Tectonic and depositional setting of the lower Cambrian and lower Silurian marine shales in the Yangtze Platform, South China: Implications for shale gas exploration and production

Marine shales of the Lower Cambrian Qiongzhusi and Lower Silurian Longmaxi formations are extensively distributed in the Yangtze Platform. Detailed depositional and tectonic analyses and regional mapping indicated that the organic-rich Qiongzhusi-equivalent shale was deposited in an intra-shelf low-slope environment in a passive margin setting. The Longmaxi-equivalent shale was mainly deposited in a widely-distributed intra-shelf low setting in the Upper and Middle Yangtze, and in a distal foredeep area away from elastic dilution in the Lower Yangtze Platform. The transgressive and early highstand system tracts in both Qiongzhusi and Longmaxi shales produced the best-quality shale reservoirs, with high total organic matter (TOC) content (> 2%) and high quartz content (> 45%). Tectonics has substantial effects on coeval deposition of organic-rich shale and its hydrocarbon accumulation and production. Later tectonism deformed the shales and disrupted early shale gas accumulation. Recent shale gas resource exploration and geologic analysis suggested that the high rate of shale gas production is due to overpressured marine shale reservoirs with high TOC and high gas content in the tectonically-stable area. Shale gas production occurred at reduced rates in slightly underpressured shale reservoirs with lower gas content in the syncline of the tectonically-transitional area deformed by three tectonic movements. There was no gas production from shales in the tectonically-active area disrupted by at least four tectonic movements and complex faults. The results of this analysis indicate that local tectonically-stable areas in the Middle and Lower Yangtze platforms may have shale gas accumulation and production potential.