Sapropelic gas could be derived from the cracking of both sapropelic kerogen and sapropelic crude oil. The differences between these two different kinds of genetic gases are: 1 the value of C

2

/C

3

of the gas derived fron kerogen remains constant while that of the gas derived from crude oil increases gradually; the difference of carbon isotope of (δ

13

C

2

-δ

13

C

3

) is opposite to the above memtioned values. It is considered that the gas in Jilak

Sangtamu and Jiefangqu regions in North Tarim Basin is derived from sapropelic kerogen while the gas in Lunnan

Yinmali regions is derived from crude oil.