鄂尔多斯盆地延长组长6致密油成藏机理分析

Analysis of Accumulation Mechanism of Chang 6 Tight Oil in Yanchang Formation,Ordos Basin

根据野外露头、岩心观察、实验模拟、分析测试等资料,结合模拟软件,对长6致密油成藏过程与成藏模式进行分析。结果显示,长6致密油成藏动力为长7烃源岩生烃产生的过剩压力,主要成藏期为距今160 Ma和120~100 Ma,两期成藏阶段烃源岩生烃增压都大于10 MPa,储层充注孔喉下限分别为18、16 nm。恢复了长6不同沉积微相储层的孔隙度演化历史,原始孔隙度水下分流河道为39.59%、水下分流间湾为40.17%、浊积扇为37.70%,在距今180 Ma左右变为致密储层,早于致密油成藏时间,属于先致密后成藏。岩心和野外露头观察发现长6致密油具有团块状石油局部富集、沿层理层状石油富集、沿裂缝线状富集和裂缝交叉处块状石油富集等4种富集模式。室内实验模拟显示,无裂缝参与时,致密油呈整体弥散型运移;有裂缝时裂缝控制了致密油的富集。近源超压强充注、裂缝垂向沟通与侧向短距离运移调整共同形成大面积整体含油、局部甜点富集的致密油藏。

According to the data of outcrop,core observation,experimental simulation and analytical test,combined with the simulation software,the tight oil accumulation process and hydrocarbon accumulation model of Chang 6 reservoir were analyzed.The results show that the Chang 6 tight oil accumulation dynamic is the excess pressure generated by the hydrocarbon generation of the Chang 7 source rock.The main accumulation period is160 Ma and 120~100 Ma,and the hydrocarbon generation pressure of the source rock in the two stages of accumulation is greater than 10 MPa.The minimum size of pore throat of the reservoir filling in the two stages accumulation is 18 nm and 16 nm.The evolution history of porosity of different sedimentary microfacies reservoirs of Chang 6 reservoir was restored by experimental simulation.The original porosity of the underwater distributary channel is39.59%,the underwater diversion bay is 40.17%,and the turbidite fan is 37.70%.After compaction and cementation,it becomes a tight reservoir at around 180 Ma.At this time,hydrocarbon generation has not yet begun.The hydrocarbon generation pressurization during the main accumulation period promotes the filling of tight oil.It is first densified and then hydrocarbon accumulation.Core and outcrop observation revealed that Chang 6 tight oil has four enrichment modes including agglomerate local enrichment,layered enrichment along the bedding,linear enrichment along cracks at different angles,and block enrichment along the intersection of different fractures.The indoor experimental simulation shows that the tight oil migration is an integral diffusion type when there is no crack but the crack controls the enrichment of tight oil when there are cracks.Chang 6 tight oil reservoir characterized by large area distribution,overall oil content and local accumulation is formed by the combination of three modes of action:Near-source overpressure filling,vertical communication of cracks and lateral short-distance migration adjustment.