Stable carbon and hydrogen isotopic fractionation of individual n-alkanes accompanying biodegradation: evidence from a group of progressively biodegraded oils

Seven crude oils of known source and maturity, representing a natural sequence of increasing degree of biodegradation, were collected from reservoirs in the Liaohe Basin, NE China, in an effort to determine the magnitude and direction of isotopic shift of carbon and hydrogen in individual n-alkanes during microbial degradation. The results show that biodegradation has little effect on the carbon isotopic composition of the whole oil. However, a sequential loss of n-alkanes leads to C-13 depletion of the bulk residual saturate fraction. The stable carbon isotopic compositions of aromatics and macromolecular organic matter (resins and asphaltenes) follow a pattern, with an overall trend towards C-13 enrichment of 0.8-1.7parts per thousand. in the residues. The stable carbon and hydrogen isotope values of individual n-alkanes demonstrate that they follow different trends during biodegradation. No significant carbon isotopic fractionation occurs for n-alkanes during slight to moderate biodegradation. However, there is a general increase of up to 4parts per thousand in the delta(13)C values of low molecular weight n-alkanes (C-15-C-18) during heavy biodegradation. In contrast, no isotopic fractionation occurs for higher molecular weight n-alkanes ( greater than or equal to C-19). The conservative character of C-13 concentration for high molecular weight n-alkanes during biodegradation indicates that these compounds are effective tracers for source identification of heavily biodegraded oils. The hydrogen isotope compositions of n-alkanes show a significant fractionation as moderate biodegradation proceeds, resulting in an enrichment in D of up to similar to35parts per thousand upon heavy biodegradation. The dynamic fractionation in the stable carbon and hydrogen isotopes of n-alkanes accompanying biodegradation opens the possibility of quantitative estimation of the extent of biodegradation in subsurface petroleum reservoirs by introducing an isotopic kinetic modelling approach. (C) 2004 Elsevier Ltd. All rights reserved.