关键词: 低渗透储层, 剪切裂缝, 扩张裂缝, 拉张裂缝, 应力状态, 地质条件

Abstract: 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.

Key words: low-permeability reservoir, shear fracture, extension fracture, tension fracture, stress state, geological condition