The distribution and δ34S values of organic sulfur compounds in biodegraded oils from Peace River (Alberta Basin, western Canada)

Abstract

The heavy sulfur-rich oils of the Peace River oil sands were formed by severe anaerobic biodegradation. To investigate the impacts of biodegradation on organic sulfur compounds (OSCs) we have measured the distribution and δ34S values of alkylated thioaromatics detected in four oils from each of two Peace River wells (A and B). The fluid composition of the Peace River oil sands can be highly complex due to the variable contributions of sources, thermal maturity and secondary alteration. Nevertheless, a previous multi-molecular investigation of samples from well B oils (PR2 oil-leg of Marcano et al., 2013, Organic Geochemistry 59, 114–132) indicated that the oils had biodegradation levels (BLs) of 5–6, with biodegradation increasing down the oil column. The hydrocarbon composition of oils isolated from well A were biodegraded to a lesser degree than well B (i.e., the presence of isoprenoids and high molecular weight [MW] n-alkanes suggests BLs of ∼3–4). The concentrations of C1–C3 alkylated benzothiophenes (BTs) and C1–C3 alkylated dibenzothiophenes (DBTs) declined sharply through both wells consistent with increased biodegradation, although other alteration impacts (e.g., water washing) might also contribute. The rates of decrease recorded for selected isomers showed variable responses (e.g., 3&4-mBT > 2-mBT; 4-eDBT > 1,3-dmDBT) which implies different susceptibilities to biodegradation and which may contribute additional useful molecular parameters for evaluating the BL of biodegraded S-rich petroleum. The S-isotopic measurements showed a 34S enrichment in several alkyl BTs down the oil leg in well B, with their δ34S values increasing by up to 6‰, consistent with the microbial utilisation of 32S (n.b., the δ34S values of alkyl BTs were not measured in well A oils because of low aromatic sub-fraction concentrations). In contrast, there was little variation in the δ34S values of alkyl DBTs down both wells indicating negligible S-isotopic fractionation of these OSCs in the deeper more biodegraded oils, possibly because of their higher MW and resistance to biodegradation. The δ34SDBT (and bulk δ34S) values of the well B oils were generally slightly enriched than the well A oils probably due to differences in the relative contribution of the major Devonian and Jurassic source rocks.