鄂尔多斯盆地长7段致密油成藏机理分析

Analysis of tight oil accumulation mechanism of Chang 7 member in the Ordos basin

鄂尔多斯盆地是我国首个亿吨级致密油赋存盆地,三叠系延长组长7段油层组是该盆地致密油的典型代表,但对长7段致密油成藏机理的分析较为缺乏。通过对长7段致密储层特征及石油运聚动力的分析,储层成岩演化、物性演化及其与生烃演化匹配关系分析与储层润湿性分析,综合对长7段致密油成藏机理展开研究。研究表明,优质烃源岩的生烃增压及泥岩欠压实产生的剩余压力为致密油运移提供了动力;长7段致密油为两期充注,对应于晚侏罗世中晚期及早白垩世中晚期,第一期致密油充注时期储层尚未完全压实,第二期致密油大规模充注之前发生一期溶蚀,改善了致密储层的物性,两次油气充注成藏时期储层物性均未达到如今的致密程度,相对较好的储层条件为致密油提供了良好的运移和储集条件;致密储层以中性和亲油性储层为主,减小了油气充注的阻力;高源储压差与高温高压的地层条件使得低黏度的原油得以进入致密储层成藏。这对鄂尔多斯盆地下一步致密油勘探开发有重要意义。

Ordos basin is the first 100 million ton tight oil bearing basin in China. The Chang 7 member of the Triassic Yanchang Formation is a typical representative of the tight oil in this basin. However, there is limited data on the tight oil accumulation mechanism in the Chang 7 member.The tight oil accumulation mechanism of the Chang 7 member is comprehensively studied in order to clarify the reservoir forming mechanism of tight oil in the Chang 7 member, through the analysis of tight reservoir characteristics and oil migration and accumulation dynamics, combined with reservoir diagenetic evolution, physical property evolution and its corresponding relationship with hydrocarbon generation evolutionand reservoir wettability analysis.The results show that high-quality source rock’s hydrocarbon generation pressurization and the residual pressure of mudstone undercompaction provide power for tight oil migration. The tight oil in Chang 7 member shows two-stage filling, corresponding to the middle-late period of Late Jurassic and middle-late period of Early Cretaceous. During the first stage of tight oil filling, the reservoir has not been fully compacted, and the dissolution occurred before the large-scale filling of the second stage, which improved the physical properties of the tight reservoir. The physical properties of the reservoir in the two stages of oil and gas filling and accumulation were not as tight as they are today, and the relatively good reservoir conditions provided good migration and reservoir conditions for tight oil. The tight reservoir mainly consists of neutral and lipophilic reservoirs, which reduces the resistance to oil and gas filling. The high source reservoir pressure difference and high-temperature and high-pressure formation conditions allow low-viscosity crude oil to enter tight reservoirs and form reservoirs. This result is of great significance for the next tight oil exploration and development in the Ordos basin.