Abiogenic formation of alkanes in the Earth's crust as a minor source for global hydrocarbon reservoirs

Abstract

Natural hydrocarbons are largely formed by the thermal decomposition of organic matter (thermogenesis) or by microbial processes (bacteriogenesis). But the discovery of methane at an East Pacific Rise hydrothermal vent and in other crustal fluids supports the occurrence of an abiogenic source of hydrocarbons. These abiogenic hydrocarbons are generally formed by the reduction of carbon dioxide, a process which is thought to occur during magma cooling and-more commonly-in hydrothermal systems during water-rock interactions, for example involving Fischer-Tropsch reactions and the serpentinization of ultramafic rocks. Suggestions that abiogenic hydrocarbons make a significant contribution to economic hydrocarbon reservoirs have been difficult to resolve, in part owing to uncertainty in the carbon isotopic signatures for abiogenic versus thermogenic hydrocarbons. Here, using carbon and hydrogen isotope analyses of abiogenic methane and higher hydrocarbons in crystalline rocks of the Canadian shield, we show a clear distinction between abiogenic and thermogenic hydrocarbons. The progressive isotopic trends for the series of C1-C4 alkanes indicate that hydrocarbon formation occurs by way of polymerization of methane precursors. Given that these trends are not observed in the isotopic signatures of economic gas reservoirs, we can now rule out the presence of a globally significant abiogenic source of hydrocarbons.

Laboratory and field-based investigations of subsurface geochemical processes in seafloor hydrothermal systems

“…The use of stable isotope measurements has become an important tool in elucidating processes of abiotic hydrocarbon production (SHERWOOD LOLLAR et al, 2002;. Chapter 4 represents an experimental investigation of the feasibility of hydrogen isotope exchange between low molecular weight hydrocarbons and water and the findings of this chapter have important implications for the utility of hydrogen isotopes in elucidating abiotic hydrocarbon production, in addition to contributing to our understanding of factors influencing the hydrogen isotope composition of hydrocarbons in petroleum systems.…”

“…Recently, studies of 2 H/ 1 H ratios have been expanded to the low molecular weight (LMW) n-alkanes (C 1 to C 5 ), which represent an important class of compounds in a variety of high temperature fluids. The formation of abiogenic LMW n-alkanes associated with serpentinization of ultramafic igneous crust is currently the subject of extensive interest SEEWALD, 2001, 2007;FU et al, 2007;) and δ 2 H measurements been used to infer mechanistic information regarding abiogenesis (SHERWOOD LOLLAR et al, 2002). Moreover, C 1 to C 5 n-alkanes are abundantly present in oil and natural gas formed by thermal maturation of sedimentary organic matter (TISSOT and WELTE, 1984;HUNT, 1996) and there is interest in developing their 2 H/ 1 H signatures as maturation and correlation parameters for petroleum characterization (TANG et al, 2007).…”

“…Though the exact mechanism for production of n-alkanes in highly-reducing crustal fluids is uncertain, it is assumed to be analogous to the industrial Fischer-Tropsch reaction and involves both reduction of inorganic carbon (CO 2 or CO) to methyl/methylene moieties and subsequent polymerization on a catalytic mineral surface (see review in . In the last decade, 2 H/ 1 H ratios have been used to elucidate this phenomenon in addition to stable carbon isotope ratios (SHERWOOD LOLLAR et al, 2002;FU et al, 2007;. Because of the larger relative mass difference between 2 H and 1 H, substantial kinetic H isotope fractionation could occur between homologues during abiogenic polymerization.…”

“…Because of the larger relative mass difference between 2 H and 1 H, substantial kinetic H isotope fractionation could occur between homologues during abiogenic polymerization. As a result of investigations of putative abiogenic C 1 -C 4 n-alkanes recovered from saline fracture waters in crystalline basement rocks of the Canadian Shield, SHERWOOD LOLLAR et al (2002) proposed a characteristic H isotope trend with carbon number whereby δ 2 H meth values (often below -400‰) are extremely low relative to the C 2 -C 4 alkanes (δ 2 H ~ -220 to -320‰), which show a trend of increasing δ 2 H with increasing carbon number (Figure 4.8). This trend has subsequently been observed in several other Precambrian shield gases (SHERWOOD LOLLAR et al, 2006, 2008 and is postulated to reflect preferential cleavage of 12 C-1 H bonds relative to 12 C-2 H bonds during the abiogenic polymerization process, putative kinetic effects associated with 13 C/ 12 C fractionation during polymerization.…”

“…The hydrogen isotope composition of n-alkanes have been utilized to elucidate such diverse processes as abiogenic hydrocarbon formation (SHERWOOD LOLLAR et al, 2002;FU et al, 2007;, formation and biodegradation of petroleum constituents (SCHOELL, 1983;WHITICAR et al, 1985;TANG et al, 2005;SCHIMMELMANN et al, 2006;BOREHAM et al, 2008) and paleoenvironmental changes (SCHEFUSS et al, 2005;SACHSE et al, 2006;EGLINTON and EGLINTON, 2008). However, use of δ 2 H measurements to infer primary isotopic influences (i.e.…”