Discussion on the carbon isotopic reversal of alkane gases from the Huanglong Formation in the Sichuan Basin,China

The Huanglong Formation has become the exploration and development focus of the Sichuan Basin since the late 1980s,and it is also the main reservoir and producing formation in the Sichuan Basin. In 2008,it produced 68.1×108m3 natural gases,accounting for more than 40% of the total output of the basin in that year. The geochemical characteristics of the natural gas from the Huanglong Formation are as follows:(1) Helium isotope is of the typical crustal type,and most of the R/Ra ratios are 0.01; (2) The content of hydrogen sulfide is very low,with the average of 0.27%,so the natural gas reserves in the Huanglong Formation are of low-carbon energy with high quality; (3) The gas is very dry. The average content of methane is 96.71% and the content of C2+ is very low,with the average of only 0.71%; (4) All the carbon isotopes of methane and ethane are in a reverse succession,i.e.δ13C1δ13C2δ13C3. The carbon isotopic reversal of natural gases from the Huanglong Formation was attributed to the mixing of the light associated gas generated by the Silurian source rock in the early stage and the heavy-oil-cracked gas formed in the late stage.