| Taking Longmaxi Formation in Sichuan Basin and its surrounding areas as an example, this paper analyzes the
plane distribution characteristics and reversal of alkane carbon isotopes, quantitatively studies the relationship between
alkane carbon isotope value and thermal evolution degree, burial depth and gas content, and also discusses the main
reasons for the difference of alkane carbon isotope reversal degree in different blocks. The results show that: (1) Longmaxi
shale gas has typical dry gas characteristics: the content of CH4 ranges from 95.32% to 99.59%, with an average of
98.44%; the content of C2H6 is relatively low, ranging from 0.09% to 0.74%, with an average of 0.52%; the content of
C3H8 is generally very low. (2) The carbon isotopes of alkanes gradually lighten from the edge to the center of the basin,
with δ13C1 value ranging from -36.9‰ to -26.7 ‰ with an average of -30.27 ‰, δ13C2 value ranging from -42.8‰ to -31‰
with an average of -34.9 ‰, and δ13C3 value ranging from -50.5‰ to -33.1‰ with an average of -37.28 ‰. (3)The
Longmaxi shale gas is characterized by complete reversal of alkane carbon isotopes (δ13C1＞δ13C2＞δ13C3), which is mainly
caused by the mixing of kerogen pyrolysis gas and residue hydrocarbon pyrolysis gas. (4) The isotope quantitative
fractionation model shows that the proportion of residue hydrocarbon pyrolysis gas in shale gas is more than 60% in most
blocks. It indicates that the difference of mixing ratio of two kinds gas is the main reason for the difference of alkane
carbon isotope reversal degree. With the increase of content of residue hydrocarbon pyrolysis gas, the δ13C2 value
decreases, the alkane carbon isotope reversal degree increases, and the gas content of shale increases gradually. |
