A COMPARATIVE STUDY OF RESERVOIRS OF THE CHANGXING AND FEIXIANGUAN FORMATION BETWEEN PUGUANG GASFIELD AND JIANNAN GASFIELD IN THE SICHUAN BASIN

Puguang Gasfield and Jiannan Gasfield are located in Northeast Sichuan Basin.As the main gas bearing zones,the Permian Changxing and Triassic Feixianguan Formations are all reef-bank deposits.However,the reservoir scale and property are greatly different between Puguang Gasfield and Jiannan Gasfield.The reservoirs in Puguang Gasfield are mainly a large set of dolomites with the characteristics of dissolution pores,great thickness,medium porosity and medium permeability,high porosity and high permeability.However,the reservoirs in Jiannan Gasfield are mainly limestones and a little dolomites with the characteristics of thin thickness,low porosity and low permeability.Based on the comparative study on the formation conditions,the primary factor for differences towards the reservoirs between Puguang Gasfield and Jiannan Gasfield is the origin of the dolomites.The former well developed penecontemporaneous dolomitization and formed a large set of dolomites while the latter developed burial dolomitization and formed a little dolomites.Correlation analysis results showed that there is a close correlation to the differences of the sedimentary environment.The reef-bank facies are adjacent to the restricted platform facies or evaporate platform facies in Puguang Gasfield,however,which are adjacent to the open platform facies in Jiannan Gasfield.As a result,the former is easier to acquire saline water of high Mg/Ca values and beneficial to penecontemporaneous dolomitization while the latter is not avail to penecontemporaneous dolomitization.Consequently,it is suggested that the reef-bank facies adjacent to the sedimentary environment such as restricted platform facies or evaporate platform facies are advantageous to forming quality reservoirs.