Molecular and Carbon Isotopic Variation during Canister Degassing of Terrestrial Shale: A Case Study from Xiahuayuan Formation in the Xuanhua Basin, North China

• Main Authors: Jia Tao, Jinchuan Zhang, Junlan Liu, Yang Liu, Wei Dang, Haicheng Yu, Zhe Cao, Sheng Wang, Zhe Dong
• Format: Article
• Language: English
• Published: MDPI AG 2021-08-01
• Series: Minerals
• Subjects: molecular variation; isotope variation; terrestrial shales; core degassing; reservoir physical properties; micropore parameters
• Online Access: https://www.mdpi.com/2075-163X/11/8/843

Molecular and carbon isotopic variation during degassing process have been observed in marine shale reservoirs, however, this behavior remains largely unexplored in terrestrial shale reservoirs. Here, we investigate the rock parameters of five terrestrial shale core samples from the Xiahuayuan Formation and the geochemical parameters of thirty natural gas samples collected during field canister degassing experiments. Based on these new data, the gas composition and carbon isotope variation during canister degassing are discussed and, further, the relationship between petrophysics and the carbon isotope variation is explored. The results show that methane content first increases and then decreases, the concentrations of carbon dioxide (CO₂) and nitrogen gas (N₂) peak in the early degassing stage, while heavier hydrocarbons gradually increase over time. Shale gas generated from humic source rocks contains more non-hydrocarbon and less heavy hydrocarbon components than that generated from sapropelic source rocks with similar maturity. Time-series sampling presents an upward increase in δ¹³C₁ value during the degassing process with the largest variation up to 5.7‰, while the variation in δ¹³C₃ and δ¹³C₂ is insignificant compared to δ¹³C₁. Moreover, we find that there is only a small variation in δ¹³C₁ in shale samples with high permeability and relatively undeveloped micropores, which is similar to the limited δ¹³C₁ variation in conventional natural gas. For our studied samples, the degree of carbon isotope variation is positively correlated with the TOC content, micropore volume, and micropore surface, suggesting that these three factors may play a significant role in carbon isotope shifts during shale gas degassing. We further propose that the strong ¹³C₁ and C₂⁺depletion of shale gas observed during the early degassing stage may have resulted from the desorption and diffusion effect, which may lead to deviation in the identification of natural gas origin. It is therefore shale gas of the late degassing stage that would be more suitable for study to reduce analytic deviations. In most samples investigated, significant isotopic variation occurred during the degassing stage at room temperature, indicating that the adsorbed gas had already been desorbed at this stage Our results therefore suggest that more parameters may need to be considered when evaluating the lost gas of shales.