Study On Reservoir Characteristics And Genetic Mechanism Of Low Permeability Sandstone In Zhuhai Formation Of Wenchang Sag

With the development of oil-gas exploration theories and production technology,methods and techniques,low permeability sandstone reservoirs are valued for their great potential of oil-gas resources.Low permeability sandstone reservoirs typically underwent complex diagenesis which makes them very tight,and the physical properties of the reservoirs are generally characterized by low porosity and low permeability.However,under certain conditions,the physical properties of the reservoirs can also be preserved to form effective reservoirs,that is,relatively high quality reservoirs.Low permeability reservoirs are widely distributed in sedimentary basins in China and have become an important area of oil-gas exploration.Wenchang A Sag is rich in oil and gas resources,and Zhuhai formation is an important oil-gas exploration in the sag.However,the exploration results show that Zhuhai formation is a typical low porosity and low permeability reservoir with large burial depth,complex diagenesis and strong reservoir heterogeneity.Therefore,it is the focus of oil and gas exploration in Zhuhai formation to search for high quality reservoirs with relatively higher porosity and permeability.Strengthening the understanding of reservoir development characteristics and diagenetic evolution process,and clarifying the main control factors and pore development mechanism in the study area are the basis for the prediction of high-quality reservoirs,which is of great significance to the exploration and development of low permeability reservoirs in Wenchang A Sag and even the whole Pearl River Mouth Basin.This thesis is based on the previous research and taking the theories of reservoir sedimentology,petroleum geology,reservoir petrology,and well logging geology as the guidance.Based on the core observation,thin section,scanning electron microscopy,physical properties,X-ray diffraction,cathode luminescence,carbon and oxygen isotopes,capillary pressure test and other experimental analysis methods,the reservoir characteristics and diagenesis characteristics of Zhuhai formation were analyzed,the pore evolution model was established,the main controlling factors of low-permeability reservoirs were identified,the genetic mechanism of high-quality reservoirs was discussed.The results are showed as follow:The study shows that Zhuhai formation is low in compositional maturity and moderate in textural maturity.The storage space is dominated by feldspar dissolved pores and residual intergranular pores,followed by intergranular dissolved pores and mold holes.The mercury injection parameters were medium to high discharge pressure and median pressure,the mercury intake saturation was high and the mercury removal efficiency was low,indicating that the reservoir throat was narrow and the pore throat connectivity was poor.The average porosity is 11.5%,the average permeability is24.18×10^-3?m²,which,overall,is low to ultra-low porosity and permeability porous reservoir.Zhuhai formation is in the middle diagenetic A-B stage,and the burial depth is generally greater than 3000m,with the current average burial depth of 3800m.The reservoir mainly underwent mechanical compaction,cementation,dissolution and fracture.Strong compaction is the main cause of reservoir porosity reduction.According to the diagenesis type,strength and characteristic minerals,diagenetic facies are divided,and the diagenetic facies vertical identification and plane distribution prediction are realized by using neural network technology.The results show that south fault zone mainly develop Weak compaction weak cementation primary intergranular pore facies and medium compaction weak cementation strong dissolution facies in the medium-deep-depth,while strong compaction and medium-strong dissolution facies developed in the deep layer.In sixth fault zone,medium-strong compaction chlorite coating facies and medium-strong compaction strong dissolution facies are relatively developed.Through comprehensive research,it is concluded that the development of low permeability reservoirs in Zhuhai formation is mainly controlled by sedimentary environment,diagenesis type and intensity,and fluid activity channel.Differences in sedimentary environments lead to differences in the original rock composition of the reservoir.Mechanical compaction is the main cause of formation densification in Zhuhai formation.Carbonate cementation,quartz growth and late authigenic illite are the key to formation densification.The control of thermal fluid activity by the fault resulted in a large amount of precipitation of late carbonate cement near the fault zone.Secondary dissolution can effectively improve the reservoir performance.The existence of micro-fractures not only improves the seepage ability of the reservoir,but also provides a good migration channel for acidic fluid activities,which is conducive to the dissolution.The genesis mechanism of relatively high quality reservoirs in the low permeability background of Zhuhai formation lies in the preservation of primary pores and the development of secondary dissolution pores.The preservation conditions of primary pores include favorable sedimentary environment,chlorite coating and early hydrocarbon filling.Underwater distributary channel and tidal channel developed under strong hydrodynamic conditions have good sand body continuity,high sandstone maturity and high content of rigid particles,which are conducive to the preservation of primary pores under deep burial conditions.Chlorite coating inhibits the growth of quartz secondary augmentation and is an important mechanism for primary pore preservation.Early hydrocarbon filling inhibited the precipitation of carbonate cement and protected the primary intergranular pores to a certain extent.The secondary pores in the study area are relatively developed due to the multi-stage dissolution,and the existing secondary pores are mostly related to organic acid dissolution and thermal fluid dissolution.According to the comprehensive study,the sandstone with coarse grain,weak compaction,strong dissolution and chlorite envelope are the relatively high quality reservoir development zones.