Lithofacies and cyclicity of the Lower Cambrian Niutitang shale in the Mayang Basin of western Hunan, South China

• Eight shale lithofacies were identified.
• Three cycles were organized in Niutitang shale.
• Cycle 1 holds the largest shale gas potential in the study area.

Marine black shale is widely distributed in the Lower Cambrian Niutitang Formation in Mayang Basin, Western Hunan. Detailed outcrop descriptions, continued sampling and measurement of the Niutitang shale were conducted. According to the thin section observation, total organic carbon (TOC) analysis and mineral composition by X-ray diffraction (XRD), eight shale lithofacies were identified. On the basis of detailed lithofacies description and the analysis of the vertical changes in the TOC, sponge spicule and mineral (clay) composition, three cycles were recognized in Niutitang shale by using a high-resolution sequence stratigraphic approach. The results reveal that thickness of the Niutitang shale is 105–315 m in the study area. In the lower Niutitang shale (cycle1), the most common lithofacies is siliceous carbonaceous mudstone with the TOC contents of 4.7% on average. In the middle Niutitang shale (cycle2), the carbonaceous silty mudstone with an average TOC content of 2.0% is widely developed. In the upper Niutitang shale (cycle3), the argillaceous mudstone is the dominant lithofacies with more clay minerals and less TOC content of 1.51% averagely. The measurements of TOC, Ro (vitrinite reflectance), mineral (clay) composition, porosity, permeability, and methane isothermal adsorption of the three cycles show that cycle 1 holds the largest shale gas potential in the study area.