Isotope reversals and universal stages and trends of gas maturation in sealed, self-contained petroleum systems

Isotope geochemistry is now a tool for shale gas exploration, largely due to the association of isotope reversals with mature, highly productive shale gas. Its utility, however, depends on an understanding of the isotope systematics for the particular region of interest, as well as for shale gas maturation in general. This paper reviews and re-examines isotope data from four published papers that include shale gas from the Barnett and Fayetteville Shales (Rodrigez and Philp, 2010; Zumberge et al., 2012), and gas from fractured reservoirs in the Appalachians (Burruss and Laughrey, 2010) and the Foothills of the Western Canada Sedimentary Basin (WCSB) (Tilley et al., 2011). New shale and tight sandstone gas data are also presented for the WCSB. Comparisons of these data show that the progression through three stages of gas maturation (pre-rollover zone, rollover zone and post-rollover zone) is universal in sealed, self-contained petroleum systems and that each zone has characteristic isotopic relationships and trends that are seen in all areas examined. Gases in the pre-rollover zone are isotopically normal (delta C-13(methane)