The Well Anye(AY)1 of north Guizhou

in the Sichuan Basin

is a shale gas well of major breakthrough in oil and gas exploration of the basin.The Lower Silurian Songkan Formation of this well has good resources of potential of oil and gas.To better understand the genesis of natural gas in the gas reservoir of the formation

we distinguished the Lower Silurian Songkan and Shiniulan Formation in the area

and made a series of experiments in geochemistry

gas composition

gas isotopes.The experimental results show that(1)under special lithologial combinations

the average content of TOC of the Songkan Formation in this area is 0.16%

the R

o

value varies from 2.75% to 2.92% with the average of 2.95%

and the kerogen type is mainly Type Ⅱ;meanwhile its gas accumulation is dominated by methane(over 96%)

plus a small amount of ethane

propane

and other hydrocarbon gases;(2)the natural gas drying coefficient of the Songkan Formation in the study area is much smaller than that of the Wufeng-Longmaxi Formation

and the CO

2

content of the Xintan Formation is significantly higher than that of the Songkan and Wufeng-Longmaxi Formation;(3)the δ

13

C

1

value of gas isotopes in the Songkan Formation ranges from -33.2‰ to -33.9‰

with the average of -33.5‰;δ

13

C

2

value is in the range between -36.5‰ and -37.0‰

and the average is -36.8‰

while the δD(CH

4

) value is within the range between -145.8‰ and -156.6‰

averaging at -150.0‰

and the kerogen carbon isotope value in the formation is from -32.9‰ to -32.6‰

with the average of -32.75‰.Combining CO

2

tracer gas migration theory

gas drying coefficient and gas source correlation characteristics

we conclude that the natural gas of the Songkan Formation strata from Well AY1 is mainly derived from the formation's mudstone shale gas after heat cracking.