Global estimates of hydrate-bound gas in marine sediments: how much is really out there?

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Abstract

It is generally assumed that oceanic gas hydrates contain a huge volume of natural gases, mainly methane. The most widely cited estimate of global hydrate-bound gas is 21×1015 m3 of methane at STP (or ∼10,000 Gt of methane carbon), which is proposed as a “consensus value” from several independent estimations. This large gas hydrate reservoir is further suggested as an important component of the global carbon cycle and as a future energy source. Here, I present a revised and updated set of well-justified global estimates and discuss how and why they changed over time. It appears that the global estimates of hydrate-bound gas decreased by at least one order of magnitude from 1970s–early 1980s (estimates on the order of 1017–1018 m3) to late 1980s–early 1990s (1016 m3) to late 1990s–present (1014–1015 m3). The decrease of estimates is a result of growing knowledge of the distribution and concentration of gas hydrates in marine sediments and ongoing efforts to better constrain the volume of hydrate-bearing sediments and their gas yield. These parameters appear to be relatively well constrained at present through DSDP/ODP drilling and direct measurements of gas concentrations in sediments. The global estimate of hydrate-bound gas that best reflects the current knowledge of submarine gas hydrate is in the range (1–5)×1015 m3 (∼500–2500 Gt of methane carbon). A significantly smaller global gas hydrate inventory implies that the role of gas hydrates in the global carbon cycle may not be as significant as speculated previously. Gas hydrate may be considered a future energy source not because the global volume of hydrate-bound gas is large, but because some individual gas hydrate accumulations may contain significant and concentrated resources that may be profitably recovered in the future.

Introduction

Natural gas hydrate—a crystalline mineral composed of water and gases (mainly methane)—has been subjected to numerous laboratory and field studies in the past decade including three dedicated Ocean Drilling Program (ODP) Legs: 146 (Westbrook et al., 1994), 164 (Paull et al., 1996) and 204 (Tréhu et al., 2003). The main driving force in gas hydrate research is the common assumption that the global gas hydrate inventory contains a huge amount of methane carbon (Kvenvolden, 1999) and therefore may be both a potential energy resource (Collett, 2002) and a significant player in the global carbon cycle Dickens, 2001b, Kvenvolden, 2002. However, the global estimates of hydrate-bound gas in marine sediments vary by several orders of magnitude and are thought to be highly uncertain Kvenvolden, 1999, Lerche, 2000.

Makogon (1966) was apparently the first to publish a methodology of estimating hydrate-bound gas in the subsurface, although the first gas hydrate samples were recovered much later (Yefremova and Zizchenko, 1974). Around 20 global estimates of submarine gas hydrate have been published over the last 30 years, the earliest by Trofimuk et al. (1973) and the latest by Milkov et al. (2003). Kvenvolden (1999) analyzed a subset of the global estimates (Table 1) and suggested that 21×1015 m3 of methane (or ∼10,000 Gt of methane carbon, Kvenvolden and Lorenson, 2001) should be considered as a “consensus value” because some independent estimates (e.g., by Kvenvolden, 1988, MacDonald, 1990) converge around that value. The value 10,000 Gt of methane carbon is currently used to justify gas hydrate research (e.g., Wood et al., 2002, Hesse, 2003) and is incorporated into the models of the global organic carbon cycle (Kvenvolden, 2002). However, Lerche (2000) attempted a statistical analysis of some estimates listed by Kvenvolden (1999) and found no systematic pattern of convergence of the published estimates. Moreover, a careful examination of the literature suggests that some estimates listed by Kvenvolden (1999) were never presented (e.g., Makogon, 1981, Dickens et al., 1997), others were cited erroneously (e.g., Trofimuk et al., 1977, Dobrynin et al., 1981) and some early estimates were not previously summarized (e.g., Trofimuk et al., 1973, Trofimuk et al., 1975, Trofimuk et al., 1979, Cherskiy and Tsarev, 1977). In addition, two new global estimates of hydrate-bound gas in the ocean were presented recently Soloviev, 2002, Milkov et al., 2003. A detailed historical analysis of the evolution of global gas hydrate estimates, as well as the approaches, assumptions and data used to produce them, appears necessary to better understand how much gas may actually be concentrated in gas submarine hydrate.

