鄂尔多斯盆地南部延长组中段剪裂缝方向分析

Shear fracture direction and mechanical characteristics of the middle Yanchang Formation, southern Ordos Basin, China

鄂尔多斯盆地南部延长组中段长6~长8段发育6组高角度天然构造剪性缝,组成3组共轭裂缝系统,分别为走向60°和120°的共轭剪裂缝,最大主应力为E-W向;走向150°和75°的共轭剪裂缝,最大主应力为NWW-SEE向;走向30°和90°的共轭剪裂缝,最大主应力为NEE-SWW向.由于延长组地层的各向异性,走向约75°的裂缝为延长组中段的优势裂缝.地震各向异性计算和成像测井识别得到的裂缝走向玫瑰图均显示：和长6与长8相比,长7中的优势裂缝走向顺时针偏转了约5°.而各层的平面裂缝分布图也显示各层的裂缝密度分布发生了改变.根据常规测井曲线计算划分岩性,分别统计成像测井的图像识别结果中泥页岩和砂岩中的裂缝.走向玫瑰图结果显示,同一构造背景下,砂岩中的优势裂缝走向和泥岩相比较,逆时针方向旋转了约10°,和野外露头的节理测量结果一致.由常规测曲线的值带入公式和前人总结的经验数学关系中,计算得到泥岩的内摩擦角较砂岩的大18.6°,因此泥岩的剪裂角较砂岩小约9.2°.根据库伦破裂准则,一组共轭剪裂缝的锐角平分线方向与最大挤压应力方向一致,剪裂缝和最大挤压应力的夹角为剪裂角.而延长组地层在鄂尔多斯盆地南部近水平,成像测井中识别的剪性天然构造张开缝均为高角度近垂直,由于优势裂缝走向约75°,对应的最大主应力方向为NWW-SEE向,因此砂岩中的NEE至E-W向的裂缝较泥岩中的向逆时针方向发生偏转.根据单井中自然伽马值（GR）的分布具有正态分布的特点,进行了各层砂泥比计算：长6砂泥比0.72;长7砂泥比0.24;长8砂泥比1.04.将长7作为纯泥岩段,长8作为砂泥互层段,计算得到的剪破裂角和成像识别出的裂缝走向差异一致.

Field observations, fractures identified from micro-resistivity logging images from six wells, and seismic anisotropy analysis all show that six sets of vertical shear fractures are developed in the middle Yanchang Formation in the southern Ordos Basin, comprising three orthogonal fracture systems. Of these, System I fracture sets predominate, trending approximately 75° and 150°, the result of NWW-SEE directed maximum compressive stress. System II fracture sets trend approximately 60° and 120°, the result of E-W directed maximum compressive stress, while System III fracture sets trend approximately 30° and 90°, the result of NEE-SWW directed maximum compressive tress. Fractures with a strike of 75° are most common in the middle Yanchang Formation because of rock heterogeneity, while three reservoir sets are present, comprising fluvial medium-to-fine grained blocky sandstones and thin interbeds of sandstone and shale from lacustrine-delta facies. Based on a combination of data from natural gamma ray and interval transit time curves from six vertical wells, we classify the stratigraphic lithology of the middle Yanchang Formation and extract shear fractures recognized from logging images within the sandstones and shales. Fractures plotted in striking rose diagrams demonstrate the dominant strike of fractures in sandstone deflects approximately 10° counterclockwise compared to the orientation in shale. Field observation also provides the evidence of 10° deflection of dominant fracture trending in sandstone and shale. Conventional logging values are substituted with empirical correlations in order to determine and quantify the mechanical properties of the sandstones and shales in the middle Yanchang Formation of the southern Ordos Basin. As the result puts, shale has averaged Poisson＇s ratio of 0.268, compression strength of 3.18 MPa and internal friction angle of 34.6°. Sandstone has averaged Poisson＇s ratio of 0.279, compression strength of 19.76 MPa and internal friction angle of 16.3°. The internal friction angle in sand is 18.3° more than that in shale, thus the shear angle in sand is 9.2° more than in shale. The classic Coulomb-Mohr failure criterion predicts the development of two conjugate shear planes that are inclined at shear angle to the axis of maximum compressive stress. Striking rose diagrams of natural shear cracks within each set are plotted respectively from seismic anisotropy analysis and image logging interpretation, showing that Chang 6 to Chang 8 sets develops dominant fracture trending approximately 75°, which is one set of conjugated shear fractural system of 150° and 75° in responding to NWW-SEE maximal principal stress. This leads to the counterclockwise deflection of the shear fracture in sand. Compared to Chang 6 and Chang 8, the dominant striking of fracture deflects clockwise of nearly 5 degree. Considering the same dominant fracture striking in Chang 6 and Chang 8, tectonic setting isn＇t responsible for the trending deflection. On the basis of normal distribution of GR value of single well from Chang 6 to Chang 8 set, we calculated the sand to shale ratio in each reservoir set： 0.72 within Chang 6, 0.24 within Chang 7, 1.04 within Chang 8. According to the sand to shale ratio from natural gamma ray logging calculation, take Chang 7 group as pure shale section and Chang 8 group as interbedded sandstone and shale, the shear angle from calculation provided approximately 6° difference consistent with the fractural strike difference from logging images and seismic data.