DIAGENESIS EFFECTS OF LOWER TRIASSIC FEIXIANGUAN FORMATION RESERVOIR IN PUGUANG GAS FIELD, NORTHEAST SICHUAN

The Lower Triassic Feixianguan Formation reservoir in the Puguang Gas Field of the Northeast Sichuan mainly consists of platform margin oolitic beach sediments.The main lithology is dolomite.The main diagenesis effects include: micritization,compaction and pressure solution,consolidated fill,dolomitization,nascent deformation and recrystallization,solution and structural disruption.Among which,dolomitization,solution and structural compression work as the key effects for reservoir formation.According to dolomite types,order degree and C,O isotope analyses,dolomite reservoir generates in high salinity fluid and quick crystallization environment.It is the result of dolomitization caused by seepage reflux during the penecontemporaneous stage.Three stages of solution effects have taken place in the reservoir.From the early diagenesis stage to the early middle diagenesis stage,pores generated from fresh water solution and primary intragranular pores are filled by consolidations.The present effective pores are buried ones.The burial solution is divided into two stages.The first stage is after structural compression but before oil filling.The second stage is when the filled oil has transferred to bitumen.Structural compression is another controlling element for reservoir formation in the Feixianguan Formation.On the one hand,structural compression is favorable for recrystallization;on the other hand,it forms fracture and cracks crystals,hence provides pathways for acid fluid.In this way,the contact surface for oil and rock is enlarged,which helps the generation of burial solution.