Gravity-flow deposits and implications for unconventional oil and gas in the Ordovician- Silurian Wufeng-Longmaxi Formation, Sichuan Basin, South China

The Ordovician-Silurian black shales in the Wufeng-Longmaxi Formation within the Sichuan Basin, South China, have yielded the largest shale gas production in China. Such black shales are commonly agreed as being deposited in deep-water shelf facies. This study integrates a series of data sets, including outcrops, cores, wire-line logs, fullbore formation microimager (FMI) data, and seismic data, and finds that sedimentary gravity -flow deposits were developed, intercalating with black shales. Turbi-dites, slumps, and slides, are widely developed in the Sichuan Basin. Turbidites are featured with typical Bouma sequence, groove clasts, and sole marks at bases of sandy intervals. The binary and parallel interbedding of muddy and sandy laminations and convolute laminations can be decently recognized on FMI loggings. Slumps show deformed fabrics, and slides display sparse slickensides on cores and FMI loggings.The occurrence of turbidite, slump, and slide deposits sup-ports the assumption that the paleogeography changed from shelf to slope to bathyal plain, therefore challenging the cur-rently dominating arguments that deep-water shelf is prevalent, and shales are solely developed.Slumps and turbidites are developed along all the uplifts sur-rounding the Sichuan Basin, suggesting sediments containing silt and sand were sourced from these uplifts and transported basin -ward. Muddy sediments were gradually unloaded on shelf and slope, whereas silty and sandy sediments were accelerated along slopes due to gravity, and then settled in bathyal plain. More sediment gravity deposits could be reasonably inferred in the basin center and hence, may provide new implications for hydro-carbon exploration and development in the Sichuan Basin and its periphery.