Pattern of Vertical Fracture Development Controlled by Mechanical Stratigraphy in Carbonate Layer in Xikeer Outcrop Area of Xinjiang Autonomous Region
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摘要: 为研究裂缝力学层控发育模式, 进而实现对裂缝型油气藏碳酸盐岩层系裂缝发育特征有效预测, 通过对新疆西克尔露头区地层产状平缓奥陶系碳酸盐岩层系3个野外露头剖面(21号剖面(39°50'17.34″N, 77°28'36.12″E)、32号剖面(39°50'18.18″N, 77°28'25.62″E)以及43号剖面(39°50'10.88″N, 77°28'26.1″E))垂向裂缝生长特征的统计分析, 结合沉积旋回的划分结果, 揭示了不同剖面力学界面的分布特征以及力学单元控制下各剖面的裂缝密度.结果表明, 21号剖面的平均裂缝密度为7.1条/m, 远大于32号剖面的2.2条/m以及43号剖面的2.9条/m.分别从岩性特征、力学单元厚度以及应力状态等方面探讨其对于3条剖面裂缝密度差异分布的影响与控制.碳酸盐岩层系裂缝的密度随灰岩中泥晶含量的增加而递增, 颗粒灰岩中裂缝的密度明显低于泥晶灰岩; 受压力影作用的影响, 力学单元的厚度与裂缝的密度呈负相关, 力学单元越厚则裂缝密度越低.43号剖面与32号剖面的裂缝饱和度均大于0.8, 接近裂缝饱和状态; 而裂缝平均密度最大的21号剖面其裂缝饱和度仅为0.6, 为不饱和状态, 裂缝的平均密度与其饱和度值呈负相关.Abstract: It is supposed that we can effectively predict the fracture density in carbonate layer by studying the pattern of vertical fracture development controlled by mechanical stratigraphy. The latitude of stratum is smooth in Ordovician carbonate in Xikeer outcrop area of Xinjiang autonomous region. Opening-mode fractures (joints) that are perpendicular to bedding are typically controlled by mechanical stratigraphy. After computing amounts and analyzing characteristics of vertical fractures in combination with the results of dividing sedimentary cycle in stratum section of Number 21 (latitude 39°50'17.34″N, longitude 77°28'36.12″E), number 32 (latitude 39°50'18.18″N, longitude 77°28'25.62″E) and number 43 (latitude 39°50'10.88″N, longitude 77°28'26.1″E) in researching outcrop area, we identify the distribution characteristics of different mechanical interfaces and based on which we calculate the fracture density controlled by different mechanical units in each section. The average of fracture density of section 21 is 7.1 per meter which is larger than 2.9 per meter of section 43 and 2.2 per meter of section 32. At the same time, we study the dominating factors of fracture density differences in different section from lithological characteristics, thickness of mechanical unit and stress state. It shows that fracture density increases as the content of mudstone is higher in limestone in carbonate stratigraphy and fracture density is sharply lower in grain limestone than that in mudstone. On the other hand, because of the influence of stress shadow, the thickness of mechanical unit has negative correlation with fracture density: the thicker the mechanical unit is, the less the fracture density is. Fracture saturation in section 43 and section 32 is larger than 0.8, which shows they have approached fracture saturated state. However, the section 21 that has the largest fracture density is of unsaturated state, with a fracture saturation of mere 0.6. Therefore, the average fracture has negative correlation with fracture saturation.
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图 1 力学单元及力学界面示意(Underwood et al., 2003)
Fig. 1. The mechanical unit and mechanical interface
图 3 力学界面与非力学界面上的裂缝节点差异
Fig. 3. The difference of the fracture tip's numbers between mechanical interface and non-mechanical interface
图 4 野外剖面裂缝发育模式
a.21号点剖面裂缝发育模式；b.32号点剖面裂缝发育模式；c.43号点剖面裂缝发育模式
Fig. 4. The pattern of the fracture development
图 5 西克尔地区鹰山组地层剖面镜下岩性照片
a.21号剖面中发育的纹层状鸟眼构造的泥晶灰岩；b.21号剖面发育的粉砂屑泥晶灰岩；c.32号剖面中发育泥晶砂屑灰岩；d.32号剖面亮晶砂屑灰岩；e.43号剖面中发育砂屑泥晶灰岩；f.43号剖面中发育的泥晶砂屑灰岩(可见藻类和生物碎屑壳体)
Fig. 5. Photos of the rock sample from Yingshan Fromation under the microscope in Xikeer area
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