The gas fields in Central Ordos Basin are in an unified and broad block.The gas in the fields is mainly from Upper Paleozoic coal measures.The principal evidences for this are:the δ

13

C values of the gas from Ordovician reservoirs are basically the same in wide areas and are well-matched or only have about 2‰ deviation with that of the gas from the Upper Paleozoic;the δ

13

C values of the gas-condensate light hydrocarbon monomeric molecule are consistent with that of condensate from Permo-Carboniferous;the δ

13

C val-ues are generally heavier than -25‰ and relatively enriched in heavier carbon isotope

13

C;the carbon isotopes of the n-alkane

pristane and phytane all suggest that they are the products of high-matured coal measure source rocks and have a close parental rela-tionship with Upper Paleozoic coal measures.Besides

the compact rocks of O

1

m

5

4

-O

1

m

5

6

are overlained by gas productive rocks

whereas the compact rocks of O

1

m

5

6

-O

1

m

5

7

gas reservoir are poorly gas-bearing;bitumen are rarely present in the Lower Paleozoic rocks.