塔里木盆地塔东2油藏形成历史——原油稳定性与裂解作用实例研究

Oil accumulation history in Tadong 2 oil reservoir in Tarim Basin, NW China-A case study of oil stability and cracking

塔里木盆地塔东2井寒武系储集层中的原油密度大、黏度高、非烃和沥青质含量高,发现了一些高浓度的特殊化合物(包括4-甲基甾烷、甲藻甾烷及其芳构化产物等),它们与寒武系岩石抽提物中的生物标志化合物的分布非常一致,却又明显不同于中、上奥陶统烃源岩及轮南油区的稠油,指示该原油起源于寒武系.该油藏经历了次生蚀变过程,大量生液态烃在中、晚奥陶世,生油过程在约10Ma内快速完成,此后地层继续埋深,经历了200～210℃古地温,灰岩孔、洞、缝甚至泥岩中大量存在的干沥青、原油中高含量的稠合芳烃以及重碳同位素组成是高温过程对油藏影响的重要证据.泥盆纪以后塔东2油藏抬升至地下1730m深度,其储集层温度降至约70℃(盐水包裹体均一化温度主要为60～90℃).新形成的断裂和构造裂缝沟通了寒武系早期裂缝系统中聚集的油气,导致油气向上扩散,遭受生物降解和蒸发分馏作用,而使原油进一步稠化,原油中高浓度25-降藿烷的存在以及原油的高含蜡是两种作用存在的主要证据.400Ma前形成的油藏在经历超过200℃的高温之后还能保存至今,表明原油具有高度的稳定性.

The oil from the Well Tadong 2 ( TD2 ) in the Cambrian reservoir in eastern Tarim Basin contains high concentration of 4-methyl steranes, dinosteranes and their aromatic dinosteroids, which is different from heavy oils discovered in the Lunnan area and their Middle-Upper Ordovician source rocks, suggesting a clear affinity with the Cambrian source rock extracts but distinctly. Present maturity of the Cambrian source rocks is as high as 2. 67%-2. 75% VRE. Burial history and hydrocarbon generation history analysis suggest that major liquid hydrocarbon generation time from this source rock suite was in the Middle and Late Ordovician and petroleum generation process was rapidly completed within 10 Ma. All these indicate that the oil accumulation has been severely affected by high temperature processes, which caused oil densification and cracking to gas. During subsequent stratigraphic elevation after the Devonian, TD2 oil pool was uplifted to 1730m undersurface and reservoir temperature dropped to about 70℃( brine inclusion homogenization temperature of 50-90℃). Newly formed fractures and tectonic fissures linked up early accumulated oil and gas in the Cambrian fissure system, leading to oil and gas diffusion upward and biodegradation. High concentration of 25-norhopanes occurrence in oil is one obvious result of biodegradation, which causes further oil densification. Discovery of TD2 heavy oil inspire some new considerations. An oil pool formed more than 400Ma ago, which has experienced paleotemperature of higher than 200℃, can be preserved till present, indicating much higher oil stability than previously believed.