The origin of gaseous hydrocarbon is discussed on the basis of geochemical featuresof natural gas in Sanshui Basin

Guangdong Province.The Cretaceous-Eogene Systems inthe basin have a thickness greater than 7600m.The Buxin Formation(E

1-2

b)contains morethan 500m thick source rocks in which kerogen is mainly types Ⅱ and Ⅲ.Vitrinite reflec-tivity(R

o

)is between 1.2% and 1.9%.Eleven samples of natural gas have been analyzedfor gas components and isotopic composition of carbon and hydrogen.The results showthat geochemical characteristics of ignitable gases are:1.hydrocarbon-rich natural gascontains N

2

He

C

O

2

but not H

2

s;2.the value of dry-wet indices (C

1

/ (C

2

+C

3

))has the range of 2.1-196 which can be easily separated into two groups:

>

18 and

<

8;3. iC

4

/nC

4

ratios Change between 0.42 and 0.87;4.the value of δ

13

C

1

and δ

13

C

2

(PDB) ranges from -61‰ to -44‰ and -33‰ to -30‰ respectively;5.δDC

1

value(SMOW) has the range of -237‰ to -174‰ which reflects the genetic process ofmethane and salinity of sedimentary water.Dry gas found in the highest part of structureis distributed in the area in which oil field water is of NaHCO

3

type with a lower intensityof mineralization (25-50 g/l).The carbon and hydrogen isotope composition(δ

13

C=-60‰ and δD=-236‰--174‰) of methane in dry gas are identical with those of bio-genetical methan.Wet gas (C

1

/ (C

2

+C

3

)

<

8)in company with crude oil is rich in

13

C(δ

13

C

1

=-48‰--44‰

δ

13

C

2

=-33‰--30‰).The difference between δ

13

C

2

and δ

13

C

1

changes from 14.7‰ to 15.2‰.The isotopic compositions of wet gas conform tothose of oil-type gas from Zhongyuan and Sichuan Oilfields.Wet gas is found in the areathat has Na

2

SO

4

type of oilfield water with a higher intensity of mineralization (50—100g/1).All of the geochemical evidences confirm that dry gas in Sanshui Basin is generatedby microbe altering organic matter (including oil and gas) after the period of oil genera-tion

wet gas originated from E

1-2

b source rocks.