Diagenetic Differences And Its Genesis Of Low Permeability Sandstone Reservoirs In The Yanchang Formation,Ordos Basin

In this paper,the core observation and thin section identification are taken as the basic data.Combined with abundant logging data,carbon and oxygen isotope data,homogenization temperature data of fluid inclusions and the application of scanning electron microscopy and cathodoluminescence equipment.The diagenetic differences and its genesis of low permeability sandstone reservoirs are systematically analyzed in the three areas of Jiyuan,Huanjiang and Longdong in Ordos Basin,the following major achievements have been made.1?The feldspar is the highest content of debris particles,the second is quartz.Compared with the Jiyuan and Huanjiang areas,the sandstone in the Longdong area is finer in grain size.The sandstone classification is moderate to good in the three regions,but has some differences within each region.Through the observation and analysis of the core in the study area,Summarizing 10 types of lithofacies and 9 types of lithofacies combination,Besides,the underwater distributary channel and distributary interchannel sedimentary microfacies are mainly developing in the In the Jiyuan and the Huanjiang area,in addition,the estuary dam microfacies are also developing in Longdong area.2?The sandstone reservoirs of Chang 8-Chang 9 experienced a variety of complex diagenesis processes in the study area.mainly including differential mechanical compaction,two-stage carbonate cementation,authigenic chlorite cementation and two-stage quartz cementation,Dissolution of unstable components and metasomatism as well as other diagenesis types.3?The diagenetic differences is as follows in the plane,for underwater distributary channel microfacies,the content of chlorite cement is high,the content of carbonate cement is low in the Huanjiang area,but the content of carbonate cement is high,the content of chlorite cement is low in the Longdong area.The diagenetic differences is as follows in vertical dimension,carbonate usually cementes intensely in the lower part of the positive grain sequence sandstone combination,while the chlorite usually cementes intensely in the upper phase sequence.For the vertical combination mode of reverse grain sequence sandstone,the content of carbonate and chlorite cements are usually higher in the upper phase sequence.4?Huanjiang area is controlled by western provenance,which provides high content of dark particles such as volcanic rock debris and biotite,also providing a large amount of iron and magnesium ions during the later self-generated chlorite cementation process.The provenance of the Jiyuan area is mainly from the ancient land of Alashan in the direction of the north west,which provides shorter content of dark particle than Huanjiang area.The provenance of the Longdong area mainly comes from the ancient land of Longxi in the direction of south west,which provides less content of dark particle.Besides,the dissolution of feldspar particles can provide a large amount of siliceous source for quartz cement,Whereas the phenomenon of high siliceous cementation due to the intense compression is often observed in the Huanjiang and Longdong areas.In addition,the difference in grain size affects the size and preservation of interparticle volume,The grain size of Longdong area is small as well as the compaction effect is intensely.The analysis of carbon and oxygen isotopes in carbonate cements shows that the Jiyuan area is greatly affected by organic fluids.and the organic fluids are less affected in the Longdong area.A development pattern of diagenetic differences is established finally.