V P/ V S ratio anomalies in fractured rocks and implication for dolomite gas reservoir identification in the northwest Sichuan Basin

Abstract The dolomite gas reservoir of the Permian Chihsia Formation has recently become the main target of hydrocarbon exploration in the northwest Sichuan Basin. Owing to its substantially low porosity and permeability, the dolomite gas reservoir has become a great challenge for geophysicists in terms of distinguishing gas-filled dolomite thin layers. We have used limestone and dolomite core samples from the Permian in the northwest Sichuan Basin to investigate the physical characteristics of the dolomite gas reservoir in a laboratory. Dolomite has a comparatively higher permeability than limestone due to its higher fracture development. X-ray CT scanning indicates that a few large pore spaces dominate the pore space in limestone that cannot provide good pore connections, whereas small pores and fractures dominate the pore spaces in dolomite. The fracture development in the dolomites provides insignificant porosity that substantially increases the permeability. The VP/VS ratio estimated from well-logging data in the northwest Sichuan Basin indicates significantly lower anomalies. These low VP/VS anomalies validate the rock-physics analysis from the laboratory data and indicate that the VP/VS ratio could be a good indicator of gas reservoirs in dolomite layers.