Characteristics And Forming Mechanisms Of Paleogene Reservoir Of Huagang Formation With Low Porosity And Low Permeability In Xihu Depression

Based on observation and description of the cores and the rock thin sections,after reading quite a few references,combined with analyzed data and test results,the sedimentary background,petrography,diagenesis and physical properties of the Paleogene reservoir of Huagang Formation in Xihu Depression have been studied according to several structural belts,and we have also summarized such influences to the reservoir caused by depositional facies,texture characteristics of rock particles and diagenesis.Finally,find out the controlling factors of reservoir development.It can be divided into 5 structural belts from west to east in Xihu Depression,they are western slope belt,western sub-sag,central anticline belt,eastern sub-sag and estern fault and fold belt.According to well positions,it also can be divided into 6structural areas,pingbei,pinghu,western sub-sag,central-southern of central anticline belt,yuquan and chunxiao-tianwaitian.Huagang formation ais mainly developed braided delta and lakedepositional facies,the reservoir rock are with high content of feldspar and rock fragments,the main rock types are feldspar lithic sandstone and lithic feldspar sandstone with low compositional maturity and low textural maturity.According to SEM and thin section observations,we found that the reservoir rock has went through moderate compaction,the porosity is decreasing with increasing depth.The cement of the sandstones is relatively simple,they are mianly carbonate minerals,such as calcite,ferrocalcite,dolomite,ankerite,and also some amount of kaolinite and authigenic quartz.The dissolution of the reservoir is quite strong,which has made developed secondary porosities.In particular,the feldsparintragranular pores and the kaolinite crystalline pores play important roles in the study of reservoir dissolution.According to carbon and oxygen isotope data analysis,we have confirmed that calcite cement of the reservoir had generated from early diagenetic stage B to mid diagenetic stage B.Another point we have studied is the porosities made by dissolution are far more than which filled by authigenic minerals.And the meteoric water is dominated in dissolving media.Through observation and analysis,the average porosity in different structural areas of the study area is 3%¹3%or so,which belongs to low porosity reservoir,20%porosity layers appear rarely.The permeability is ranging from 0.1×10^-3?m²to10×10^-3?m² or so,which belongs to low-super low permeability reservoir.Reservoir development was influenced by deposition,compositional features of skeletal grains,diagenesis and tectonic movement and so on.The depositional characteristics before sandstone generation are the fundamental factors towards physical properties of sandstones,whereas skeletal grain features and diagenetic features are the after-controlling factors,which would influence reservoir rocks the most and would decide whether it is a good reservoir or not.