Characteristics of black shale in the Upper Ordovician Wufeng and lower Silurian Longmaxi formations in the Sichuan Basin and its periphery, China

ABSTRACT

Geochemical and mineralogical analyses, in addition to isothermal adsorption experiments on field samples, are used to characterise the sedimentary environments, reservoirs and adsorbed gas of the Upper Ordovician Wufeng–lower Silurian Longmaxi formations in the Sichuan Basin and its peripheral areas. The sedimentary environment of the Wufeng and the lower part of Longmaxi formations is a deep-water shelf with five different lithologies identified: siliceous shale, black shale, siltstone, biolithite limestone and bentonite. The black shale in the Wufeng and the lower part of Longmaxi formations is 50 m thick, with an average organic carbon content (TOC) of 3.81 wt% and a maturity (Ro) of 1.62%. Quartz comprises 54.94 vol% of the shale and positively correlates with the TOC. Micropores in the black shale include intergranular pores, intragranular pores, organic matter pores and microfractures. Among these pores, spaces between clay sheets and organic molecules represent a favourable storage space for the accumulation and preservation of oil and gas. The Langmuir volume parameter ranges between 1.52 and 3.01 cm³/g, with an average value of 2.33 cm³/g. The presence of organic matter pores and pores between clay sheets in the black shale is the main and controlling factor for accumulated gas.

KEYWORDS:

• Sichuan Basin
• Wufeng Formation
• Longmaxi Formation
• black shale
• sedimentary environments
• organic geochemistry
• reservoir characteristics
• gas resource

Acknowledgements

This research was supported by Science and Technology of the Sichuan Province support planning project (15ZC1390). We acknowledge the help of Luba Jansa in correcting the English in this manuscript. The authors appreciate the reviewers’ comments, which improved the quality of the paper.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Science and Technology of Sichuan Province support planning project [grant number 15ZC1390].