Shale-gas accumulation and pore structure characteristics in the lower Cambrian Niutitang shales, Cen-gong Block, South China

Abstract

To evaluate pore networks of the lower Cambrian Niutitang Shale in the Cen-gong Block, multiple techniques, including geochemical analysis, low-pressure gas (CO₂ and N₂) adsorption, mercury intrusion capillary porosimetry (MICP) and field-emission scanning electron microscope (FE-SEM), were performed to investigate pore structure, including pore-size distribution (PSD), pore volume and surface area of black organic-rich shales from two wells to better understand the mechanism of shale-gas accumulation. Geochemical analysis shows that equivalent vitrinite reflectance ranges from 2.1 to 3.5% and are overmature. Surface areas and pore volumes of micropores calculated from CO₂ adsorption, which vary from 8.23 to 37.39 m²/g and from 3.06 $\times$ 10⁻³ to 10.89 $\times$ 10⁻³ cm³/g, respectively, can provide large amounts of adsorptive sites for shale-gas accumulation. N₂ adsorption shows reversed S-shaped isotherms, which are classified into type H2 and H3 hysteresis loops, indicative of ink-bottle pores and slit-or plate-like pores. N₂ adsorption and MICP analysis indicates that the inorganic-associated pores in shales may play a significant role in the storage and production of hydrocarbons. High-resolution FE-SEM analysis suggests that OM pores (OMPs) are common in Niutitang samples with a dominant diameter less 50 nm, but a sample with the highest total organic carbon (TOC) of 10.01 wt% has few OMPs, whereas a sample with the TOC of 7.51 wt% has more OMPs. The percentage of OMP area vs total organic matter area (Øs) ranges from 1.70 to 7.92%. The pore structure characteristics of TX1 well, calculated by multiple techniques, are better than samples from TM1 well. In addition, pore types are dominated by interparticle, intraparticle and organic matter pores, and the minor amounts of OMPs in TM1 well indicates that most of the organic matter may have lost the potential to generate hydrocarbons owing to the release of formation pressure with some graphitisation and high-angle fractures larger in the TX1 well. TOC content, mineral composition and tectonically related thermal maturity may be the main controlling factors of the pore structure and the lower Cambrian Niutitang Shale demonstrates the greatest potential for shale-gas production in South China.

Keywords:

• Niutitang Shale
• pore structure
• gas adsorption
• mercury intrusion
• FE-SEM
• controlling factors

Acknowledgements

The authors would like to extend their appreciation to the Key Laboratory of Tectonics and Petroleum Resources Ministry of Education, China University of Geosciences, Wuhan and Oil & Gas Survey Center of China Geological Survey. We also express our most sincere appreciation to Dr Keyu Liu his work on this paper.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

China Geological Survey Project Grant (No. DD20160185); China National Science and Technology Major Projects (Nos. 2016ZX05034002-003, 2017ZX05005001-008); the National Natural Science Foundation of China (Nos. 41690134, 41672139); and the Programme of Introducing Talents of Discipline to Universities (No. B14031).