Molecular and isotopic composition data of over hundred gas samples were combined with analytical data of source and reservoir rocks to deal with geochemical uncertainties of gases in the Feixianguan-Changxing Formations in Puguang

Yuanba and Tongnanba regions in northeastern Sichuan Basin.Discussion was focused on differentiation of cracking gases from oil and source rocks

causation of carbon isotope reversals in hydrocarbon gases and genetic relationship between CO

2

and H

2

S.The results show that oil cracking gases from vuggy reservoirs abundant in solid bitumen in Puguang and Yuanba regions are relatively enriched in propane and have low ln(C

2

/C

3

) value (

<

3.0) as a diagnostic signature.In contrast

the source rock cracking gases (sourced from kerogen and bitumen cracking) from fractured reservoirs in Tongnanba region

exhibit a co-varying ln(C

1

/C

2

) and ln(C

2

/C

3

)

and display higher values (

>

3.0) of ln(C

2

/C

3

)

distinguishing from typical oil cracking gas.Isotope reversals

resulted possibly from various factors

appear extensively in hydrocarbon gases from the Formations.For the paraffin gas from the Feixianguan Fm of Hebachang gas field in Tongnanba region

the isotope reversal can be attributed to a mixture of post-mature dry gas generated from the Upper Permian Longtan source rocks and gas with slightly higher wetness coming from the Silurian source.Two non-hydrocarbon gas composition modes

namely high H

2

S content-low CO

2

content and low H

2

S content-high CO

2

content

are recognized in the gases experienced TSR from the Feixianguan-Changxing reservoirs.The relative proportions of these two non-hydrocarbon gases may be associated with overall interaction system of reservoir fluids and rocks.CO

2

with somewhat negative carbon isotope is possibly resulted from hydrocarbon oxidizing in high H

2

S reservoirs

whereas

isotopically heavier CO

2

could be mainly formed by chemical degradation of carbonate when it presents in abnormally high amount.