Change in hydrogen isotope composition of n-alkanes, pristane, phytane, and aromatic hydrocarbons in Miocene siliceous mudstones with increasing maturity

Abstract

Variation in the hydrogen stable isotope ratios (D/H ratios) of n-alkanes, pristane, phytane, and aromatic hydrocarbons with increasing maturity was investigated using a set of 3

Isotopic compositions and biomarkers in crude oils from the Tarim Basin: Oil maturity and oil mixing

“…5), which was commonly reported in previous work (Li et al, 2001;Schimmelmann et al, 2004;Dawson et al, 2005Dawson et al, , 2007Lu et al, 2005;Asif et al, 2009;Aboglila et al, 2010). Such a trend was not observed in the immature sediments, however, n-alkanes in the sediments coming into the oil window were characterized by D enrichment with increasing carbon numbers (Pedentchouk et al, 2006;Kikuchi et al, 2010). Of the many possible reasons for D enrichment with increasing carbon number, the kinetic isotopic effects in the generation of oils might be the most plausible one (Schimmelmann et al, 2004).…”

“…Generally, n-alkanes are more hydrogen isotopically conservative than total oils because they are mostly bound to macromolecular structures by nonpolar and strong covalent bonds, therefore, a relatively high activation energy for H exchange is required (Schimmelmann et al, 2006). Hence, D/H ratios of n-alkanes could be less influenced by hydrogen exchange with environmental water or minerals than other compound classes as suggested in several reports on D/H ratios for various organic molecules with progressive maturation (Schimmelmann et al, 2004;Dawson et al, 2005Dawson et al, , 2007Radke et al, 2005;Pedentchouk et al, 2006;Kikuchi et al, 2010). Therefore, the change of relationship between the oil and n-alkanes may mainly be caused by oil not n-alkanes.…”

“…Radke et al (2005) reported that nalkanes were much more depleted in D in the sediments, with Ro values of 1.30%, relative to those in less mature sediments (Ro = 1.14%). Pedentchouk et al (2006) and Kikuchi et al (2010) also show that there were relatively large variations in dD values of n-alkanes from the low maturity sediments, and n-alkanes might not be always enriched in D with increasing maturity. Moreover, Schimmelmann et al (2006) stated that the D/H ratios of organic matter often, though not always, increase with increased maturity.…”

“…Compared with the correlation shown by carbon isotopes, the correlation of dD values of n-alkanes with oil maturity is relatively weak, which indicates that the hydrogen isotopic compositions of n-alkanes could also be affected by other factors. Generally, the n-alkanes in the oils and sediments could be progressively enriched in D by $40‰ during maturation (Li et al, 2001;Dawson et al, 2005Dawson et al, , 2007; however, n-alkanes extracted from the sediments might exhibit more complicated variations (Radke et al, 2005;Pedentchouk et al, 2006;Kikuchi et al, 2010). Radke et al (2005) reported that nalkanes were much more depleted in D in the sediments, with Ro values of 1.30%, relative to those in less mature sediments (Ro = 1.14%).…”

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.…”