Heteroatom compounds in oils from the Shuntuoguole low uplift, Tarim Basin characterized by (+ESI) FT-ICR MS: Implications for ultra-deep petroleum charges and alteration

Abstract

Eighteen oil samples from the Shuntuoguole (STGL) low uplift of the Tarim Basin were characterized using positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT–ICR MS) to investigate the distribution of heteroatomic compounds, with emphasis on sulfur-containing compounds. The N1, N1O1, O1S1, S1, and S2 compound species were generally identified within the Shunbei oils; conversely, the N1S1, O1, O2, and O2S1 species were either absent or present as a negligible proportion of all compounds. The proposed maturity parameters of detected N1 species (DBE9-12/DBE15-18-N1), N1O1 species (DBE4-7/DBE8-14-N1O1), O1S1 species (DBE6/DBE4-5), S1 species (DBE9/DBE1-22 and DBE1-8/DBE9), and S2 species (C22-/C23+DBE14) indicate that oils located at the northeast-striking fault (NE oils) share an overall higher maturity than those with oils distributed in the north–northwest direction (NNW oils), which is in accordance with the aromatic hydrocarbon maturity parameters. The NE oils generally display a relative higher abundance (87.5%–92.9%) of S1 species than the NNW oils (57.4%–86.7%), with a dominant DBE (double-bond equivalence) of 9 (i.e., dibenzothiophene) and a carbon number range of 13–14 for the S1 species. In terms of the S1 class of the Shunbei oils, the parameters of DBE1-8/DBE9 and DBE9/DBE12 vary along a trend of thermal maturity, which differs from the previously reported results for oils that suffered thermochemical sulfate reduction (TSR). This difference indicates that high thermal maturity rather than TSR results in the enrichment of dibenzothiophene. Oils from the Shuntuo and Shunnan fields are also enriched in low DBE S1 species that are absent from petroleum fluids of equivalent or even higher maturity that have not experienced TSR, thus implying the possible mixing of TSR–altered oils. The overall positive correlation between the concentration of H2S and the maturity parameters of sulfur–containing compounds and aromatic hydrocarbons suggests that thermal maturity associated with the differential petroleum mixing is the main factor controlling both the H2S content and heterogeneity of the NSO-compounds between the NE and NNW reservoirs.