Conventional Production of Unconventional Shale Gases in the Sichuan Basin: Preliminary Evidence from Isotope “Rollover” of Produced Gases

The production and geochemical data of many conventionally produced gas fields in the Sichuan Basin, SW China show chemical and stable carbon isotope compositions that are incompatible with those predicted using the methods for conventional gas reservoirs. Significant reversal has been observed in the δ13C values of coproduced methane and ethane (i.e. δ13Cmethane > δ13Cethane values), together with an ethane isotope rollover (i.e. gases with the lowest wetness tend to display the lowest δ13Cethane values). Quantitative comparison of carbon isotope composition of methane, ethane and propane in the gases from the Wolonghe fields with those in North American shale gas and other high maturity gas reservoirs, along with isotope kinetic fractionation, indicate that these gases produced from the conventional gas reservoirs may contain significant components of shale gases derived from in-situ thermal cracking of high-mature organic matter and the consequent desorption process is a highly viable mechanism for the observed isotope reversal. This interpretation is also consistent with the general isotope trends observed from different stratigraphic successions of the producing field.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California