δ13C compositions of saturate and aromatic fractions of lacustrine oils and bitumens: evidence for water column stratification

Intact microbial fossils in the Permian Lucaogou Formation oil shale, Junggar Basin, NW China

“…An alternative explanation for depleted δ 13 C values for the saturated hydrocarbon fraction is that they signal the presence of a methane cycle with hydrocarbons such as hopanes derived from methanotrophic bacteria living near the chemocline (Collister et al, 1992;Schoell et al, 1994). Collister and Wavrek (1996) demonstrated a correlation between hopane concentration and the difference between saturated and aromatic isotopic values in several ancient lacustrine systems. This led Carroll (1998) to postulate that methanotrophs were not significant contributors to organic matter in the Lucaogou Fm., because the saturated-aromatic hydrocarbon isotopic difference for Junggar extracts does not correlate with hopane concentration.…”

Biodegradation of 25-norhopanes in a Liaohe Basin (NE China) oil reservoir

“…The average isotopic difference between aromatic and saturated hydrocarbon fractions is as large as 4.9‰, typically characteristic in lacustrine oils (Collister and Wavrek, 1996). The stable isotopic compositions of resin and asphaltene are only slightly heavier than that of aromatic hydrocarbon fraction with an average d 13 C value of À29.0‰ and À28.5‰, respectively.…”

Jurassic non-marine source rocks and oils of the Porcupine Basin and other North Atlantic margin basins

“…Figure 10A shows a range plot for the difference between the carbon isotope values of the aromatic and saturate fractions (A<5 13 C) for Porcupine Basin and other western European oils and source-rock extracts. Collister & Wavrek (1996) have shown that this difference is related to the input to the saturate hydrocarbon fraction of isotopically light (more negative) organic biomass from methanogenic (methanotrophic) bacteria in highly stratified, reducing lacustrine water columns. The highest A6 13 C values are associated with oils worldwide which are derived from source rocks deposited in lacustrine environ-ments.…”

“…In marine systems, when the water or sediment column becomes anoxic, sulphate is available to fuel sulphate-reducing bacteria which become the dominant organisms in the sequence of diagenetic processes. In lacustrine fresh/brackish water systems, where sulphate concentrations are low, methanogenic (methanotrophic) bacteria become the dominant diagenetic organisms (Collister & Wavrek 1996).…”