۱- Introduction

۲- Regional geological setting

۳- Data and methodology

۴- Results of the depositional setting and distribution analysis of the Lower Paleozoic shales

۵- Results of the tectonism in the Yangtze Platform and control on lower Paleozoic shale gas generation, accumulation and preservation

۶- Discussions

۷- Conclusions

References

Abstract

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 clastic 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.

Introduction

Advances in hydraulic fracturing, horizontal drilling, geological characterization, and characterization of shale properties have greatly expanded shale gas production in the United States, and have revolutionized the US energy market (Curtis, 2002; Steward, 2007; EIA, 2013). In China, shale gas exploration is only just beginning. In 2013, commercially-viable shale gas was produced from the Lower Silurian Longmaxi marine shale in the Fuling Shale Gas Field in the southeast Sichuan Basin, making China the only country outside North America that has reported commercial production (Jiang, 2014). Resource assessment, exploration activities and preliminary production indicate that the Lower Paleozoic marine shales distributed throughout the Yangtze Platform hold the most shale gas reserves (Zhang et al, 2008; Zou et al., 2010; Zhang et al., 2012; Du et al., 2015; Jiang et al., 2015, 2016). Exploration and analyses indicated promising results were mainly from individual pockets in the Sichuan Basin and adjacent areas in the Upper Yangtze Platform located to the west of Huangling anticline in Hubei Province. There was virtually no evidence for shale gas production in the Lower Yangtze Platform (located to the east and southeast of Tanlu Fault), and results indicated limited production in the Middle Yangtze Platform located between Upper and Lower Yangtze Platforms (Tan et al., 2014; Jiang et al., 2015, 2016).