacific; Fluid phase; Gas hydrates; Gaseous phase; Geochemistry; Hydrate Ridge; Hydrochemistry; Isotope ratios; Isotopes; Leg 204; Marine sediments; Mid-ocean ridges; Models; Molecular dynamics; Movement; Natural gas; North Pacific; Northeast Pacific; ODP Site 1244; ODP Site 1245; ODP Site 1247; ODP Site 1248; ODP Site 1250; Ocean Drilling Program; Ocean floors; Oregon; Pacific Ocean; Petroleum; Properties; Sediments; Southern Summit; Stable isotopes; United States
Coordinates: N443400 N443500 W1250400 W1250900
N443500 N443500 W1250700 W1250700
N443500 N443500 W1250900 W1250900
N443500 N443500 W1250900 W1250900
N443400 N443400 W1250900 W1250900
Record ID: 2005026951
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands