Geological conditions evaluation and favorable areas selection of the shale gas from Longmaxi Formation in the Northeast of Yunnan

According to measured profiles and lab tests in the Longmaxi Formation of the northeast of Yunnan, TOC and maturity of the samples were determined. And also, the shale gas reservoirs were investigated by XRD, high pressure mercury intrusion, gas absorption and scanning electron microscope. Besides the above methods, isothermal absorption is also used to evaluate the favorable area in the study area. The results show that: (1)the black shales are distributed in the lower segment of Longmaxi Formation, and the average thickness is more than 30 m and it gradually increases from southeast to northwest in the study area. With Yanjin and Daguan counties as the center, the burial depth of strata increases outward. (2)Regionally, TOC of shale increases from south to north, and it is more than 2% in the northwest of Yanjin and the north of Weixin. Vertically, TOC of shale is more than 2% in lower segment of Longmaxi Formation, but less than 1% in the upper segment of Longmaxi Formation. The measured average vitrinite reflectance of shales is 3.6%, showing that the shales are at the over-mature stage. (3)The shales have more brittle minerals in the study area, so the reservoirs are easy to be artificially fractured. Compared with the major gas shales in the north American, clay mineral contents are higher but brittle minerals (quartz, et al.) and carbonate minerals (calcite, et al.) are less in the study area. (4)Mercury injection experiments show that the pores are mainly transition pores and micro pores, but the pore opening degree decreases from the bottom to top in the Longmaxi Formation. Liquid nitrogen adsorption experiments indicate that pores with diameters less than 20nm are better developed in the transition pores and micro pores. Moreover, the two ends of pores are open, with the shapes of cylinders, cones, parallel plates, and a certain amount of flask pores. Measured pore specific surface area is 6.479-17.329cm2/g, with the mean of 11. 425cm2/g. The pore volume is 0.006-0.016cm3/g. The pore average size is 3.256-4.367nm. (5)Isothermal adsorption experiments show that maximum absorbed gas content of the shales in Longmaxi Formation can attain 3.21cm3/g. Comprehensively, this paper has selected three favorable areas for shale gas, including the area between Yanjin and Suijiang county, the northeast of Daguan and the southwest of Weixin. ©, 2015, Science Press. All right reserved.