Diagenetic Analysis And Seismic Prediction Of Gas-bearing Sand Of Zhujiang Formation In Baiyun Sag

The deep-water fan sandstone of Zhujiang formation in Baiyun sag of pearl river estuary basin is the main production layer in the deep-water area.With good petrophysical property and stable distribution,the sandstone generally contains gas in structural traps.On seismic data,the gas-bearing sandstone presents "bright spot" response,amid to which the earlier well drilling obtained a productive achievement.With the deepening of exploration,almost all structural traps and "bright spot" gas reservoirs have been drilled.And gas-bearing sandstone with non-bright type has been found,while the AVO anomaly type presents class Ⅱ as well as class Ⅲ.Due to the lack of structural traps and "bright spot" gas reservoirs,the exploration of Baiyun sag turns to the identification of lithologic gas reservoirs.The seismic response of gas-bearing sandstone in Zhujiang formation is mainly affected by diagenesis.The burial depth of Zhujiang formation changes greatly and the types of diagenesis change rapidly.Compaction,cementation and dissolution are the main diagenesis types developed in sandstone.Diagenesis changes the rock physical properties of the gasbearing sandstone and then affects the seismic response.Compaction significantly reduces the porosity of sandstone,tightens the contact of mineral particles,thus increases elastic modulus and decreases the difference of rock physical properties between sandstone and mudstone,resulting in weak seismic response;Cementation has relatively slight influence on porosity,but can significantly enhance the rigidity of sandstone,then increases elastic modulus and weakens seismic response;Dissolution increases porosity by dissolving grains and cements,reduces the contact area between grains,thus decreases the elastic modulus of sandstone,enlarges the difference of rock physical properties between sandstone and mudstone,resulting in an obvious seismic response.Therefore,the study of diagenesis becomes the key to seismic prediction of gas-bearing sandstone.The development degree of different diagenesis is mainly related to burial depth.According to the difference of main controlled diagenesis,the study area is divided into three diagenetic facies zones,which are weak compaction—weak dissolution—strong cementation facies,medium compaction—strong dissolution—weak cementation facies,and strong compaction—medium dissolution—weak cementation facies.The problem of uncertain AVO response of gas-bearing sandstone has been solved by carrying out AVO analysis on each diagenetic facies zone.In order to quantitatively predict gas-bearing sandstone,the contact-cement model is selected for rock physical modeling of sandstone in each diagenetic facies zone.And two elastic parameters,P-wave impedance and ,which are sensitive to gas sand,are selected to make rock physical templates.Verified by logging data,the rock physical templates of each diagenetic facies zone can well identify gas and water sands.Combining with elastic parameter data from simultaneously pre-stack inversion,the distribution of gas-bearing sandstone in Zhujiang formation in the whole study area is quantitatively predicted,which provides technical support for further exploration.