Abstract: The distribution of seismic quality factor Q and conditions for shale gas preservation of the lower shale interval of Wufeng-Longmaxi Formations were studied in Nanchuan area based on seismic and geologic data as well as seismic attributes.In regard to the faulted fracture zones in the Nanchuan shale gas block, the workflow and its iterative algorithm were proposed to work out the stable seismic quality factor Q and its limits.Results show that the internal features of the faulted fracture zones and distribution of different types of shales can be recognized rapidly and directly from the distribution of seismic quality factor Q.Thereinto, the seismic quality factor Q of the faulted fracture shale zones is generally low, with its value mostly below 0.2 while locally over 0.5, and shows chaotic or porphyritic distribution; the shales are distributed in the shape of stripes, trending north-east, but ripped and crushed by the fault zones, resulting in poor preservation conditions.The seismic quality factor Q of the stably-distributed shale zones is higher, with its value being more than 0.7, and shows stable distribution in plane without any abrupt changes, leading to good preservation conditions.The seismic quality factor Q of the shale zones with well-developed micro-fractures tends to change gradually, with its value diminishing regularly from 0.7 at the margin to 0.1 at the centre of the shale zones, implying the best preservation conditions.Practices have shown that most high-yield shale gas wells are located within the stably distributed shale zones or stable shale zones with well-developed micro-fractures, and the method of evaluating shale with seismic quality factor Q is a practical tool for assessing preservation conditions for shale gas and optimizing the deployment of development wells.

Key words: seismic quality factor, iterative algorithm, microfracture, faulted fracture zone, preservation condition, shale gas, Wufeng-Longmaxi Formations, Sichuan Basin