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

The ultra-deep Ordovician reservoirs in 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-C5 and 2M-/3M-C_(6) ratios varied in the ranges of 1.41‒1.81 and 0.79-1.09,respectively,and the iC_(5)/nC_(5) and 3M-C_(5)/nC_(6) 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 K_(1) 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/nC_(7) and nC_(7)/MCC_(6) ratios range from 0.10-0.38 and 1.50‒1.80,respectively.The close correlation between P_(3) and P_(2)+N_(2) and between P_(2) and N_(2)/P_(3) 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 Lower Cambrian Yu’ertusi Formation.Meanwhile,the source rocks in different fault zones slightly differed in organic facies.