Strong gas shows are discovered in drilling the Lower Cambrian Shuijingtuo Formation shale in Yichang area

Middle Yangtze

and high commercial gas flow was tested following hydraulic fracturing of the Shuijingtuo shale in Well Yiye1 in the study area

which is a prospective area for shale gas exploration in the Lower Cambrian shale outside the Sichuan Basin.Nine shale gas samples were collected after hydraulic fracturing to analyze the geochemistry and origin of shale gas in the Shuijingtuo Formation

following the study of shale gas composition

as well as carbon

hydrogen

and He isotopic compositions.The results show that methane is the main constituent of the shale gas and varies in concentration from 87.17% to 92.75%

the content of ethane ranges from 0.83% to 0.94%

and the propane is in trace amount; the drying coefficient (C

l

/C

l-5

) is about 0.99

indicating typical dry gas.The content of nitrogen in non-hydrocarbon gases is slightly higher

with an average of 7.73%

the carbon dioxide content is less than 1%

and there is no H

2

S.The δ

13

C methane in the Cambrian shale gas varies between -33.8‰ and -33.1‰ in content

the δ

13

C ethane value ranges from -39.2‰ to -36.0‰

δ

13

C propane value ranges from -39.4‰ to -38.5‰

and the δ

13

C value for CO

2

ranges from -16.8‰ to -14.6‰.The δD

CH4

methane in the shale gas ranges from -133.8‰ to -128.5‰ in content

while the δD

C2H6

ethane ranges from -168.0‰ to -146.1‰.The stable carbon isotopic distribution of the shale gas shows a totally reversed distribution pattern with δ

13

C

1

>

δ

13

C

2

>

δ

13

C

3

and δD

CH4

>

δD

C2H6

.The R/R

a

ratio of helium in the Shuijingtuo shale gas ranges from 0.04 to 0.08

indicating the crustal origin of the helium.Considering the shale gas compositions and their ratios

as well as the reversed distribution of carbon and hydrogen isotopes at the high-over mature stage in thermal evolution

we believed that the shale gas of the Lower Cambrian Shuijingtuo Formation in Yichang area belongs to the oil-derived gas

featuring an origin of secondary cracking

which is similar to the Qiongzhusi shale gas in Sichuan Basin.