Abstract:

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 δ¹³C 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 δ¹³C values of the gas-condensate light hydrocarbon monomeric molecule are consistent with that of condensate from Permo-Carboniferous;the δ¹³C val-ues are generally heavier than -25‰ and relatively enriched in heavier carbon isotope ¹³C;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 O₁m₅⁴ -O₁m₅⁶ are overlained by gas productive rocks,whereas the compact rocks of O₁m₅⁶ -O₁m₅⁷ gas reservoir are poorly gas-bearing;bitumen are rarely present in the Lower Paleozoic rocks.