Earth Science Frontiers ›› 2023, Vol. 30 ›› Issue (3): 1-13.DOI: 10.13745/j.esf.sf.2022.5.31

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Main factors controlling the shale gas content of Cambrian shales of southern China—a discussion

ZHANG Tongwei1(), LUO Huan2, MENG Kang3   

  1. 1. Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78713, USA
    2. School of Earth Sciences/Key Laboratory of Mineral Resources in Western China (Gansu Province), Lanzhou University, Lanzhou 730000, China
    3. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
  • Received:2022-04-06 Revised:2022-04-30 Online:2023-05-25 Published:2023-04-27

Abstract:

The Cambrian shale gas resource potential of southern China is expected to be enormous with its widely distributed high-maturity organic-rich shales; however, success in shale gas exploration is uneven across the region. In this paper, we analyze the current research progress in the study of Cambrian shale gas geology in southern China, and discuss the key factors that determine the shale gas content, including reservoir characteristics, total organic carbon (TOC) content, porosity as well as tectonic conditions for gas preservation. The organic-rich Cambrian shales of the middle-upper Yangtze regions are mainly distributed in the intracratonic troughs in the Mianyang-Changning area and western Hubei, and in the deepwater shelf-slope area bordering Hunan, Guizhou and Chongqing. The porosity of Cambrian shales commonly ranges from 1% to 6%, and the pores are dominated by micropores and mesopores, mainly isolated, spongelike organic pores, which are very different from the widely-developed bobble-like organic pores in the Silurian Longmaxi shale. Organic pores are well developed in the Cambrian Qiongzhusi/Shuijingtuo shales of the troughs, with average organic porosity ranging between 20%-50%, and inorganic pores are also abundant and contribute to the total porosity. The shale gas content ranges from 1.5 to 5.5 m3/t, which is 10%-30% (in some samples as high as ~50%) of the estimated total gas generation, indicating favorable tectonic preservation conditions in the troughs. In contrast, organic pores are poorly developed in the Cambrian Niutitang shale of the deepwater shelf-slope area, with average organic porosity less than 20%. The shale gas content is below 1.0 m3/t, or less than ~10% of the estimated total gas generation, indicating poor preservation conditions and large-scale gas loss in the area. The gas loss might be caused by regional unconformities developed at the bottom of the organic-rich Cambrian shale formation as well as severe post-depositional tectonic uplift in the middle-upper Yangtze. However, interbedded carbonate-rich thin layers in troughs might act as a barrier to oil migration and gas loss. As a result, the two troughs should be the key target for Cambrian shale gas exploration, which is consistent with the current success in Cambrian shale gas exploration in China.

Key words: shale gas, gas content, Cambrian, organic pores, tectonic preservation conditions, southern China

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