Based on detailed observations of cores and thin sections,combined with the application of field emission scanning electron microscopy (FESEM),physical property and mercury intrusion analysis,X-ray diffraction,carbon and oxygen isotope analysis,the causes of low porosity and permeability sandstone and favorable reservoir in the Huagang Formation of Xihu sag in East China Sea are analyzed.It reveals that the sandstones in the Huagang Formation mainly consist of detritus feldspar sandstone and feldspar detritus sandstone with dominant secondary pores.The cause of deep reservoirs is discussed based on research of sedimentary facies,digenesis of rocks and abnormal pressure and it is considered that the main reason for the low porosity and low permeability in deeply buried reservoirs is the late stage compaction and cementation.The former is responsible for the low porosity and low permeability of reservoirs.Development of ferrocalcite and illite cementation leads to deterioration of reservoir quality,while silk thread shaped illite occurred in pore throat is also decreasing the permeability of reservoirs.It shows that the sedimentary facies,digenesis and overpressure are main controlling factors on deeply buried reservoirs of high quality.Uncompaction resulted from corrosion and overpressure,organic acid,meteoric water and hydrothermal liquid are main factors for the formation of deeply buried high-quality reservoirs.In general,deposition is the basis,diagenesis is the key and overpressure is the important reason for the development of favorable reservoir in the Xihu sag.
Xihu sagcharacter of deep section reservoirsuncompactiondissolution
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