Elevated Mango's K-1 Values Resulting from Thermochemical Sulfate Reduction within the Tazhong Oils, Tarim Basin

摘要

Mango proposed a significant invariance of four isoheptanes, in which a ratio of sums (K-1) 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 K-1 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 K-1 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 K-1 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 K-1 values. Methylhexanes (MHs) are assumed to be more susceptible to TSR than dimethylpentanes (DMPs), and the terminal methyl groups of C-7 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 K-1 values. A positive correlation between the increased 2-MH/3-MH ratios and the elevated K-1 values has validated this hypothesis.