Natural fractures in tight gas volcanic reservoirs and their influences on production in the Xujiaweizi depression, Songliao Basin, China

Lower Cretaceous volcanic rocks that contain major natural fractures are the main natural gas reservoirs in the deep Xujiaweizi depression of the northern Songliao Basin in China. Natural fractures are mainly tectonic fractures, in addition to some shrinkage, dissolution, and cryptoexplosive fractures. Analyzing various sources of data revealed that fractures are generally better developed in the rhyolites and trachytes of the crater and near-crater group. Also, tectonic fractures are developed to a higher degree in the hanging wall of faults, with the degree of development decreasing with increasing distance from the fault planes. More than 85% of these fractures are found to be unfilled and, when connected by pores, provide the main effective storage volume for these volcanic reservoirs (up to ∼75%). Natural gas production is closely related to the linear density (P10) of unfilled tectonic fractures and the angle between the orientation of unfilled tectonic fracture and the corresponding direction of the maximum horizontal stress component. The results show that natural gas production is elevated when the average P10 exceeds 2 m−1, and the average included angle is less than 30°. Finally, it was concluded that within the study area, unfilled tectonic fractures of near east-west strikes, which have smaller included angles (<30°) with respect to the maximum horizontal stress direction, are under minimal compressive stresses and make the best fluid-flow pathways in the reservoir.