关键词: 低压成因, 天然气, 流体动力场, 扶杨油层, 松辽盆地

Abstract: In Songliao Basin,there are three subnormal pressure areas developed in Fu-Yang oil-bearing formation in the northeasternpart and Shiwu fault-depression on the southeastern edge,with the pressure coefficients of 0.93~1.00,0.65~0.96,and 0.96~0.55 respectively.They are main natural gas accumulation areas.The main reason,which has induced the subnormal pressure,isthat the decrease of energy due to natural gas leakage was larger than the increase of energy due to natural gas and undergroundwater recharge.In Fu-Yang oil-bearing formation at northeastern part of Songliao Basin,the reservoir thickness,porosity and permeabilityall increased in the direction of natural gas migration.The difference between the decrease of energy due to natural gas leakageand the increase of energy due to natural gas and underground water recharge increased in the direction of gas migration,resulting in decrease of the pressure coefficients.In Shiwu fault-depression of Songliao Basin,the fluid pressure profile may be divided into 3 zones vertically.They are normal pressure zone (from 0 to 1250m),subnormal pressure zone (deeper than 1500m),and transitional zone (between the othertwo),which corresponds to free replacing zone,obstructive replacing zone and lagged zone respectively in hydrodynamics.Gasloss is very obvious in the depression,which could have induced ground water and meteoric water recharge.In free replacingzone,the pressure keeps normal because recharge and discharge of pore fluids is in equilibrium.In obstructive replacing zone,there may be a formation with low permeability,through which gas diffusion is slow,and the recharge of subsurface water is difficult.The amount of gas loss in the obstructive replacing and lagged zones is greater than that of subsurface water and gasrecharge,which may cause the fluid pressure dicreasing and subnormal pressure developing at last.The recharge of subsurfacewater becomes smaller and stops quickly,while gas diffusion becomes larger as depth increasing.This resultes in the increase ofpressure coefficients along gas migration.

Key words: origin of subnormal pressure, gas, fluid dynamics, Fu-Yang oil-bearing formation, Songliao Basin