There are two types of gas pools in the Upper PermianLower Triassic in northwestern Sichuan Basin

namely porous and fractured gas pools.They show obvious differences in bitumen content and chemical compositions of gas.Geological

geochemical and basin modeling methods are used to analyze their gas sources

carrier systems and hydrocarbon charging history as well as their influences on hydrocarbon accumulation.The carrier system of poroustype gas pools is composed of faults and porous reservoirs and its formation is in good timing with peak hydrocarbon generation of source rocks

leading to the formation of paleooil/gas pools.During subsequent deep burial

the paleooil pools became gas pools due to pyrolysis.In the same time

H

2

S and CO

2

were generated because of thermochemical sulfate reduction between hydrocarbon gases and sulfates in pore fluids.In contrast

the carrier system of fracturedtype gas pools consists mainly of faults and fractures and their formation is later than the peak hydrocarbon generation but is good timing with the late stage of hydrocarbon generation.Therefore

only pyrolysis gas from kerogen accumulate in the reservoirs.Later deep burial of source rocks and tectonic evolution are primary causes for the differences between these two types of gas pools.