Study Of Diagenesis And "Sweet Spot" Prediction Of Sandstone Reservoir In The Huagang Formation,Xihu Depression

The north-central N gas field in the central reversal zone of the Xihu Depression in the East China Sea shows great potential for deep natural gas development.However,the main sandstone reservoir has a large buried depth span and the overall heterogeneity is strong.The reservoir control factors are complex and the understanding of the "sweet spot" is the key to gas reservoir development.This paper studies the microscopic characteristics and diagenetic evolution of the main sandstone reservoirs in the N gas field.The type of logging diagenetic facies are divided and predicted in the study area,and the main controlling factors of the reservoir's "sweet spot" are summarized,and finally integrated the prediction results of the diagenetic facies and the data of the actual trial gas production,the development of the reservoir "sweet spot" are predicted in order to provide guidance for the next exploration and development.The deep burial span of the study area is mainly characterized by the development of three distinct pore zones,of which the main sandstone reservoirs are developed at depths ranging from 3500 to 4600 m.Through core observations,thin section analysis,cathodoluminescence,scanning electron microscopy,particle size analysis,the X-ray diffraction analysis and other technical means,combined with related physical property data and geochemical data,the study result shows that:When the buried depth is less than 4100 m,there mainly develops the I-III reservoir "sweet spot",and the diagenetic facies mainly consist of medium compaction-medium to strong dissolution facies,medium to strong compaction-medium dissolution facies,and medium to strong compaction-medium to weak dissolution facies.The origin of the I and ? type reservoir "sweet spot" is similar,the impact of early undercompaction overpressure is stronger.The favorable stratigraphic structure and depth range make the reservoir undergo the early strong undercompaction overpressure,with a result of slowing the mechanical and chemical compaction,promoting the development of chlorite coat and working together to restrain quartz cement.And the well preservation of primary pore facilitates the late large-scale organic acid dissolution,with a relative high porosity existing partly.Type III reservoir "sweet spot" is less affected by the early undercompaction and overpressure,which is mainly controlled by sedimentary factors and subsequent diagenesis.Because it is formed in medium-strong hydrodynamic channels,it is dominated by mass medium-coarse sandstones with a higher quartz content and a lower clay content detrital composition.At the same time,the structure exhibits better sorting and rounding,even if the effect of the overpressure pore preserved is weak,the relatively high composition and structure maturity high can also inhibit compaction to a certain extent,so the primary porosity can be preserved partly.When the buried depth is greater than 4100 m,there mainly develops the ? reservoir "sweet spot",and the diagenetic facies mainly consist of medium to strong compaction-medium dissolution facies and medium to strong compaction-medium to weak dissolution facies.With a relative greater buried depth and high digenetic degree,much porosity has lost by intense compaction in the reservoir before undergoing the early uncompaction overpressure,so porosity preservation by overpressure and the late large-scale organic acid dissolution will be difficult to occur.In addition,further development of various cement in alkaline diagenetic environment make the reservoir tend to be tighter.Taking into account the next-step fracturing development requirements,the medium-coarse sandstone with higher quartz content,lower clay content,and relatively good physical properties can be the first choice of "sweet spot" at this depth.