Abstract: The regular distribution of oil, gas and water due to gravitational differentiation tends to the fact that a gas reservoirforms in structural trap at higher part, while an oil reservoir in structural trap at lower part. However, there exist gas reservoir intrap at lower part and oil reservoir in trap at higher part due to differential hydrocarbon accumulation. Many typical cases haveindicated this abroad.In long-axis anticline of Honggang field in Songliao basin, it is indicated from formation tests that Hong-l31 trap belongs to a gas reservoir which lies in lower sub-layer G13 of Gaotaizi sand layer-II, Hong-7 trap is an oil-gas reser-voir at higher part, and in Hong-l6 trap merely contains water which lies at the top. Based on detailed analyses of the structuraland reservoir characteristics, it is shown that the basic conditions for differential hydrocarbon accumulation could appear in layerGl3 and can be used for well interpreting its oil-gas distribution. So the sub-layer G13 is found to be as a first and typical caseof the differential hydrocarbon accumulation in China.

Key words: Songliao basin, Honggang oilfield, structural feature, grav itational differentiation, hydrocarbon accumulation, distribution