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.