Monoterpanes as molecular indicators to diagnose depositional environments for source rocks of crude oils and condensates

A total of 74 crude oils and condensates originating from various deposits were collected in Tarim, Junggar, Beibuwan and Bohai Bay basins and analyzed by comprehensive two dimensional gas chromatography-time of flight mass spectrometry (GC × GC-TOFMS) in order to investigate the occurrence and distributions of C9 and C10 hydrocarbons. 3-Ethyl-2-methylheptane and 1,1,2,3-tetramethylcyclohexane, two C10 compounds possibly derived from monoterpenoids via cracking and reduction, may be formed under anoxic conditions during diagenesis. We suggest that the ratio of 3-ethyl-2-methylheptane to 3-methylnonane (MT1) coupled with the ratio of 1,1,2,3-tetramethylcyclohexane to propylcyclohexane (MT2), are useful parameters to infer depositional environments and to distinguish oil families. The oils and condensates formed from source rocks deposited under anoxic conditions show high 3-ethyl-2-methylheptane and 1,1,2,3-tetramethylcyclohexane levels (both MT1 and MT2 > 0.5). Whereas, the hydrocarbons derived from source rocks deposited in oxic conditions contain low amounts of 3-ethyl-2-methylheptane and 1,1,2,3-tetramethylcyclohexane (both MT1 and MT2 < 0.5). These monoterpane parameters may be useful in petroleum exploration, especially for light oils and condensates where most high molecular weight biomarkers could be cracked or eliminated through fractionation.