文摘
Mango proposed a significant invariance of four isoheptanes, in which a ratio of sums (K1) remains at approximately 1, regardless of the absolute concentrations. Gas chromatographic analyses of 70 oils from the Tarim Basin demonstrated that most of these oils share invariant K1 values close to 1.0, even if they are derived from different sources and have varying degrees of maturity and biodegradation. However, a few unexpectedly high K1 values occurred in certain oils from the Tazhong-4 Oilfield in the Tazhong Uplift, and these oils are characterized by abnormally high contents of dibenzothiophenes (DBTs). An excellent positive correlation between the elevated K1 values and the abnormally high DBT contents indicated that the thermochemical sulfate reduction (TSR) alteration, which contributed to the increased DBT contents, likely resulted in the elevated K1 values. Methylhexanes (MHs) are assumed to be more susceptible to TSR than dimethylpentanes (DMPs), and the terminal methyl groups of C7 compounds are more resistant than their mid-chain isomers in the TSR-altered oils. Therefore, in the oils affected by TSR alteration, a decrease in the 2,3-DMP/2,4-DMP ratios will not be sufficient to offset the increased 2-MH/3-MH ratios, which results in elevated K1 values. A positive correlation between the increased 2-MH/3-MH ratios and the elevated K1 values has validated this hypothesis.