Abstract:

Based on the observation and description of Niutitang shale core fractures in southeast Chongqing and north Guizhou, the effectiveness of fractures was analyzed quantitatively by using logging and regional geological data, meanwhile, the controlling factors of natural fractures effectiveness were researched qualitatively. The relationship between shale gas content and fractures effectiveness was discussed based on the desorption experiments of shale core. The results show that fractures are abundant in Niutitang shale core. The significant difference between deep laterolog resistivity values and shallow laterolog resistivity values revealed the high effectiveness of natural fractures with wild aperture, steep angle and far extending in longitudinal. On the contrary, the low effectiveness of natural fractures due to low angle, narrow aperture, mineral sealing and closely extending in longitudinal, caused the unconspicuous difference between deep laterolog resistivity values and shallow laterolog resistivity values. The effectiveness of fractures is mainly controlled by mineral sealing, denudation, abnormal fluid pressure and tectonic movement. Among them, mineral sealing in fractures caused major damage to the effectiveness, denudation and high abnormal fluid pressure improved the porosity and permeability of shale, thereby improved the effectiveness. The regional tectonic activity produced different effects to the fractures which are in different parts of tectonic position. The effective fractures contributed to the desorption of shale gas, hence the free gas content increased. Moreover, the effective fractures could be the channels for gas migration and exploitation, which improved the permeability of shale reservoir. However, natural fractures with large scale would destroy the sealing property and overpressure of shale reservoir, which will result in the loss of natural gas, therefore, it is not conducive to the preservation of shale gas.

Key words: southeastern Chongqing, northern Guizhou, Niutitang, shale, fracture, effectiveness