Characteristics and controlling factors of the reef shoal reservoirs from the Changxing Formation-Feixianguan Formation in the Xuanhan-Daxian zone, northeastern Sichuan

The Xuanhan-Daxian zone in northeastern Sichuan is believed to be an important locality for the development of gas pools in China. The present paper deals, in detail, with the rock type, porosity type, porosity structure, physical property and distribution of the reef shoal reservoirs in terms of physical data, well logs and pressure-injection data with the aid of thin section examination and SEM techniques. The high-quality reservoirs in the study area consist of biohermal dolostone and oolitic dolostone characterized by the enrichment of intergranular solution openings due to recrystallizarion and polystage dissolution. The average porosity equals to 5.4% and average permeability equals to 93.41×10^-3μm² for the Changxing Formation reservoirs, while the average porosity equals to 8% and average permeability equals to 132.76×10^-3μm² for the Feixianguan Formation reservoirs. Sedimentary facies tends to control rock types and distribution, whereas diagenesis especially the dissolution tends to control the physical properties of the reservoirs. The fissures caused by structural processes may have dual impacts on the carbonate reservoirs.