Gas source of the Ordovician weathering crust gas reservoirs in Jingbian gas field is restudied by analyzing gas source of Well Yutan1 in Ordos Basin.An analysis of the geochemical features of source rock and a comparative analysis on carbon isotopes of natural gases are performed.The total organic carbon (TOC) of the Middle Ordovician Wulalike Formation is 0.30%-1.16% (averaging at 0.51%) and the thickness of dark mudstone is 52.59m.These two parameters indicate that this formation can be effective source rock.The carbon isotopes of methane in natural gas from Yutan-1 well are significantly lighter with δ

13

C

1

value in the range of -39.11‰--38.92‰

which is similar with the pyrolytic methane from carbonate source rocks.In contrast

the carbon isotopes of ethane are relatively heavier with δ

13

C

1

value in the range of -27.26‰ and -27.17‰

which seems to be the characteristics of coal-derived gas.However

the calculated gas maturity (R

o

=1.86%-1.89%) from source rock thermal simulation experiment is coincided with the measured gas maturity (R

o

=1.83%-1.92%) from source rock

both indicating high thermal maturity.In addition

the overlying thick Ordovician mudstone of the gas reservoir makes it difficult for the coal-derived gas from the Upper Paleozoic to migrate into the reservoir.All these evidences show that the Ordovician gas of Yutan-1 well should be oil-generated gas.Using the Ordovician gas of Yutan-1 well as the end member of oil-generated gas in Ordos basin

a comparative analysis is performed on the carbon isotopes of gas from the central-northern and southern Jingbian gas field.The following results are obtained.(1) Carbon isotope of methane should be considered as the major gas source index for the Ordovician gas

and δ

13

C

1

<

-38‰ is a index for identifying oil-generated gas in Jingbian weathering crust gas field;(2) The gas source in the entire Jingbian gas field is still dominated by mixed coal-derived gas of high maturity

but the proportion of oil-generated gases is higher in the southern Jingbian gas field than that in the central-northern Jingbian gas field;(3) Secondary pyrolysis of ethane may be the major factor causing heavier ethane carbon isotope of the oil-generated gas in the Ordovician.