Application of the monoterpane ratio (MTR) to distinguish marine oils from terrigenous oils and infer depositional environment in northern Tarim Basin, China

Six monoterpenoid hydrocarbons including two acyclic, two monocyclic and two aromatic components were identified and quantified in 49 oil and condensate samples using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-TOFMS). Among them, 2,6-dimethyloctane (2,6-DMO) and 2-methyl-3-ethylheptane (2-M-3-EH) are the most abundant compounds and geochemically significant. Used in conjunction with other geochemical parameters such as pristane/phytane (Pr/Ph) and dibenzothiophene/phenanthrene (DBT/P) ratios, the 2-M-3-EH/2,6-DMO ratio (monoterpane ratio, MTR) provides a useful tool for distinguishing marine oils and condensates from those of terrigenous origin. Oils and condensates derived from marine source rocks have high MTR (> 0.4), low Pr/Ph and high DBT/P values, whereas those generated from terrigenous source rocks have lower MTR (< 0.3), higher Pr/Ph and lower DBT/P values. Differences in MTR values are mainly caused by variations in the concentrations of 2-M-3-EH as the concentrations of 2,6-DMO are in the same range in different sample groups. The 2-M-3-EH is enriched in marine oils and condensates from highly anoxic depositional environments, but depleted in terrigenous oils and condensates from oxic/sub-oxic depositional environments. The MTR may serve as a new parameter for assigning the source rock depositional environment. MTR values higher than 0.4 may indicate reducing conditions while MTR values lower than 0.3 may suggest an oxic/sub-oxic condition.