山东东营凹陷八面河油田稠油成因分析

Origin of the Heavy Oils from the Bamianhe Oilfield, Dongying Depression

东营凹陷八面河地区原油物性呈规律性的变化,偏离生油中心的构造高部位主要分布稠油,靠近生油中心的构造低部位主要分布正常油.对原油族组成与化学成分的分析表明,八面河油田稠油具有低饱芳比、饱和烃含量低、链烷烃与低分子量萘、菲等轻质馏分严重缺失等轻度-中等降解油特征,邻区草桥油田稠油含较为完整的生物降解标志物--25-降藿烷系列,系严重降解油,反映该区稠油的形成与次生变化有关.同区具有相同或相似油气成因的沙子岭原油的成熟度(C29甾烷ααα20S/(S+R)值为0.24～0.25)低于八面河的(C29甾烷ααα20S/(S+R)值为0.31～0.44),为典型未熟-低熟油.沙子岭的轻度或未降解油同样表现为正常油,反映八面河地区低温成烃与稠油无必然的联系,进一步验证八面河稠油主系次生成因.处于构造高部位的油藏由于埋深浅、保存条件差,导致水洗、生物降解等次生变化相对较强,最终形成稠油.

Obvious variations of oil properties were observed from the oils of the Bamianhe region, Bohai Bay Basin, eastern China Heavy oils are mainly distributed in high position tectonically whereas normal oils are in low position with relatively short distance from the depo center of source rock Detailedly geochemical investigations of 31 oil samples show that the heavy oils of Bamianhe oilfield are characterized by low concentration of saturated hydrocarbons with low ratios of saturated/aromatic hydrocarbons, and low content of hydrocarbon fractionation with low molecular weight i e alkyl alkanes, naphthalene and phenanthrene, suggesting light to middle degree of secondary alteration Oils from adjacent Chaoqiao oilfield are featured by occurrence of 25 norhopane series besides the similar characteristics of biodegradation in the Bamianhe oilfield It is suggested that secondary alteration is crucial for the formation of the heavy oils in the Bamianhe area Additionally, the immature oils in the Shaziling oilfield with similar source rocks to the Bamianhe display chacteristics of normal oils rather than that of the heavy oils, which infers that early hydrocarbon degradation does not result in indigenous heavy oils, which further proves secondary alteration is the main factor responsible for the heavy oils in the Bamianhe oilfield Oil pools with shallow burial depth are liable to be destroyed under relatively bad reserve conditions