沉积盆地异常低压的成因

CAUSES OF SUBNORMAL PRESSURE IN THE SEDIMENTARY BASINS

张立宽 王震亮 于在平

ZHANG Li-kuan WANG Zhen-liang YU Zai-ping

西北大学, 地质学系, 陕西, 西安, 710069

Department of Geology, Northwest University, Xi'an, Shaanxi 710069, China

将异常低压的形成机制归纳为4大类:1)构造抬升和地层剥蚀、孔隙水化学作用引起的孔隙体积增大;2)封闭层压力的释放、物性差异引起的非均匀流、渗析作用和轻烃扩散作用等流体排出造成的压力降低;3)饱和天然气藏的深埋;4)承压面低于地表。分析了每一类成因机制对形成异常低压的贡献大小。其中,构造抬升引起一定程度的孔隙回弹和温度降低,从而导致地层压力下降,是构造抬升强烈的沉积盆地中形成异常低压的主要原因。而在抬升幅度较小的地区,低压的形成则常与轻烃扩散和饱和天然气藏的深埋有关。孔隙水的化学作用和渗析作用受自身因素的制约,对异常低压贡献不大。除物性差异引起的非均匀流外,其它的低压形成机制都需要以好的封闭条件为前提,否则地层压力将衰减至常压。在中国西部盆地,因地形起伏较大,承压面低于地表,使得地层压力偏低,但并非真正意义上的低压,只是由于表达方式(起算深度是否等于地表)的原因而造成。

The forming mechanisms of subnormal pressure can be divided into 4 types: 1)increase in pore volume caused by tectonic uplift and formation erosion, or chemical effects of pore water; 2)fluid flowing out due to the release of overpressure in seal formation, unhomogeneous flow, osmosis across semipermeable shale membranes and diffusion of light hydrocarbon; 3)deep burial of saturated gas reservoir; 4)piezometric surface lower than ground surface. In some sedimentary basins in which large-scale tectonic uplift has taken place, the main cause of low pressure may be the removal of overburden, which results in dilation of pore volume with the consequent reduction in stress and temperature. Subnormal pressure in the small-scale uplift area is mostly related to the diffusion of light hydrocarbon and the burial of saturated gas reservoir. The release of overpressure seems to be a probable mechanism; however, it is a question how low pressure can be preserved. The chemical effect and osmosis effect of pore water are self-limited, which may not be the major factors in subnormal pressure system. Aside from unhomogeneous flow, other mechanisms require good seal, otherwise pressure will be equalized to normal hydrostatic. The low pressure is not a true meaning subnormal pressure, because that the piezometric surface is lower than the ground surface, but people give a wrong assumption in the course of showing the relative size of pressure.

国家重点基础研究发展规划(天然气"973")项目(2001CB209103).

https://doi.org/10.11781/sysydz200405422