DIAGENESIS AND POROSITY EVOLUTION OF SHAXIMIAO FORMATION IN LUODAI GAS FIELD OF THE WEST SICHUAN

From the analysis of thin section,it shows widely growing primary porosity and complicate secondary porosity in Shaximiao Fm of Middle Jurassic,Luodai Gas Field of the West Sichuan.Diagenesis and interstitial matter have great influence on porosity.Based on the X ray diffraction of Clay mineral and the scanning electron microscopic,the diagenetic stages here are Classified as earlier diagenetic A,B and middle diagenetic A,B.In the earlier diagenetic A,the primary porosity decreased rapidly to 10%～15% because of compaction and quarts growth Ⅰ.And in the earlier diagenetic B,the porosity is evaluated to hit 10% because of arise of carbonate and muriacite cementations.Then quarts growth Ⅱ occurred in heavy acidulous water and authigenic kaolinite deposited in the middle diagenetic A.Until the middle diagenetic B,the porosity were improved by the enhancement of denudation.During the whole middle digenetic stage,the porosity descented to 4%～9% after constant decrease and increase.After a series of diagenetic stages,the reservoirs were densified to low porosity and low permeability ones.