八面河地区“未熟—低熟油”成因探讨

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

采用先进的多馏分绝对定量、非烃技术 ,配合有机地球化学常规分析 ,在地质、地球化学综合研究的基础上 ,主要从分子地球化学角度对东营凹陷八面河“未熟—低熟油”成因进行了剖析。成熟度、单体烃绝对定量、多馏分与多参数综合对比揭示 ,以往研究确认的埋藏较浅的有机质富集层段—沙四段未熟—低熟页岩段并非该区“未熟—低熟油”的主力烃源岩 ,该区原油主要来自已进入生油门窗的成熟烃源岩 ,未熟—低熟烃源岩供应的油气量非常有限。八面河“未熟—低熟油”并非真正意义上的未熟—低熟油 。

Detailed source rock investigation of 'immature oil' from Bamianhe oilfield, Jiyang depression, Eastern China has been carried out not only through normal organic geochemical analysis, but also through absolute quantitative analysis of biomarkers in several fractions by co injection of standard compounds. The oils analyzed are derived from reducing hypersaline source depositional environment, and have characteristics of oils from some other rift basin of eastern China such as low Pr/Ph, high abundance of Gammacerane and MTTC. Results indicate that sterane parameters of C 29 ααα20S/(S+R) of the oils in Bamianhe fracture belt is in the range of 0.31～0.44 and the oils in Tanhai tectonic belt is lower than 0.30 showing relatively low isomerization. However, other maturity parameters such as n alkane carbon number preference index, C 31 hopane 22 S/(22S+22R) ratio indices and so on generally show slight difference between the oils, and indicate all the oils have characteristics of mature oil. Oil source correlation reveal that the lacustrine shales of Sahejie Formation distributed above the conventional oil windows, which were considered as effective source intervals in a number of literature reports, is not effective petroleum source beds. The so called immature oils are still mostly from shale and mudstone probably with buried depth higher than 2700 meter, which is within the conventional oil windows. It is suggested that some low maturation characteristics of the oils such as relatively low C 29 sterane ααα20S/(S+R) value and the existence of 5 β (H) cholestane are results from the mixing of a little amount immature hydrocarbon or oils with large amount of mature oils. Then, the genetic mechanism of the so called immature oil of Bamianhe oilfield still obey the degradation theory of kerogen proposed by Tissort.