PORE EVOLUTION OF RESERVOIRS OF FEIXIANGUAN FORMATION IN PUGUANG GASFIELD IN NORTHEASTERN SICHUAN

The reservoirs of Feixiangaun Formation in Puguang gasfield in northeastern Sichuan are mainly consisted of sediments of platform marginal ooilite beach,with principle rocks of ooilite dolomite and crystalline dolomite.Due to fresh water dissolution in the early stage of diagenesis,some secondary solutional voids were developed.However,these voids and the primary pore were largely filled by early cement.Small amount,normally less than 1%,of intercrystalline pore was formed as a result of dolomitization.Present effective pores mainly are solutional voids and fractures formed during the burial period.There were two stages of burial dissolution,with stage Ⅰ was developed after structural compression but before oil filling.Dissolution was related to acid flow with rich organic acid and carbon dioxide.The pores formed in this period accounted for 40% of present porosity of reservoirs.Stage Ⅱ dissolution was developed after the filled oil was transferred to bitumen.The acid flow might be related to the pro-ducts of themochemical sulfate reduction(TSR).Its porosity accounted for 60% of present reservoirs.In terms of pore evolution,the favorable diagenesis for formation of good reservoirs included dolomitization,structural compression and early and late burial dissolution.