鄂尔多斯盆地西南部三叠系延长组7段页岩层系层理缝发育特征与主控因素

Development characteristics and main controlling factors of bedding⁃parallel lamellated fractures in shale in 7th member of Triassic Yanchang Formation,southwestern Ordos Basin

鄂尔多斯盆地西南部三叠系延长组7段(以下简称长7)页岩层系中层理缝普遍发育,对该区甜点优选、压裂施工和开发部署等具有重要意义。通过对盆地西南部庆城—华池地区的地表露头和岩心观察,结合其有机质含量、矿物组分和纹层特征等分析测试资料,明确了长7页岩层系不同岩性中层理缝的发育特征,并分析了该区层理缝发育的主控因素。研究结果表明,鄂尔多斯盆地西南部研究区长7页岩中层理缝的形态和分布主要受纹层控制,因纹层特征的不同而呈现连续平直、波状弯曲、分叉等特征。砂岩层理缝大多顺黑云母纹层分布,连续性好,开度大,普遍未被充填,而页岩层理缝在黑色页岩中发育程度最高,多沿有机质纹层构成的页理面分布,少数被方解石、有机质局部或完全充填,开度较砂岩更小但密度更大。层理缝还受有机质含量、岩性、矿物组分、纹层结构的控制。砂岩层理缝主要受黑云母含量及其形成的纹层控制,当砂岩分选好、黑云母含量多且呈层状分布时,层理缝发育程度高。随着纹层密度的增加,层理缝的发育程度随之增加,而页岩层理缝主要发育于有机质纹层和凝灰质纹层中,受有机质含量及矿物组分控制,层理缝密度随纹层密度先增加后降低,薄纹层较厚纹层中层理缝密度更高。

Bedding⁃parallel lamellated fractures are widely developed in shale in the 7th member of Triassic Yan⁃chang Formation(hereinafter referred to as Chang 7)in the southwestern Ordos Basin,which holds significant importance for sweet spot selection,fracturing operations,and development planning.In this paper,based on the surface outcrop and core observations in the Qingcheng to Huachi region of the southwestern basin,combined with analysis and testing of organic matter content,mineral composition and fabric characteristics,the develop⁃mental characteristics of bedding⁃parallel lamellated fractures in different lithologies in the Chang 7 shale were identified,and the main controlling factors of fracture development were analyzed.Results show that the morphology and distribution of the bedding⁃parallel lamellated fractures are mainly controlled by the laminates,exhibiting characteristics such as continuous flatness,wavy bending and branching due to the different characteristics of the laminae.Sandstone bedding⁃parallel lamellated fractures are mostly distributed along the biotite laminae,with good continuity and large aperture,and are generally unfilled.Shale bedding⁃parallel lamellated fractures are most developed in black shale,mostly distributed along the bedding laminates composed of organic matter layers,with a few partially or completely filled by calcite and organic matter.The aperture is smaller than that of sandstone,but the density is higher.Bedding⁃parallel lamellated fractures are also controlled by organic matter content,lithology,mineral composition,and laminate structure.The sandstone bedding⁃parallel lamellated fractures are mainly controlled by the content of biotite and the laminates formed by it.When sandstone sorting is good and biotite content is high with a layered distribution,the degree of fracture development is high.As the density of the laminates increases,the degree of development of bedding⁃parallel lamellated fractures also increases.Shale bedding⁃parallel lamellated fractures mainly develop in organic matter laminates and tuffaceous laminates,and are controlled by organic matter content and mineral components.Fracture density increases first and then decreases with the density of the layers.Fracture density in thin laminates is higher than that in thick laminates.