In order to determine the source of natural gas and to reveal the factors controlling the accumulation and distribution of natural gas in the Ordovician reservoirs of the study area,we studied the composition of the natural gas through geochemical analysis and comprehensive geologic studies.The Ordovician gases are revealed to be entirely dry gas and their dryness indices range from 0.958 to 0.986.Geochemical analysis indicates that the Ordovician gases are mostly derived from the Carboniferous-Permian coal measure source rocks,whereas the marine Ordovician source is assumed to be a minor contribution.This is also supported by the Ordovician source rock analysis,which has revealed that the organic matter content of the mudstones and the carbonates is commonly not high.The average TOC of the mudstones in the Wulalike,Kelimoli,Lashizhong formations are only 0.54%,0.52% and 0.43%,respectively,and that of the carbonates in the above three formations are 0.35%,0.31% and 0.25%,respectively.Comprehensive studies suggest that the gas accumulation and distribution is principally controlled by structure,reservoirs,cap rocks,quality source rocks and migration pathways.Gases mainly enrich in places where the following 5 factors couple,namely structural highs of the slope,highly developed reservoirs with dissolution fractures and vugs,cap rocks of the Wulalike and Lashizhong formations,quality source rocks,and migration pathways for downward charging of gas from the Upper Paleozoic source rocks.