Determination of maturity for source rocks in Biyang Sag of Nanxiang Basin and its significance

19 source rock samples of lacustrine facies have been collected from the Hetaoyuan Formation in 15 drilling wells in the Biyang Sag of the Nanxiang Basin, and their maturities have been determined by means of fluorescence alteration of multiple macerals (FAMM) technique. The vitrinite reflectance of source rocks from the Hetaoyuan Formation is obviously suppressed, and the suppression corrected value is from 0.08% to 0.39%. The level of vitrinite reflectance suppression is relatively higher when source rock quality is better. The maturity of 19 samples has entered oil window (EqVR from 0.58% to 1.29%) instead of 0.38%-1.05% from vitrinite reflectance. According to the maturity of source rocks at different depth in 4 wells, paleogeothermal gradient is 36-45℃/km, and the depth of threshold for oil generation is 1 800-1 400 m. The correct determination of maturity for the source rocks from the Hetaoyuan Formation provides the basis to evaluate the potential of hydrocarbon reasonably.