Characteristics and geochemical implications of light hydrocarbons from ultra-deep Ordovician oils in the North Shuntuoguole area, Tarim Basin

Abstract

The ultra-deep Ordovician reservoirs in the North Shuntuoguole Oilfield (or Shunbei Oilfield) of SINOPEC have achieved annual production of one million ton, and the oil & gas in different faults show different physical properties and fluid phases. In this study, the 28 oil samples from the ultra-deep Ordovician were analyzed using whole oil chromatography. The heptane and isoheptane values of the oil samples were in the range of 29.79%‒46.86% and 1.01%–3.06%, respectively, indicating the oils are high mature. The maturity that calculated based on light hydrocarbon values was higher than which calculated by using aromatic hydrocarbon parameters, suggesting the light hydrocarbon maturity mostly reflects the maturity of the late charged hydrocarbon. The 2M-/3M-C ₅ and 2M-/3M-C₆ ratios varied in the ranges of 1.41‒1.81 and 0.79–1.09, respectively, and the iC₅/nC₅ and 3M-C₅/nC₆ ratios were 0.31‒0.90 and 0.16–0.37, respectively, indicating that ultra-deep Ordovician reservoirs have not experienced biodegradation. The Mango parameter K₁ of the oil samples ranges 0.96‒1.01 except for the oil from Well SB4, which suggests that most of the reservoirs have not suffered thermochemical sulfur reduction (TSR). Meanwhile, the oils have not experienced evaporative fractionation since the toluene/nC₇ and nC₇/MCC₆ ratios range from 0.10–0.38 and 1.50‒1.80, respectively. The close correlation between P₃ and P₂ + N₂ and between P₂ and N₂/P₃ indicates that the oils from different faults have the same origin. According to the characteristics of LHs rich in n-alkane, as well as other biomarkers, such as aryl isoprenoids, and aromatic hydrocarbon parameters, the oil originated from the source rock of the Lower Cambrian Yu’ertusi Formation. Meanwhile, the source rocks in different fault zones slightly differed in organic facies.

Graphical abstract

Keywords

light hydrocarbons / biodegradation / thermochemical sulfate reduction (TSR) / evaporative fractionation / ultra-deep Ordovician reservoir / Tarim Basin

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Acknowledgments

This research was supported by the National Natural Science Foundation of China (Grant Nos. 41772153, U19B6003), and Sinopec Division of Science and Technology (Nos. P19024-4, P21085-8). The authors would like to thank three anonymous reviewers for their constructive comments on the manuscript, which improved the scientific clarity.

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