Characteristics and Main Controls of Tectonic Fractures and Their Contribution to Deep Tight Reservoirs: A Case Study of the Huoshiling-Shahezi Formations in Gudian Half-Graben, Songliao Basin, China

AbstractThe deep volcanic and clastic rocks in the Songliao Basin are typical tight reservoirs with ultralow porosity and permeability. Natural fracture determines the effective reservoirs in the deep strata. Integrated core and thin section observations, image logging, physical property measurement, X-ray diffraction, and seismic attribute analysis were applied to ascertain the characteristics and genetic mechanisms of the tectonic fractures in the Huoshiling-Shahezi Formations of the Gudian half-graben. The contribution of the fractures to deep tight reservoirs was also discussed. Results showed that tectonic fractures are mainly tension, shear, and slip fractures. The density of the fractures in andesitic tuffs with high clay minerals content is significantly higher than that in the other rock types. The main controls of the developmental degree of fractures are tectonism. Three sets of fractures were identified based on the genetic connection between fracture formation and tectonic stress. The NNW-trending tension fractures were formed in the syn-rift stage and successively extended in the post-rift stage. The regional compressional stress drove the formation of the NEE-trending shear fractures and NNW-trending “X” plane-conjugated shear fractures in the postextrusion stage. The averaged porosity and permeability of the reservoirs are generally less than 6.0% and lower than 0.06 mD. The pores can hardly independently provide migration pathways and storage spaces, but the fractures significantly improved the reservoir properties. The fractures were well developed in the regions near the Gudian fault, fault transfer zones, and volcanoes, indicating the distribution of potential reservoirs. Moreover, unraveling the types and orientations of the fractures will provide an essential basis for the analysis of hydrocarbon migration and hydraulic fracture treatments