富县地区长8致密储层微观结构及致密化

Discussion of microstructure and densification process of Chang 8 tight reservoir in Fuxian area

依据铸体薄片、场发射扫描电镜、高压压汞、X射线衍射、CT扫描、核磁共振测试等资料,分析鄂尔多斯盆地富县地区长8致密储层微观结构,探讨长8储层致密化与油气充注的关系。研究表明,长8储层孔隙类型以粒间孔、粒内溶孔和成岩微裂缝为主,以微米级尺度孔隙分布为主,孔喉非均质性强。成岩作用中,压实和胶结作用是储层致密的主要控制因素。晚三叠世—早白垩世早期,强烈的压实作用使储层孔隙度由35%降至15%,孔隙减小率为62.4%;后期绿泥石衬垫、硅质次生加大及方解石胶结,使孔隙度降至5%,孔隙减小率为23.5%;早白垩世末期生烃过程中有机酸对储层进行溶蚀,孔隙度升高至8%,改善了储层物性。储层致密化始于140 Ma,早白垩世早期(125 Ma)完全致密,早白垩世中期—末期(120 Ma)开始油气成藏,为先致密后成藏。

Based on the data such as cast sheet, field emission scanning electron microscope, high pressure mercury intrusion, X-ray diffraction, and CT scanning and nuclear magnetic resonance test, the microstructure of Chang 8 tight reservoir of Yanchang Formation in Fuxian area of Ordos Basin was analyzed to reproduce the densification process of Chang 8 reservoir. Results show that the pore type of Chang 8 reservoir is dominated by intergranular pores, intergranular dissolution pores and diagenetic micro cracks,and is characterized by continuous distribution of nanometer scale pore and strong heterogeneous pore. Compaction and cementation are the main reasons for the reservoir density. The compaction effect leads to an average reduction rate of 62.4%, and the cement reduction rate is 23.5%. In the Late Triassic-Early stage of Early Cretaceous, strong compaction reduced the porosity of the reservoir from 35% to 15%. Late chlorite liner, siliceous secondary enrichment and calcite cementation reduced the porosity to 5%. During the early stage of Early Cretaceous, the organic acids was dissolved in the organic matter, and the porosity increased to 8%. The densification of reservoirs began at 140 Ma and was completely dense in the early Cretaceous（125 Ma）. Hydrocarbon accumulation began at 120 Ma, which was later than densification.