Abstract: Studies of the composition, light hydrocarbons and carbon isotopes of natural gases from the Xihu Sag showed that the gases were a mix of coal-type (accounting for the major proportion) and oil-type gases havinghigh maturities.They were mainly derived from type II source rocks in the PinghuFormation and underlying formations during the mature and over-mature stages.Crude oils from the Xihu Sag were classified into light oils and condensates according to studies of their physical properties, saturated hydrocarbons, carbon isotopes and light hydrocarbons.These two types of crude oil showed similar physical properties suggestive of acoal origin, and were generated in fresh water lakeswith weakly anoxic conditions.Some of the crude oils were mature (R_o<1.0%)and some are high-to over mature (R_o=1.3%~2.0%).