Reservoir Characteristics And Controlling Factors Of The Paleogene Huagang Formation In The Central Inversion Tectonic Belt Of The Xihu Sag

The sandstone reservoirs of the Paleogene Huagang Formation in the central inversion tectonic belt of Xihu Sag of the East China Sea Shelf Basin are generally characterized by low porosity and low permeability,reservoirs with relatively good physical properties still developed locally.However,the understanding of the reservoir has not yet revealed the formation condition,distribution law and main control factors of the high quality reservoir in the study area,which seriously restricted the exploration and exploitation of natural gas towards deep favorable reservoirs of the Xihu Sag.Therefore,this paper focuses on the petrology,diagenesis,pore structure characteristics and pore evolution processes of the Huagang Formation in the central inversion tectonic belt of the Xihu Sag,and analyzes the controlling factors of the high quality reservoir development.And to classify and evaluate the Huagang Formation reservoirs in order to promote the exploration and exploitation of oil and gas in the Huagang Formation of the Xihu Sag.The main results achieved are as follows:?1?Based on thin section identifications,particle size analysis and measured physical properties,it is determined that the Huagang reservoir sandstone is mainly composed of fine-medium sand,with good sorting and roundness,medium compositional maturity and high structural maturity.The rock types is dominated by feldspathic and lithic quartz sandstone with coarse grains,high content of quartz,low content of matrix,low content of kaolinite and content,in general,it has petrological characteristics of"one coarse,one high and three low".The porosity of the Huagang Formation reservoirs is mainly in the range of 6%¹5%,which belongs to low to super-low porosity reservoir.The permeability is mainly in the range of 0.1¹0×10^-3?m²,which belongs to low to super-low permeability reservoir.The percentage of secondary dissolution pores in the Huagang sandstone reservoirs is increasing with increasing burial depth.The change of reservoir physical properties shows strong heterogeneity.The northern area is dominated by super-low porosity and low permeability reservoir,and the Huagang Formation in the middle area belongs to the super-low/low porosity and low permeability reservoir,while the southern area is dominated by medium porosity reservoirs.?2?According to the large number of thin section identification,physical properties analysis and combined with the actual production conditions,to determine the standard of the high quality reservoirs of the Huagang Formation in the study area:continuous distribution reservoirs of porosity of more than 6%,permeability greater than 0.5×10^-3?m² and thickness greater than 5m.In order to further study the high quality reservoir,it is determined that reservoirs with permeability ranges between0.5×10^-3?m² and 10×10^-3?m²,porosity ranges of 6%¹5%,the effective thickness is more than 5m,the reserving capability is better than that of the surrounding rock is defined as"dessert","dessert"reservoir level can be distributed of class?and?1.Vertically,the H3-H4 layer is the main development zone of high quality reservoir.Horizontally,the thickness of the reservoir is relatively large in the north but has relatively poor physical properties.Southwards,the thickness of the reservoir decreases,while the physical properties of the reservoir is becoming better.?3?The study shows that the sediment source of the study area is mainly from the northern tiger skin reef,the distance is far away,the washing is clean,the northern area is closer to the main source which causes relatively low maturity,good sorting and roundness,its depositional rate and scale of the sedimentary strata is large and the reservoirs are formed in the middle area.The source of central and southern area is dominated by the Diaoyu Islands uplift and the western seas uplift in the eastern part of the Yangtze River.The decline of the sea level leads to the rapid transfer of the coarse grain debris to the depositional center,which caused sorting and roundness are relatively poor.High content of quartz,low content of matrix,coarse particles,high primary porosity are all favorable for the development of high quality reservoirs.The presence of overpressure relives the compaction of the rock particles and preserves the pores.Oil and gas filling,changing the property of the pore fluid and slow down the cementation.In the process of diagenesis,early stage chlorite and authigenic quartz plays an anti-compaction role,which preserved the residual porosity,and it was favorable for the later dissolution to produce a large number of secondary pores and develop relatively high quality reservoirs.The multi-stage dissolution is the most important diagenetic factor for the transformation of reservoir physical properties.