A review of carbon isotopes and maturity determinations of paleozoic unconventional shale gases

Abstract

Isotope rollover and reversal have been widely reported from unconventional shale gas systems, and importantly this phenomenon is often associated with productive and highly mature gases. Although information about thermal maturity is critical to assess potential source rocks, determining maturity is often challenging for early Paleozoic source rocks due to a lack of true vitrinite. Carbon isotopes of gas compounds have been utilized to calculate maturity (vitrinite reflectance equivalent) of conventional gases based on linear correlations between carbon isotopes and maturity. As isotope rollover and reversal commonly occur in highly mature unconventional gases (e.g., Ro > 1.5%), the relationship between carbon isotopes and maturity is no longer linear, and utilizing such calculations would underestimate maturity for highly mature gases. In order to determine gas maturity, this review integrates isotopic and chemical data with traditional maturity methods from well-studied Paleozoic unconventional gas plays in the world. These gases show a common pattern of carbon isotopes of methane and ethane varying from normal, i.e., becoming 13C enriched with maturity, to rollover and even reversing the normal trend during gas maturation. Four levels of maturity are defined as vitrinite reflectance equivalent for these Paleozoic unconventional gases based on carbon isotopes: 0.5–1.5% (I), 1.5–2.0% (II), 2.0–3.5% (III), and 3.5–5% (IV). The maturity levels and the corresponding isotope trends suggest that as gas maturity increases, carbon isotopes of gas compounds may be controlled more by maturity and affected less by source rocks (e.g., different marine facies). The implication of this study is that the isotope patterns, along with maturity levels, can be utilized to help explorationists identify sweet spots. For example, it is expected that the best liquid potential is in level I, the best gas potential is between level II and level III, and a gas in level IV is beyond its peak productivity.