Geological Openness on the Chemical and Carbon Isotopic Signatures of Shale Gas: Implications From the Stepwise Pyrolysis of Bitumen

Abstract

Earlier studies have revealed that part of the shale gas is derived from the cracking of residual oils, and one evidence of this process is the presence of abundant pyrobitumen in mature and overmature gas shales. Many experiments have been carried out to investigate the chemical and carbon isotopic signatures of oil cracking gas in a closed or confined system, but most of the results failed to fully explain the characteristics of shale gas that is usually much drier and characterized with relatively heavier methane carbon isotopes than the pyrolysate gas. In fact, the shale system is probably not a completely closed or confined system when the shale gas is being generated, and therefore the shale gas could be released periodically with the natural fracturing of shales induced by gas generation. This overpressure fracturing has been recorded by the fluid inclusions of gas in some gas shales. From this point of view, the shale gas today may be considered as the residual part of a more abundant gas generated in shales. Therefore, it is quite necessary to carry out experiments that can match the geological conditions as much as possible. In this study, we performed a series of closed and stepwise experiments on a low mature bitumen (Rb=0.8%) using sealed gold tube method to investigate how the experimental openness affect the chemical composition of pyrolysates, the methane yield and its carbon isotope. The closed experiment shows that most of the ethane and propane is generated before the maturity level of EasyRo 2.3% and 1.7%, respectively. In the stepwise experiments, the residual bitumen samples that have been artificially depleted of ethane and propane potentials, were pyrolyzed again to imitate the regeneration of gas after shale fracturing and subsequent gas loss, and to investigate the late methane potential and its carbon isotopic composition without the influence of ethane and propane. The results show that there is still a maximum methane potential of about 70–102mL/g bitumen, and the methane carbon isotopic values are 3–5‰ heavier than those in the closed system and are much closer to the carbon isotopes of methane in shale gases. These results imply that (1) the cracking of wet gases in a closed system will produce isotopically lighter methane than the cracking of residual bitumen itself and (2) the shale gas today is only the residual part of gas generated in shales after its migration into conventional reservoirs.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016