Thermogenic methane

Of special interest in recent years has been the analysis of natural gas hydrates that form in marine sediments and polar rocks when saline pore waters are saturated with gas at high pressure and low temperature. Large and 5D-variations of hydrate bound methane, summarized by Kvenvolden (1995) and Milkov (2005), suggest that gas hydrates represent complex mixtures of gases of both microbial and thermogenic origin. The proportions of both gas types can vary significantly even between proximal sites. 

Sherwood-Lollar et al. 1993,2002). In contrast to thermogenic hydrocarbons where higher hydrocarbons (ethane, propane, butane) are enriched in and D relative to methane, abiogenic alkanes may be depleted in C and D relative to methane. These depletion patterns relative to methane may be produced by polymerization reactions of methane precursors (Sherwood-Lollar et al. 2002). 

Of the two sources of methane (biogenic and thermogenic) in natural hydrates, most of the gas is biogenic, that is, from bacterially generated methane, with anecdotal exceptions. 

Methane formed by biogenic processes ranges in < 13C from about — 60%c to —85%c, while methane from thermogenic processes ranges from —25%c to —55%c (Hunt 1979, p. 25). 

Figure 4 The and 8 H isotopic values for methane gas found dissolved in saline groundwaters from several sites on the Canadian and Fennoscandian Shields (samples for each area are circled). These are compared to the bacterial and thermogenic ranges of Schoell (1988). and Bp are the ranges as outlined by the boxes of bacterial methane production from CO2 reduction and acetate fermentation, respectively (Collar et al. 1993b) (reproduced by permission of Elsevier from Geochim. Cosmochim. Acta, 1993, 57, 5087-5097).

The He content of waters has rarely been investigated for hydrocarbon exploration. However, Dyck and Dunn (1987) report that broad, coincident anomalies of >2.4 pL/L He and >0.4 pL/L methane in wells and springs in the Cypass Hills district, Saskatchewan, Canada, correlate with commercial oil and gas fields and/or tectonic features. High He concentrations can distinguish thermogenic methane from biogenic methane (marsh gas). 

The most complete set of published observaTions on the sources, sinks, and distributions of methane in coastal waters is that by researchers at Texas A M University. Without detailing an exhaustive list of the observations, many of which deal with the thermogenic as well as biogenic sources of methane, the following papers are most relevant to our own discussion of coastal distributions of methane (7, 10-14). 

Biogenic and thermogenic gases can usually be distinguished on the basis of chemical and isotopic composition. Biological gas is dominantly composed of methane, which is depleted in relative to thermogenic methane (Whiticar... 

Fig. 14.8 Discrimination of biogenic and thermogenic methane sources based on A. The carbon and hydrogen isotopic composition of the methane (after Schoell 1988), and B. The ratio of methane (Cj) to higher hydrocarbons (C + C ) plotted against the carbon isotopic composition of methane (from Claypool and Kvenvolden 1983).

If methane, either from biogenic or thermogenic sources, is present in high enough concentration to stabilize the hydrate structure at thermodynamically favourable conditions (Figs. 14.3 and... 

---