Methane cracking within shale rocks: A new explanation for carbon isotope reversal of shale gas. (November 2020)

• Record Type: Journal Article
• Authors: Cheng, Bin; Xu, Jianbing; Deng, Qian; Liao, Zewen; Wang, Yunpeng; Faboya, Oluwabamise Lekan; Li, Shida; Liu, Jinzhong; Peng, Ping'an
• Is Part Of: Marine and petroleum geology. Volume 121(2020)
• Publication Date: 2020-11
• Subjects: Carbon isotope reversal -- Methane cracking -- Pyrolysis experiment -- Shale gas; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2020.104591
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: Shale gas with carbon isotope reversal (δ 13 Cmethane > δ 13 Cethane ), as a promising indicator for resource "sweet-spot", tends to occur at high-over maturity shales with high gas dryness index (Cmethane /(Cmethane + Cethane ) > 0.97). However, the cause for this reversal is still controversial and remains an open question. In the present work, we propose methane cracking within high-over matured shale rocks, as one of the possible mechanisms, to explain this reversal, which will further enhance the knowledge about carbon isotope reversal of shale gas. In order to validate this hypothesis, methane pyrolysis experiments were carried out in closed gold-tube system to investigate the mechanism of occurrence of carbon isotope reversal in shale gas. From the pyrolysates, the cracking of CH4 and the generation of higher number hydrocarbons like C2 H4, C2 H6, and C3 H8 were detected. The CH4 cracking and the subsequent formation of C2 H6 led to the residual CH4 being enriched in 13 C whereas the produced C2 H6 is depleted in 13 C. Based on our experimental results and the data analysis of 583 samples from shale gas reservoirs from different regions of the world, we suggest that CH4 will be cracked and combined into C2 H6 in high-over matured shales. When the contribution from methane-derived C2 H6 is sufficiently massive, the commonly found order of δ 13 Cmethane < δ 13 Cethane will change with increasing maturity (e.g. vitrinite reflectance, R o > ~2.0%) and a carbon isotope reversal will occur for the shale gas. The Barnett shale from Fort Worth Basin, USA, as the application representative shale gas reservoir, cover the evolution scope of the increase and subsequent decrease of δ 13 Cethane along with increase of δ 13 Cmethane and thus the beginning of the carbon isotope reversal. Highlights: CH4 cracking occurred in the pyrolysis experiments in shale gas maturity scope. In the pyrolysates residual CH4 enriched but the produced C2 H6 depleted in 13 C. Carbon isotope reversal in shale gas starts to occur at R o around 2.0%. Methane cracking may cause the carbon isotope reversal in shale gas.