鄂尔多斯盆地志靖地区原油地化特征及其对延安组成藏的启示

Geochemical characteristics of crude oils in the Zhijing area,Ordos Basin:Implication for oil accumulation in the Yan’an Formation

使用全烃气相色谱和饱和烃色谱质谱技术对志(丹)靖(边)地区延长组长6以浅的原油开展地化研究。结果发现,这些原油的生源指标相近,均揭示了混合有机质输入的生油母质以及弱氧化-弱还原、水体分层性差的源岩沉积环境。它们的成熟度指标也相近,揭示原油均已成熟。但是,延安组和延长组石油的正构烷烃分布特征和相分馏指标明显不同:前者C_(18)以下的正构烷烃分布凌乱并在C_(11)处出现“错位”,甲苯/正庚烷普遍小于0.2;后者正构烷烃分布相对规律,甲苯/正庚烷普遍大于0.2。在延安组内,古河道附近原油的成熟度普遍小于远离古河道的原油,原油成熟度在平面上的分布也无规律。研究认为,研究区长6以浅的原油同源同期,均来自湖盆中心的长7烃源岩。延安组石油异常的正构烷烃分布与石油沿断裂/裂缝垂向运移过程中发生的轻重组分分离、混合有关:源自长7的石油先在延长组内沿砂体发生北东向侧向运移,然后沿断裂/裂缝垂向运移注入延安组。导油断裂/裂缝的展布控制了延安组油藏的分布,是导致延安组油藏在远离古河道的区域仍然发育的原因。

Geochemical research on the crude oils in the strata shallower than the Chang-6 member of Yanchang Formation in the Zhidan-Jingbian area was carried out using the full hydrocarbon gas chromatography and saturated hydrocarbon chromatography-mass spectrometry.Our results show that the source-related indexes of these oils are similar,which suggest aparent material imported by mixed organic matter and a sedimentary environment of source rocks with weak oxidation and reduction and poor water column stratification.Their maturity indexes are also similar and indicate that all of them are mature oils.However,the distribution characteristics of n-alkanes and phase fractionation indexes are obviously different between the oils in the Yan’an and Yanchang formations.The former were characterized by the disorderly distribution of n-alkanes below C_(18),"dislocation"occurring at C_(11),and the values of toluene/n-heptane generally less than 0.2.The latter were characterized by the relatively regular distribution of normal alkanes and the values of toluene/n-heptane generally greater than 0.2.In Yan’an Formation,the maturities of crude oils near the ancient river channel are generally lower than those far from the ancient river channel,and the oil maturity distribution on the plane is very disorderly.According to the analysis,the studied oils in the area are expected to originate from the Chang 7 source rocks in the central basin.The abnormal n-alkanes distribution of the oils in Yan’an Formation is related to the separation and mixing of light-heavy components of oils during the vertical migration along faults/fractures.The oils from the Chang 7 source rocks first migrated laterally through sand body towards the northeast within Yanchang Formation,and then migrated vertically along faults/fractures and charged into Yan’an Formation.The distribution of oil reservoirs within Yan’an Formation is controlled by the distribution of oil-migration-related faults/fractures,which is the cause that oil reservoirs are still developed in Yan’an Formation far away from the ancient rivers.