Hydrogen Isotopic (D/H) Composition of Organic Matter During Diagenesis and Thermal Maturation

Abstract

Changes in the D/H ratio of sedimentary organic matter (SOM) during thermal maturation have been difficult to interpret because the effects of hydrogen exchange and kinetic fractionations are confounded in natural samples. Recent analytical developments have significantly improved our understanding of the responsible mechanisms. In this paper, we review experimental and field data that document a progressive increase in the D/H ratio of most organic hydrogen at the bulk and molecular levels, and suggest that the transfer of hydrogen from water to organic matter is the most important mechanism leading to those changes. SOM and water in natural petroleum systems approach a pseudoequilibrium D/H fractionation of about −80 to −110‰. D/H ratios of organic hydrogen can preserve quantitative information about paleoclimate throughout diagenesis, and some qualitative information through catagenesis.

The effect of oil expulsion or retention on further thermal degradation of kerogen at the high maturity stage: A pyrolysis study of type II kerogen from Pingliang shale, China

“…D enrichments in n-alkanes, by 30-40‰, have also been reported for natural oil samples at various maturity stages (Li et al, 2001;Jia et al, 2013). However, n-alkanes in extracts from natural sediments and rocks might display more complicated variations in hydrogen isotopes during maturation, although a general trend of D enrichment for n-alkanes has been observed (Dawson et al, 2005(Dawson et al, , 2007Radke et al, 2005;Pedentchouk et al, 2006;Schimmelmann et al, 2006;Kikuchi et al, 2010). This is due to the diversity of sources of n-alkanes in sediments and rocks, and in some cases hydrogen exchange with inorganic hydrogen either in water or minerals has taken place.…”

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

“…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.…”

“…kinetic or equilibrium isotope effects associated with hydrocarbon formation) implicitly assumes the resistance of organic hydrogen to secondary exchange processes over pertinent geologic timeframes. There is growing evidence to suggest that exchange or incorporation of water-derived H into organic molecules can obscure primary isotopic signatures but there are still significant gaps in our understanding of this process SCHIMMELMANN et al, 2006). 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.…”

“…In addition to its obvious ubiquity in hydrothermal systems, water may be a reactive source of H not just during the generation of petroleum hydrocarbons but also during subsequent migration and accumulation processes (LEWAN, 1997;SEEWALD, 2003;SCHIMMELMANN et al, 2006). While aliphatic H is typically considered to be the most isotopically conservative moiety under low temperature conditions (SCHIMMELMANN et al, 2006), aqueous reaction paths at elevated temperatures could facilitate reversible exchange with water-derived H. Experiments conducted under hydrothermal conditions have shown that n-alkanes (below C 7 in length) and their corresponding n-alkenes rapidly attain reversible states of redox-dependant metastable equilibrium under redox-buffered conditions according to the following reaction (SEEWALD, 1994(SEEWALD, , 2003:…”

Late Quaternary climate and environmental reconstruction based on leaf wax analyses in the loess sequence of Möhlin, Switzerland

“…They are straight, saturated carbon-hydrogen chains (C n H 2n+2 ), stable over geological timescales (Eglinton and Eglinton, 2008;Schimmelmann et al, 2006) and thus preserved in various sedimentary archives. Their homologue pattern depends on the type of vegetation (Cranwell, 1973;Marseille et al, 1999;Schwark et al, 2002) and can provide information about whether deciduous trees or grasses were dominant in the research area, whereas deciduous trees produce mainly n-C 27 and grasses predominantly n-C 31 and n-C 33 (Zech et al, 2010;Schäfer et al, 2016b;Schwark et al, 2002).…”