Shale Gas Generation And Variation In Stable Carbon And Hydrogen Isotope Compositions

The isotope rollover or reversal occurrced in wells which have high production rate made shale gas generation a hot spot scientists pursuring. Pyrolysis technology has already been widly used and been an effective tool in researching hydrocarbon generation.Of course, deeply understanding shale gas generation aslo depend on simulation experiment.A pyrolysis experiment was conducted with low maturity kerogen isolated from typeⅡorganic-rich shale in a vacuum MSSV(Micro scale sealed vessel) System to simulate gas hydrocarbon generation at three heating programs 2℃/h,6℃/h,20℃/h,respectively. The stable carbon and hydrogen isotope value of generated gas hydrocarbon were measured to discuss their characteristics with increasing temperature.The results shows that, isotopevalue of methane becomes more negative with increasing temperature until reaching the lightest point, then it changes more positive while isotope value of ethane and propane always shows a positive trend with elevated temperatures. According to the statistic data of four shale gas plays from published papers, gas isotope characteristics with its wetness decreasing were discussed, aslo the possibilities result in isotope reversal or rollover. The main factors impact on isotope of shale gas may include : type of organic matters,maturity of the shale and micro-migration of gas in shale systems. Although it is still unclear the reason lead to isotope reversal or rollover,we can conclude that a isotope reversal or rollover phenomenon wouldn’t happen in a single thermogenic gas reservoir according to our prolysis experiment results and other experimental data from published papers, thus it must be involved with some other geochemistry process.To know well the contribution of primary gas and secondnary gas in shale gas, hydrocarbon gas generated from kerogen in the short tube and in long tube before their maturity up to 1.2(Easy Ro)were treated as primary gas, in which stage gas carbn isotope from two kind tubes have little variations, then we predicted gas isotope of higher maturites. Gases cracked from n C22 are adjusted to be used as secondary gas. Finally, with the two end members we calculated the contribution of primary gas and secondary gas in the Appalachain shale gas at different maturites.