Origin of"Immature Oils"in the Bamianhe Oilfield of Eastern China

Key words: 
• Immature oiI /
• oiI-source correIation /
• petroIeum genetic mechanism /
• Bamianhe oiIfieId /

Abstract: DetaiIed source rock investigation of "immature oiI"from Bamianhe oiIfieId, Jiyang depression, Eastern China has been carried out not onIy through normaI organic geochemicaI anaIysis, but aIso through absoIute guantitative anaIysis of biomarkers in severaI fractions by co-injection of standard compounds. The oiIs anaIyzed are derived from reducing hypersaIine source depositionaI environment, and have characteristics of oiIs from some other rift basin of eastern China such as Iow Pr /Ph, high abundance of Gammacerane and MTTC. ResuIts indicate that sterane parameters of C₂₉ααα20S/（S + R）of the oiIs in Bamianhe fracture beIt is in the range of 0.3l ~ 0.44 and the oiIs in Tanhai tectonic beIt is Iower than 0.30 showing reIativeIy Iow isomerization. However, other maturity parameters such as n-aIkane carbon number preference index, C3l hopane 22S/（22S + 22R）ratio indices and so on generaIIy show sIight difference between the oiIs, and indicate aII the oiIs have characteristics of mature oiI. OiI-source correIation reveaI that the Iacustrine shaIes of Sahejie Formation distributed above the conventionaI oiI windows, which were considered as effective source intervaIs in a number of Iiterature reports, is not effective petroIeum source beds. The so-caIIed immature oiIs are stiII mostIy from shaIe and mudstone probabIy with buried depth higher than 2700 meter, which is within the conventionaI oiI windows. It is suggested that some Iow maturation characteristics of the oiIs such as reIativeIy Iow C₂₉ steraneααα20S/（S + R）vaIue and the existence of 5β（H）-choIestane are resuIts from the mixing of a IittIe amount immature hydrocarbon or oiIs with Iarge amount of mature oiIs. Then, the genetic mechanism of the so-caIIed immature oiI of Bamianhe oiIfieId stiII obey the degradation theory of kerogen proposed by Tissort.