Diagenesis Researches Of Natural Gas Reservoir With Gyprock And Salt Bed In Minfeng Area Of The Lower Part Of Number 4 Of Sha Hejie Formation In Dongying Depression

As the data collected from relevant department,much gas reservoir was buried deeper than 3500meters underground in Jiyang Sag.especially in the glutenite with gyprock and salt bed in the north of Dongying Depression.With the burial depth increased the influence of diagenesis on reservoir becomes more and more obvious.So it is urgent for us to know the diagenetic environment in deep formation to predict favorable reservoir.Cores of 11 wells,about 150 meters,were observed and 60 glutenite samples were derived from Minfeng area of the lower part of number 4 of Sha Hejie formation.The techniques applied in this study include thin-section observing,stained–thin section identification,Scanning Electronic Microscope observation,X-ray diffraction,Fluid inclusion heating-cooling experiment and mercury intrusion pore size distribution and etc,As the results of experiments show,the lower part of number 4 of Sha Hejie reservoirs composed of three kinds of rocks,sandstone,pebbled sandstone and conglomerate,they almost occurred in the branch channels,lobate sand body and main channels.The characters of diagenesis is particular because of the strong influences of gyprock and salt bed.Several digenesis processes have been observed :some area is less compaction,cementation degree is greatly different,much clay mineral transform into illite and chlorite,dissolution process and rejuvenation of crystals are common.Zeolite and pyrite indicates alkaline diagenetic environment.Diagenesis stage is confirmed that glutenite developed in the transition stage from late diagenesis A to B.Considering the burial history,the mode of the diagenesis is established.Diagenetic facies can be concluded as four types:less compaction digenetic facies,dissolution diagenetic facies, high compaction digenetic facies and high cementation digenetic facies,Storage spaces were divided into two kinds:pore and cleavage including primary origin and secondary origin.The existence of abnormal overpressure,gyprock and salt and the lower geothermal gradient is positive to preserve the original pore.Secondary pore produced by dissolution improved the ability of reservoir.Based on all study,we concluded that:Beside structural setting and sedimental micro-facies,the influence of the evolution of pore fluid in gyprock and salt bed and overpressure on diagenesis is more seriously.We draw a conclusion: In this area,less compaction digenetic facies and dissolution diagenetic facies,which develop on the sueface of branch channels and lobate sand body of submerged fan,are favour of gas reservoir formation.