The objectives of the present study are to: (1) present an updated inventory of global estimates made over the last 30 years; (2) demonstrate how these estimates changed as a function of time and growing knowledge of gas hydrate distribution and concentration in marine sediments; and (3) assess the range of global estimates that would reflect the current understanding of gas hydrate. The significance of the presented data and conclusions for various speculations and models that consider the size of the global gas hydrate reservoir are discussed.

Section snippets

Review of global estimates in a chronological order

A list of 20 estimates of the global volume of hydrate-bound gas is presented in Table 2. Only original estimates (i.e., estimates that were not taken from previous works) are included. The widely cited estimate of Dobrynin et al. (1981) is not included in Table 2 because it was taken (and erroneously converted to cubic meters) from the work of Trofimuk et al. (1979). In the following analysis, I consider only the estimates in which the method of estimation is clearly presented and the

Decrease of global estimates as a function of time and the growing understanding of natural gas hydrate

The well-justified global estimates of hydrate-bound gas in the marine sediments described above are plotted in Fig. 1a as a function of year in which the estimate was made. It appears that the global estimates consistently decreased from 1970s to present. The difference between the earliest and the latest estimates is very significant (3–4 orders of magnitude). The negative correlation between the estimated volumes of hydrate-bound gas and the number of published research papers on gas hydrate

Gas hydrate as a component of the global carbon cycle

Several recent studies emphasized that gas hydrate may be an important component of the global carbon cycle Kvenvolden, 2002, Dickens, 2001b. This conclusion was largely based on the assumption that the gas hydrate reservoir of methane carbon is “enormous when compared to the sizes of the other organic carbon reservoirs” (Kvenvolden, 2002, p. 302). The comparison of various reservoirs of organic carbon in the Earth (Fig. 4) indeed suggests that the global gas hydrate inventory contains more

Conclusions

Global estimates of hydrate-bound gas in submarine environments have decreased over the last 30 years from 1018–1017 to 1014–1015 m3 at STP as a result of growing knowledge of gas hydrate distribution and concentration in sediment. It appears that the widely cited and used “consensus value” of 21×1015 m3 of methane (or ∼10,000 Gt of methane carbon) may represent a consensus only for the estimates made in the late 1980s–early 1990s when most studies assumed that gas hydrates occur over large

Acknowledgements

Many views presented in this paper originated in the Laboratory of Gas Hydrate Geology of VNIIOkeangeologia (Saint-Petersburg, Russia) where I was first exposed to the problem of gas hydrates by late G.D. Ginsburg and by V.A. Soloviev. I am thankful to R. Sassen, G.R. Dickens, G. Claypool, Yu.F. Makogon and E.D. Sloan for numerous discussions and encouragement. I also wish to thank A.M. Tréhu because a certain discussion with her convinced me that this paper would be useful for many

Alexei V. Milkov received his BSc (1996) and MSc (1998) degrees in Petroleum Geology from Saint-Petersburg State University in Russia. His BSc thesis on the Messoyakha gas hydrate(?) field in Western Siberia and the MSc thesis on the hydrate-bearing Håkon Mosby mud volcano in the Norwegian Sea were the beginning of his research on gas hydrates and mud volcanoes. He received a PhD in Geology from Texas A&M University (2001) where he studied gas hydrates and hydrocarbon seepage in the Gulf of

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    Alexei V. Milkov received his BSc (1996) and MSc (1998) degrees in Petroleum Geology from Saint-Petersburg State University in Russia. His BSc thesis on the Messoyakha gas hydrate(?) field in Western Siberia and the MSc thesis on the hydrate-bearing Håkon Mosby mud volcano in the Norwegian Sea were the beginning of his research on gas hydrates and mud volcanoes. He received a PhD in Geology from Texas A&M University (2001) where he studied gas hydrates and hydrocarbon seepage in the Gulf of Mexico. He participated in ODP Leg 204 (offshore Oregon) that was dedicated to the study of gas hydrates at an active margin. After a short post-doctoral fellowship at Woods Hole Oceanographic Insttitution, he joined BP in Houston, TX. His research interests are in marine geology, petroleum systems, migration and PVT properties of fluids in deep and shallow subsurface, geochemistry of natural gases and oils, and reservoir compartmentalization. He published 80+ papers and abstracts on gas hydrates and mud volcanoes, and delivered numerous talks on these subjects as an invited speaker at conferences, universities, and agencies.

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