Through systematic analysis of gas component

carbon isotope

thin section and inclusions

we found that the Permian and Triassic reef-bank reservoirs are dominated by natural gas from secondary cracking of crude oil.The Longtan Formation(Wujiaping Formation)is the major source rocks.The gas reservoirs experienced three stages

namely earlier oil accumulation in ancient lithologic reservoirs

ancient crude oil cracking and gas adjusting and re-accumulation.The hydrocarbon accumulation pattern is characterized by multiple hydrocarbon supply

accumulation near the source

lithology reservoirs

oil-gas conversion and later adjustment.And the present day gas reservoirs are mainly controlled by three factors.Firstly

the hydrocarbon kitchen of the Permian source rock controls the adequacy of oil-gas source.Secondly

the distribution and scale of reservoirs of reef-bank facies control the scale and location of hydrocarbon accumulation.Thirdly

later preservation conditions determine the finally accumulation of gas.By systematically summarizing the sedimentary characteristics and hydrocarbon accumulation conditions of the discovered reef-bank gas reservoirs(outcrops)

we believe that reservoirs of platform margin reef-bank facies

intra-platform shoal facies and the Lower Permian shoal facies are the favorable targets for further gas exploration.