Impact of biodegradation on methyltrimethyltridecylchromans (MTTCs) in crude oils from the Bohai Bay Basin, China

Methyltrimethyltridecylchromans (MTTCs), a class of aromatic hydrocarbon molecular marker compounds, provide an assessment of paleosalinity and maturity in sediments and crude oils. However, little is known about the effect of biodegradation on MTTCs. In this study, the saturated and aromatic fractions of a suite of oils from the Neogene Guantao (N1g) and Minghuazhen (N1m) Formation in the Miaoxi Depression, Bohai Bay Basin, were analyzed. Distriubutions of the more bioresistive biomarker show that the oils are derived from similar organic matter (dominantly from higher plants) and depositional environments (low salinity), and have similar maturity (in the middle oil window). Two biodegradation scales, were used to evaluate the level of biodegration. The samples ranged from PM 1 to 8 and MN2 from 140 to 983 (slight to heavy-severe biodegraded). MTTC concentrations decrease with the increasing biodegradation levels and are or almost completely removed at PM = 8. The degree of alkylation affect their relative susceptibilities to biodegradation: trimethyl-MTTC (alpha-MTTC) > dimethyl-MTTCs (including beta-MTTC, gamma-MTTC and-zeta MTTC) > monomethyl-MTTC (delta-MTTC). The relative susceptibilities of dimethyl-MTTC isomers are gamma-MTTC > beta-MTTC >-MTTC. The beta/gamma-MTTC, a maturity indicator, is significantly impacted by biodegradation. Paleosalinity inidcators, alpha/delta-MTTC, alpha/gamma-MTTC, MTTCI and a ternary diagram of the relative abundance in different degree of alkylation of MTTCs, consistently indicate the oils are derived from sources deposited in a low samility environment. Additional research is needed to determine if biodegradation perturbs MTTC distributions in oils from other source environments to influence the inferred paleosalinity assessment.