Light hydrocarbon has abundant geochemical information and great geological significance.Using a series of light hydrocarbon parameters

the natural gas from Shenmu gas field in the Ordos Basin and piedmont zone of the southwest Tarim Basin were analyzed.The results show that the Shenmu and Akemomu gas field are mainly coal-derived gas.However

the former is mixed by partial oil-type gas.The natural gas from Kekeya-Kedong gas field is mainly mixed gas.For the first time

the alkane index parameter is put forward.When the index is above 2

it means the gas is oil-derived gas or mixed gas.When it is below 2

it means the gas is oil-derived gas.The natural gas from Shenmu gas field originated from the same source rocks according to K

1

and K

2

parameter.However

the gas from piedmont zone of the southwest Ta-rim Basin originated from different source rocks.According to Mango parameter intersection chart

the source rocks of Shenmu and Akemomu gas field were mainly from terrestrial higher plant origins.However

the source rocks of Kekeya-Kedong gas field are mixed.Based on isoheptane and n-heptane parameter

the maturity of Shenmu

Akemomu and Kekeya-Kedong gas field is low-normal stage

mature gas and high maturity gas stage

respectively.