Diagenesis and characteristics of reservoirs in the Member 3 of the Lower Triassic Feixianguan Formation in Heba gas field

Gas pools were found in the Member 3 of the Lower Triassic Feixianguan Formation in Heba area of the northeastern Sichuan Basin,indicating a great exploration potential in the Member 3 of the Lower Triassic Feixianguan Formation(T1f3 for short).Based on detailed microscopic,scanning electron microscopic and election microprobe observations of ordinary,mold and cathodoluminescence thin sections of core samples and combined with measurement results of porosity,permeability and trace element analysis of some samples,we examined diagenetic characteristics of carbonate reservoirs in the Member 3 of the Lower Triassic Feixianguan Formation in this area.The results showed that the Member 3 of the Feixianguan Formation is dominated by intra-platform shoal and inter-shoal deposits of open platform,of which the intra-platform shoal deposit is favorable for reservoir evolution.Main reservoir rocks are composed of oolitic limestones and calcarenite having intragranular dissolved pores and mould pores with low-porosity and low-permeability.The main kinds of diagenesis in reservoirs of the Member 3 of the Feixianguan Formation are micritization,dissolution,cementation or filling,compaction-pressolution,dolomitization and fracturing.The pore development of these grain carbonate reservoirs is affected by penecontemporaneous meteoric dissolution and burial dissolution,of which penecontemporaneous meteoric dissolution is a key factor that controls reservoir pore formation.Burial diagenesis including dolomitization,strontianite cementation or replacement and burial corrosion has greatly improved pores.Thus,favorable exploration zones in this area should be distributed in micro-paleogeomorphic upland and other places where faults and fractures are relatively developed.