Alexei V. Milkov

Global gas flux from mud volcanoes: A significant source of fossil methane in the atmosphere and the ocean

[1] There are yet unidentified sources of fossil methane (CH 4 ) in the atmosphere. Mud volcanoes (MVs) are a potentially significant but poorly quantified geologic source of fossil hydrocarbon gases and CO 2 to the atmosphere and the ocean not included in the current models of sources and sinks. Our statistical analysis of 36 previous measurements and estimates of gas flux from individual MVs suggests that the global gas flux may be as high as ∼33 Tg yr -1 (∼15.9 Tg yr during quiescent periods plus ∼17.1 Tg yr -1 during eruptions). Onshore and shallow offshore MVs are estimated to contribute ∼6 Tg yr -1 of greenhouse gases directly to the atmosphere. MVs may contribute 9% of fossil CH 4 missing in the modern atmospheric CH 4 budget, and ∼12% in the preindustrial budget. Large volumes (∼27 Tg yr -1 ) of gas may escape from deep-water MVs, suggesting that global gas flux from the seafloor may be underestimated.

Economic geology of offshore gas hydrate accumulations and provinces

The economic potential of well-studied offshore gas hydrate accumulations and provinces is assessed qualitatively based on consideration of geological, technological, and economic factors. Three types of gas hydrate accumulations are suggested. Structural accumulations occur where thermogenic, bacterial, or mixed gases are rapidly transported from the subsurface petroleum system to the gas hydrate stability zone along faults, mud volcanoes, and other structures (e.g. northwestern Gulf of Mexico, Hydrate Ridge, and Haakon Mosby mud volcano). These accumulations are generally characterized by high gas hydrate concentration in sediment, high resource density, high recovery factors, as well as low development and production costs. It is likely that structural accumulations provide marginal or economic gas hydrate reserves if they represent significant volumes of hydrate-bound gas. Stratigraphic accumulations occur in relatively permeable sediments and form largely from bacterial methane generated in situ or slowly migrated from depth in the section (e.g. Blake Ridge, Gulf of Mexico minibasins). These accumulations are generally characterized by low gas hydrate concentration in sediments and low recovery factor, as well as high development and production costs. Stratigraphic accumulations mainly provide a subeconomic gas hydrate resource. However, in cases such as the Nankai Trough province, high gas hydrate concentration occurs in permeable sand layers and may represent a viable exploration and exploitation target. Less geological data are available on the combination gas hydrate accumulations controlled both by structures and stratigraphy. On the global scale, gas hydrate reserves are likely to represent only a small fraction of the gas hydrate resource because the largest volume of gas hydrate is in subeconomic stratigraphic accumulations. However, some concentrated gas hydrate accumulations may be exploited profitably, and those should be subjected to detailed quantitative economic analysis.

In situ methane concentrations at Hydrate Ridge, offshore Oregon: New constraints on the global gas hydrate inventory from an active margin

The widespread presence of bottom-simulating reflectors (BSRs) on continental margins has bolstered suggestions that gas hydrates and free gas constitute a large dynamic reservoir of CH4 carbon and a vast potential source of energy. However, only a few hydrate-bearing areas have been drilled, and of these, the amount of CH4 has only been directly quantified in 18 discrete samples from 3 holes on Blake Ridge, east of Georgia. Here we report and discuss 30 direct measurements of CH4 concentration in sediments above and below the BSR at Hydrate Ridge on a tectonically active margin offshore Oregon. High CH4 concentrations (71–3127 m M ) support abundant gas hydrate (occupying an average of ∼11% of porosity) and free gas (occupying ∼4% of porosity in 1 sample) in a restricted area where hydrocarbon gases migrate from the deep accretionary complex to the seafloor. In a larger area lacking this hydrocarbon supply, lower CH4 concentrations (10–893 m M ) indicate less gas hydrate (average ∼1% of porosity) and little or no free gas. Overall, the amount of CH4 at Hydrate Ridge is significantly less than that at Blake Ridge. These results challenge certain interpretations, including the global volume of hydrate-bound CH4, which though large, may be four to seven times less than widely cited estimates. Speculations on the distribution and role of gas hydrate and free gas need revision.

Thermogenic vent gas and gas hydrate in the Gulf of Mexico slope: Is gas hydrate decomposition significant?

Samples of vent gas and gas hydrate on the Gulf of Mexico slope were collected by research submersible (∼540 m water depth) and by piston coring (∼1060–1070 m water depth). Although gas hydrate that crops out is transiently unstable, the larger volume of structure II gas hydrate in the gulf is stable or increasing in volume because gas from the subsurface petroleum system is venting prolifically within the gas hydrate stability zone. Vent gas from gas hydrate shows no meaningful molecular evidence of gas hydrate decomposition. Gas hydrate fabrics, mainly vein fillings, are typical of ongoing crystallization. Once crystallized, most hydrocarbons are protected from bacteria within the crystal lattice of gas hydrate. A leaky petroleum system is proposed to be the main source of thermogenic greenhouse gases in the central gulf. Stable gas hydrate sequesters large volumes of greenhouse gases, suggesting that gas hydrate may not be a significant factor in models of climate change at present.

