Thermogenic hydrocarbon

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). 

Magnolia fluid property variations are principally controlled by differences in the maturity of emplaced thermogenic hydrocarbons, variable biogenic methane content and phase fractionation processes. 

Faber E, Stahl WJ, Whiticar MJ (1992) Distinction of bacterial and thermogenic hydrocarbon gases. In Vially R (ed) Bacterial Gas Paris Echtions Technip, p 63-74 Foresman JB, Frisch A (1996) Exploring Chemistry with Electronic Stracture Methods. Gaussian Inc, Pittsburgh, PA... 

Only a few sites contain thermogenic hydrates (containing CH4 +higher hydrocarbons), such as in the Gulf of Mexico and in the Caspian Sea. These deposits tend to comprise large accumulations near the seafloor. 

Sassen R, Joye S, Sweet ST, DeFreitas DA, Milkov AV, MacDonald IR (1999) Thermogenic gas hydrates and hydrocarbon gases in complex chemosyn-thetic communities, Gulf of Mexico continental slope. Org Geochem 30 485-497... 

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). 

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).

In principle there is no difference which methane molecule is used to stabilize the clathrate structure. Hydrates formed from biogenic methane are thought to be more widespread in marine sediments, and these can be formed from methane gas produced in situ. In contrast, the presence of thermogenic methane requires gas migration form deeper sediment sources (e.g. in mud volcanoes). Thermogenic methane is also accompanied by the presence of higher hydrocarbons, which may result in the formation of gas hydrate structure II, whereas methane hydrate from biogenic methane is restricted to structure I. 

Conversely, a direct BCGS is prone to produce less liquid hydrocarbons initially, will generate more gas than liquid, may be either under- or over-pressured, and gas generation is the primary source of pore pressure. In addition, the gas generation is of thermogenic origin. Reservoirs in both systems are frequently proximal to the source rock, so gas migration distances are shorter and less tortuous than in a conventional hydrocarbon system. 

Therefore, coupling various ratios (for instance CPI, U/R, and so on), quantitation of molecular markers, organic geochemical parameters (like Cn,ax> UCM), and PAHs allows the determination of the main sources of the ahphatic and aromatic hydrocarbon compounds (petrochemical versus thermogenic/py-rolytic) characteristic of the SWMs and their leachates under study. However, these analyses presented only the assessments of the different sources of the MMs in most samples, not their source strengths (see Sect. 4.4.2). 

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