低渗透储层构造裂缝的成因类型及其形成地质条件

Origin type of tectonic fractures and geological conditions in low-permeability reservoirs

低渗透储层中构造成因的裂缝包括剪切裂缝、扩张裂缝和拉张裂缝3种类型。剪切裂缝是在压应力作用下形成的,理论上以两组共轭剪切裂缝的形式出现,但岩层强烈的非均质性通常可以抑制其中一组发育,而只留下另一组。剪切裂缝以高角度缝为主,具有透入性分布特征。此外,还常见有两类低角度剪切裂缝:一类为在泥质岩中发育的滑脱裂缝;另一类为在逆冲构造带中发育的近水平剪切裂缝。扩张裂缝和拉张裂缝都是垂直于最小主应力方向分布的,规模小,延伸短。但是扩张裂缝是在压应力作用下形成的,因而常与剪切裂缝同时出现;而拉张裂缝的形成要求至少最小主应力是张应力,通常呈透镜状局部发育,并被沥青、方解石等脉体充填。除了岩层弯曲派生的拉张应力形成的纵张裂缝外,沉积盆地中拉张裂缝的形成通常有异常高压流体的参与,异常高压流体可以使逆冲构造带中的挤压应力变为拉张应力,从而在挤压应力环境中形成拉张裂缝。拉张裂缝是沉积盆地古异常高压流体存在的重要指示标志。

The tectonic fractures in the low-permeability reservoir include shear fracture, extension fracture and tension fracture. The shear fractures were generated under the action of shear stress and appeared in two sets of conjugate fractures in theory. But the strong anisotropy of the strata would restrain one set of the fractures and develop the other. The shear fractures are mainly high dip-angle ones and equably distribute in the low-permeability reservoir. There are another two types of low dip-angle shear fractures, including the detachment fractures in mudstone, and the level shear fractures in medium-granular sandstone and grit stone in the thrust zone. The extension fractures and tension fractures are perpendicular to the minimum principal stress and have small scales and length. But the formation of tension fractures needs the minimum principal stress being tensional at least, and the extension fractures were formed under the compression stress. So, the extension fractures were often distributed together with shear fractures, and the tension fractures were developed partly with lens veins filled by asphaltum or calcite. Besides the longitudinal fractures formed under tension stress due to stratum being bended, the formation of tension fractures in the sedimentary basin is often related with abnormal high-pressure fluid. The compression stress can be changed into tension stress by the abnormal high-pressure fluid, and the tension fractures can be formed in the thrust zone. The existence of tension fractures is the main symbol for occurrence of the ancient abnormal high-pressure fluid in the sedimentary basin.