Thickness of the gas hydrate stability zone, Gulf of Mexico continental slope

The maximum thickness of the gas hydrate stability zone (GHSZ) at key gas hydrate study sites was estimated, and a generalized GHSZ profile across part of the central Gulf of Mexico slope was constructed. Maximum thickness of the GHSZ increases with water depth at the shallowest site (∼540 m water depth) from about 450 m to about 1150 m at the deepest site (∼1930 m water depth). The occurrence of gas hydrate in the subsurface is largely controlled by structural focussing of hydrocarbons, with gas hydrate most abundant at the rims of salt withdrawal basins and less abundant within intrasalt basins. The postulated geometry of large subsurface gas hydrate accumulations shows strong structural control. Bottom simulating reflectors (BSRs) are absent or infrequently observed in the Gulf of Mexico because of structural focusing of the free hydrocarbon gases that form gas hydrate. New insight to the geometry of the GHSZ in the Gulf of Mexico continental slope will contribute to improved application of geophysics to map the distribution of gas hydrates. Improved understanding of three-dimensional geometry of gas hydrate accumulations will contribute to better assessment of gas hydrate volumes as a future energy resource, as a geohazard, and will constrain theories relating the role of gas hydrate in past climate change.

Methane emission from mud volcanoes in eastern Azerbaijan

Methane (CH4) flux to the atmosphere was measured from gas vents and, for the first time, from soil microseepage at four quiescent mud volcanoes and one “everlasting fire” in eastern Azerbaijan. Mud volcanoes show different activity of venting craters, gryphons, and bubbling pools, with CH4 fluxes ranging from less than one to hundreds of tons per year. Microseepage CH4 flux is generally on the order of hundreds of milligrams per square meter per day, even far away from the active centers. The CH4 flux near the everlasting fires (on the order of 105 mg·m−2·d−1) represents the highest natural CH4 emission from soil ever measured. The specific CH4 flux to the atmosphere, between 102 and 103 t·km−2·yr−1, was similar to specific flux from other mud volcanoes in Europe. At least 1400 tons of CH4 per year are released from the investigated areas. It is conservatively estimated that all onshore mud volcanoes of Azerbaijan, during quiescent activity, may still emit ∼0.3–0.9 × 106 t of CH4 per year into the atmosphere. The new data fill a significant gap in the worldwide data set and confirm the importance of geologic sources of greenhouse CH4, although they are not yet considered in the climate-study budgets of atmospheric CH4 sources and sinks.

Geochemical evidence of secondary microbial methane from very slight biodegradation of undersaturated oils in a deep hot reservoir

A rare finding of early mature undersaturated oils with low gas/oil ratios enables us to document secondary microbial methane generation during very slight biodegradation in a deep hot reservoir in the ultradeep-water of the Gulf of Mexico. In three studied gas samples, methane is enriched in 13 C (δ 13 C is from -63‰ to -64‰) relative to pure thermogenic methane (estimated δ 13 C is from -71‰ to -67‰) and pure primary microbial methane (δ 13 C is -68‰). Carbon dioxide in gases has δ 13 C values that negatively correlate with δ 13 C values of pure thermogenic methane. Methane is unusually enriched in heavy isotope 2 H relative to associated ethane. Some extracted oils are depleted in long-chain alkyl aromatics. These lines of geochemical evidence suggest anaerobic microbial degradation of oil and subsequent reduction of resulting carbon dioxide to methane. Although specific geobiological details of secondary microbial methane generation are unclear, this process may be partially responsible for charging some of the largest gas and gas hydrate fields in the world.

Gas venting and subsurface charge in the Green Canyon area, Gulf of Mexico continental slope: evidence of a deep bacterial methane source?

Abstract Questions as to the role of modern carbon in methanogenesis and the maximum depth of methane sources in the Gulf of Mexico continental slope remain unanswered. A research submersible was used to sample mixed bacterial and thermal gas ( δ 13 C of methane=−62.8‰, δD =−176‰) venting to the water column from the Gulf slope in Green Canyon (GC) 286. The Δ 14 C value of the methane (−998‰) is consistent with fossil carbon. Another gas vent on GC 185 is 100% methane ( δ 13 C =−62.9‰, δD =−155‰) and may be from a bacterial source. The Δ 14 C (−997‰) of this bacterial methane is also consistent with fossil carbon. Fossil bacterial methane and thermal hydrocarbons are present in Pliocene to Pleistocene reservoirs (∼3509–4184 m) of Genesis Field (GC 205, 161, 160). Oil in these reservoirs is biodegraded but gas is not, suggesting that gas charge to reservoirs continues presently at 3–4 km depth. Mixed thermal and bacterial methane may charge the deep reservoirs, and fossil methane from depth may ultimately vent on the sea floor at GC 286 and GC 185. Results of this study of Green Canyon suggest that bacterial methane in gas vents and in reservoirs is from deep fossil sources.

