低幅度构造天然气成藏的闭合度下限探讨——以柴达木盆地三湖地区为例

在一定条件下,构造圈闭只有达到一定的闭合高度,气水才能有效地分离,当达到一定含气饱和度和储量规模后便形成天然气藏,因而,必然有一个成藏闭合度的下限值。该值受储层物性、盖层条件、储盖配置情况、圈闭形态和规模、水动力强度和方向等多种因素的影响,故因地而异。柴达木盆地东部三湖地区的研究表明:1)根据储层毛细管压力曲线计算天然气成藏的最小气柱高度可间接得到最小成藏幅度,不同的储层对应不同的闭合度下限,从1m至上百米不等;2)根据盖层的封闭能力计算盖层封闭的最大气柱高度可从几十米到上百米,每种类型的储层成藏的闭合度下限始终小于盖层所能封闭的最大气柱高度,因此盖层不是制约闭合度下限的因素;3)在特有的水动力条件下,如果不考虑其它因素,成藏的闭合度下限与圈闭短轴半径的关系可以涩北气田为例,与其圈闭面积相当的构造圈闭,闭合度必须大于25m才能构成气藏。据此认为储层条件和圈闭规模是该区天然气成藏下限的主要制约因素。确定低幅度构造圈闭天然气成藏的闭合度下限对今后寻找低幅度构造圈闭具有重要意义。

Discussion on minimum closure for low-amplitude structural nature gas pool: A case study from Sanhu area in the Qaidam Basin

Gas could be separated from underground water when the closure was enough high,and the nature gas pool could be formed when certain gas saturation and reserves attained.Therefore,the minimum closure must have an absolutely necessary condition for nature gas pool.The minimum closure was influenced by the reservoir property,the cap rock’s property,the allocation of reservoir and cap rock,the shape and scale of the trap,the hydrodynamic intensity and direction,and so on.Taking Sanhu area in the eastern Qaidam Basin as an example,this paper revealed that the nature gas pool could be formed when reservoir closure excessed 1 meter high in the first-rate sandstone,calculated from the capillary pressure of reservoir.More～over,according to the sealing capacity of cap-rock,gas column height could be sealed by the cap-rock from tens of meters to hundred meters.Therefore,the cap rock might not be a factor which controlled the minimum closure for nature gas pool.In a condition of the direction and angle of the gas-water interface being known,the minimum closure was controlled by the trap’s brachy-axis radius under particular hydrodynamic environment in the eastern Qaidam Basin.Calculating minimum value of closure in different trap areas,it was found that the structural trap,with a square similar with the Sebei Gasfield,could be formed a natural gas pool when its closure was higher than 25 meters.Base on the example,it was thought that the reservoir property and the scale of the trap were main factors controlled the minimum closure for nature gas pool.The results might have important practical significance for searching low amplitude structural trap in the eastern Qaidam Basin.