Abstract: According to the studies of well HY14-1-1,the maceral composition of coal-bearing source rocks in the Xihu Sag was divided into 2 types including type Ⅰ mainly of vitrinite and type Ⅱ mainly of exinite and sapropelinite.The simulations of hydrocarbon generation of the above-mentioned types indicated that the type Ⅰ mainly generated liquid hydrocarbon while the type Ⅱ mainly generated gaseous hydrocarbon.The vertical and horizontal differences between the 2 types of coal-bearing source rocks led to the present distribution of petroleum in the Xihu Sag,that is,oil in the upper formations and gas in the lower formations.