Two-dimensional modeling of gas hydrate decomposition in the northwestern Gulf of Mexico: significance to global change assessment

Abstract Thinning of the gas hydrate stability zone (GHSZ) in response to bottom water temperature increases and drops in sea level similar to those during Pliocene and Pleistocene time was modeled on two-dimensional (2D) regional scale in the northwestern Gulf of Mexico. A sea level drop of 100 m is unlikely to significantly influence the stability of gas hydrate, especially when coupled with an expected decrease in water temperature. A bottom water temperature increase of 4 °C may lead to appreciable (∼30%) thinning of the GHSZ. Neither a 100-m drop in sea level nor a 4 °C bottom water temperature increase is hypothesized to initiate significant gas flux from decomposition of gas hydrate. Hydrocarbon gases may have been released from decomposed gas hydrate to sediment at a rate considerably lower than the preliminary estimated late Pleistocene–Holocene total gas seepage from a leaky subsurface petroleum system. Potential input of greenhouse gases into the ocean is suggested to be less significant. Several processes such as recrystallization of gas hydrate in the GHSZ, trapping of free gas below the GHSZ, and microbial oxidation of hydrocarbons in sediment by bacteria and archaea contribute to sequestration and destruction of gas from gas hydrate decomposition. Gas hydrate in the Gulf of Mexico is frequently associated with enormous volumes of authigenic carbonate rock, depleted in 13 C, that sequesters a large pool of carbon in sediment and perturbs the carbon cycle. These factors appear to significantly decrease the role of hydrate-derived gas in global change. An improved understanding of how the gas hydrate system of the Gulf of Mexico responds to natural variation will contribute to better assessment of gas hydrate as an agent of global change.

Methanogenic biodegradation of petroleum in the West Siberian Basin (Russia): Significance for formation of giant Cenomanian gas pools

Approximately 1700 tcf (48 trillion m3) of dry gas (99% methane) reserves and resources occur in western Siberia, mostly in shallow (1500 m [4921 ft]) Cenomanian pools in the northern part of the basin. This dry gas constitutes about 11% of the worlds conventional gas endowment and about 17% of the annual gas production. The origin of the dry gas has been debated extensively over the last 45 yr but remains controversial. Widely discussed hypotheses on the origin include early-mature thermogenic gas from coal, primary microbial gas from dispersed organic matter or coal, and thermogenic gas from deep source rocks. However, all these hypotheses are in some ways inconsistent with the molecular or isotopic composition of the gases or the results of basin and petroleum systems modeling. Here, I present geochemical and geological evidence that a significant (although yet not quantified) part of the shallow dry gas in the northern West Siberian Basin originated from methanogenic biodegradation of petroleum. Circumstantial evidence includes the occurrence of heavily biodegraded oil legs and residual oil in many Cenomanian gas pools, as well as geochemical evidence of heavy to slight biodegradation in Jurassic–Albian reservoirs commonly underlying the Cenomanian pools. Direct evidence includes, most importantly, 13C-enriched CO2 in pools with biodegraded oil (although data are limited), which indicates 40–70 wt.% conversion of oil-derived CO2 to secondary microbial methane. Distinctive hydrocarbon molecular and isotopic compositions of most gases in Cenomanian pools (average dryness C1/(sum C1-C5) is 0.9976; average 13C of methane is 51.8) suggest that they represent mixtures of biodegraded thermogenic gases from deep, mainly Jurassic, source rocks and secondary microbial methane with an occasional small addition of primary microbial methane. Contribution of early-mature coal-derived gas is possible in areas with the most significant thermal stress of Hauterivian–Aptian sediments but remains speculative. Review of petroleum habitats of five representative oil-gas-condensate fields in western Siberia (including the worlds second largest gas field, Urengoyskoe) suggests that methanogenic biodegradation may best explain the observed distribution and properties of fluids in the shallow reservoirs of those fields. Recognition of secondary microbial gas in western Siberia helps explain the observed dominance of gas in the shallow, cool northern part of the basin, where conditions were more favorable for prolonged petroleum biodegradation than in the central and southern parts of the basin. Secondary microbial gas has been recognized worldwide and may (1) represent a volumetrically significant exploration target in shallow reservoirs (perhaps more significant than primary microbial gas) and (2) indicate effective thermogenic petroleum systems in the deeper sections. Large volumes (up to 66,500 tcf [1884 trillion m3]) of secondary microbial methane could have been generated from biodegraded petroleum accumulations worldwide. Although a part of that gas accumulated as oil-dissolved, free, and hydrate-bound gas, most gas apparently escaped into the overburden, atmosphere, and ocean and could have affected global climate in the geologic past.