A comparative study of characterization of lower Palaeozoic Niutitang shale in northwestern Hunan, China

摘要

The lower Palaeozoic Cambrian Niutitang shale and Silurian Longmaxi shale in the upper Yangtze region are the two most favourable marine shales in south China. Great achievements on shale gas development and production have already been made in the Longmaxi shale in southeastern Sichuan (SES). However, to date, economic shale gas potential has not been proved in northwestern Hunan (NWH), which is still a virgin area for shale gas exploration. Based on detailed outcrop investigation, laboratory testing and numerical simulation, the shale gas potential of Niutitang shale were evaluated in this work. Comparisons were also made through vertically dia-chroneity contrasting with the Longmaxi shale in the NWH area and horizontally isochronic contrasting with Niutitang shale in the SES area. Through analysing the evolution of the palaeoenvironment and palaeotectonics of the period from Niutitang to Longmaxi, it was found that (1) the NWH Niutitang shale was developed in the hydrothermal continental shelf paleoenvironment, which provided a favourable environment for shale deposition, so the thicker and higher total organic carbon (TOC) shale was largely developed during this time; (2) By the influence of hydrothermal igneous leaching, the element Si formed quartz, which improved the brittleness of the Niutitang shale; (3) Induced by intense tectonic movement, natural fractures were highly developed in the Niutitang shale, which improved the reservoir connectivity; and (4) By comprehensively considering the TOC, reservoir property, and gas retention conditions, the Niutitang shale is a promising target for shale gas development in this area. The results from this work warrant further exploration work in the Niutitang shale in the NWH area.