Based on the analyses of geochemical data like gas composition and carbon and hydrogen isotopes

the study is made on the geochemical characteristics

origin and source of natural gas in the Upper Triassic Xujiahe Formation in Malubei area

northeastern Sichuan Basin.The results indicate that the natural gas in the Xujiahe Formation in Malubei area is mainly composed of methane

with the content ranging from 92.60% to 99.04%

averaging out at 97.59%

and its gas dry coefficient is generally higher than 0.990 0

with the average value of 0.992 2

indicating a higher degree of thermal evolution.Compared with the Xujiahe natural gas in adjacent areas

the values of gas δ

13

C

1

in Malubei area are bigger (ranging from -33.70‰ to -28.60‰

with an average of -30.88‰)

but the values of δ

13

C

2

are smaller (ranging from -30.88‰ to -28.90‰

with an average of -33.11‰)

and the carbon isotopes of methane and ethane in the gas of the study area are mainly in reversed distribution:the carbon isotope of methane is relatively heavier while the carbon isotope of ethane is lighter.The identification of gas origin and analysis of gas-source correlation indicate that the natural gas in the Xujiahe Formation in Malubei area is the mixed thermogenic gas of coal-type gas and oil-type gas generated by kerogen Type Ⅲ and Ⅱ

and mainly sourced from the coal-measure source rocks in the Upper Triassic Xujiahe Formation and the marine source rocks in the Upper Permian Wujiaping Formation.And the carbon isotopic reversal of methane and ethane for the Xujiahe gas just results from the mixing of the coal-type gas and oil-type gas.The accumulation and high productivity of the natural gas in the Xujiahe Formation in Malubei area is closely related to hydrocarbon generation by both marine and terrestrial source rocks and the connectivity of several quality marine and terrestrial source rocks to the Xujiahe reservoir by means of deep and large faults

which have played a significant role in natural gas accumulation and later adjustment in the Xujiahe Formation in Malubei area.