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

摘要： 在松辽盆地北部东区的扶杨油层和在东南缘的十屋断陷中,发育了三个地层流体低压区,压力系数分别为0.93～1.0,0.65～0.96和0.96～0.55,都是松辽盆地主要的天然气聚集区。低压的成因机理是由于天然气的漏失减小的能量大于天然气和地下水运移补给增加的能量。在盆地北部东区的扶杨油层中,由于在天然气流动方向上储层厚度、孔渗都明显增加,流体补给增加的能量与流体散失减小的能量之间的差值在天然气的运移方向上增加,导致在天然气运移方向上压力系数减小。十屋断陷的地层压力剖面可以划分为明显的三个带,它们分别对应于三个水文地质分带,即自由交替带,交替阻滞带和交替停滞带。在自由交替带地下水供排平衡,保持正常的地层压力。在交替阻滞带和交替停滞带,由于天然气漏失损失的能量大于天然气、地下水下渗补给增加的能量,形成低压,且往下地下水的补给量迅速减小、消失,而天然气的漏失量随着埋深的增加而增加,从而使得天然气运移方向上压力系数增加。

关键词: 低压成因, 天然气, 流体动力场, 扶杨油层, 松辽盆地