Reformation mechanism of high-quality carbonate reservoirs in northeastern Sichuan Basin

The high-quality carbonate reservoirs in Puguang Gas Field of the northeastern Sichuan Basin are mainly dolomites of reef-bank facies.Secondary intragranular and intergranular pores work as the main reservoir spaces,accompanied with significant tectonic track ways.The secondary pores generated from 2 stages of dissolution during burial history.1) The 1st stage of dissolution took place during the early burial period of mesodiagenesis,which corresponded to the early and middle Jurassic and was followed by big-scale petroleum filling.2) The 2nd stage of dissolution took place during the late burial period of mesodiagenesis,which corresponded to the late Jurassic and the early Cretaceous.Organic acid generated during organic thermal evolution and H₂S (TSR origin) solution in gypsolyte strata worked as fluid medium for the 2 stages of dissolution,respectively.Burial dissolutions can be identified from micropetrologies and diagenesis marks of reservoirs.A large number of tectonic track ways in reservoir rocks resulted from stages of tectonic movement during Meso-Cenozoic.The 2 stages of tectonic movement (the early and the middle-late Yanshan movements) occurred at the same time as the 2 stages of dissolution,which gave birth to fractures and microfractures.Rock permeability was improved and acid fluids migrated into rocks.Non-selective dissolutions happened and "fracture-dissolved pore" combination formed.Tectonic stress promoted burial dissolution,and their interactions helped reservoir reformation.The reservoir generation model named "facies-controlled distribution-tectonic stress+dissolution reformation" can be used to predict favorable carbonate reservoirs.