Microbial methane generation and gas transport in shallow sediments of an accretionary complex, southern hydrate ridge (ODP Leg 204), offshore Oregon, USA

Sediments at the southern summit of Hydrate Ridge display two distinct modes of gas hydrate occurrence. The dominant mode is associated with active venting of gas exsolved from the accretionary prism and leads to high concentrations (15%–40% of pore space) of gas hydrate in seafloor or near-surface sediments at and around the topographic summit of southern Hydrate Ridge. These near-surface gas hydrates are mainly composed of previously buried microbial methane but also contain a significant (10%–15%) component of thermogenic hydrocarbons and are overprinted with microbial methane currently being generated in shallow sediments. Focused migration pathways with high gas saturation (>65%) abutting the base of gas hydrate stability create phase equilibrium conditions that permit the flow of a gas phase through the gas hydrate stability zone. Gas seepage at the summit supports rapid growth of gas hydrates and vigorous anaerobic methane oxidation. The other mode of gas hydrate occurs in slope basins and on the saddle north of the southern summit and consists of lower average concentrations (0.5%–5%) at greater depths (30–200 meters below seafloor [mbsf]) resulting from the buildup of in situ–generated dissolved microbial methane that reaches saturation levels with respect to gas hydrate stability at 30–50 mbsf. Net rates of sulfate reduction in the slope basin and ridge saddle sites estimated from curve fitting of concentration gradients are 2–4 mmol/m³/yr, and integrated net rates are 20–50 mmol/ m²/yr. Modeled microbial methane production rates are initially 1.5 mmol/m³/yr in sediments just beneath the sulfate reduction zone but rapidly decrease to rates of 100 mbsf. Integrated net rates of methane production in sediments away from the southern summit of Hydrate Ridge are 25–80 mmol/m²/yr. Anaerobic methane oxidation is minor or absent in cored sediments away from the summit of southern Hydrate Ridge. Ethane-enriched Structure I gas hydrate solids are buried more rapidly than ethane-depleted dissolved gas in the pore water because of advection from compaction. With subsidence beneath the gas hydrate stability zone, the ethane (mainly of low-temperature thermogenic origin) is released back to the dissolved gas-free gas phases and produces a discontinuous decrease in the C₁/C₂ vs. depth trend. These ethane fractionation effects may be useful to recognize and estimate levels of gas hydrate occurrence in marine sediments

Revised genetic diagrams for natural gases based on a global dataset of >20,000 samples

The origin of natural gases, in particular those containing methane (CH4 or C1), ethane (C2H6 or C2), propane (C3H8 or C3) and carbon dioxide (CO2), is commonly interpreted using binary genetic diagrams of d13C-C1 versus C1/(C2 + C3), d13C-C1 versus d2H-C1 and d13C-C1 versus d13C-CO2. These diagrams are empirical, but their currently used genetic fields were proposed around 30–40 years ago based on geographically and geologically limited datasets of tens to few hundreds gas samples. As a result, many recently collected gas samples plot outside of accepted genetic fields making these genetic diagrams partly inadequate for the purpose of gas interpretation. Here, we update the genetic diagrams using geochemical and geological data on 20,621 gas samples from a variety of geographical areas (76 countries and territories on six continents) and geological habitats (conventional and unconventional petroleum reservoirs, petroleum seeps and mud volcanoes, gas hydrates, volcanic/geothermal/hydrothermal manifestations, seeps and groundwater in serpentinized ultramafic rocks, aquifers, freshwater and marine sediments, igneous and metamorphic rocks). The revision includes genetic fields for primary microbial gases from CO2 reduction and methyl-type fermentation, secondary microbial gases generated during petroleum biodegradation, thermogenic and abiotic gases. The genetic field of thermogenic gases now includes early mature (d13C-C1 as low as 75‰) and very late mature (d13C-C1 around 15‰) gases recently recognized in various petroleum systems. Abiotic C1 is not necessarily 13C-enriched (d13C > 20‰) as was often considered in the past. The d13C values of abiotic C1 can be as negative as around 50‰, although a minor component of biotic (microbial or thermogenic) C1 is often associated with abiotic gas. In addition, the diagrams display molecular and isotopic changes that accompany post-generation processes of mixing, migration, biodegradation, thermochemical sulphate reduction and oxidation. The proposed diagrams cover the vast majority of hydrocarbon-containing gases currently known to exist in nature, are the most comprehensive empirical gas genetic diagrams published to date, and thus represent an essential tool for interpretations of natural gases. Still, holistic integration of geochemical and geological data is necessary to better interpret the origin of natural gases and processes that affected their composition.Published109-1206A. Geochimica per l'ambiente e geologia medicaJCR Journa

Gas Venting and Gas Hydrate Stability in the Northwestern Gulf of Mexico Slope: Significance to Sediment Deformation

Gas hydrate is abundant on the Gulf of Mexico continental slope because hydrocarbon gases from the subsurface petroleum system vent prolifically to the sea floor at low temperatures and high pressures within the gas hydrate stability zone (GHSZ). Compositions of gases venting from hydrate-bearing sediments show that gas hydrate decomposition is not significant at this point in geologic time. Structure II gas hydrate is commonly encountered, and contains C -C hydrocarbons that crystallize from relatively unaltered gases that have migrated from deep in the section. Most structure II gas hydrate sites studied from the ∼540-1930 m water depth range are stable, and gas hydrate is accumulating to considerable depth in sediment because of constant gas venting. Bacterial methane hydrate is also abundant in the Gulf of Mexico. Chemosynthetic communities are preferentially associated with gas hydrate. Massive accumulation of gas hydrate in the Gulf of Mexico deforms shallow sediments. Gas hydrate may be a geologic hazard because exploration and exploitation activities on the Gulf slope may cause localized decomposition to form free gas, and modify the physical properties of hydrate-bearing sediments. 1

Molecular and isotopic properties of gases from ODP Holes of ODP Leg 204

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 refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure

Exclusion of 2-methylbutane (isopentane) during crystallization of structure II gas hydrate in sea-floor sediment, Gulf of Mexico. Organic Geochemistry

Abstract Structure II gas hydrate is abundant across the central Gulf of Mexico continental slope. Sediment that directly overlies nodular gas hydrate was collected with a piston core at $1920 m water depth in the Atwater Valley (AT) 425 area of the lower continental slope. The gas hydrate has C 1 ±C 5 molecular and isotopic properties consistent with structure II gas hydrate that crystallized from relatively unaltered thermogenic vent gas. The gas hydrate contains mainly methane, ethane, propane and butanes, with 2-methylbutane (isopentane ) as a minor component (< 0.2%). Sediment that closely overlies the gas hydrate (within < 1 m) is characterized by an anomalous abundance of 2-methylbutane (as much as 9.6%). Because the molecular diameter of 2-methylbutane is too large for structure II gas hydrate, the 2-methylbutane appears to accumulate preferentially in adjacent sediment as a direct consequence of massive gas hydrate crystallization. The 2-methylbutane is interpreted to be a molecular marker of recent or ongoing net accumulation of structure II gas hydrate. Abundant 2-methylbutane in sediment also could be a precursor to the natural occurrence of structure H gas hydrate, and other new gas hydrate structures not yet discovered in the geologic environment.