Formation Mechanism of High-Obliquity Faults in Platform Areas of Chinese Western Basins and Their Control on Petroleum Migration and Occurrence
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摘要: 给出了中国西部盆地台盆区普遍发育高角度(倾角 > 45°) 断层的证据, 从断裂形成的力学机制上分析了高角度断层的成因, 认为中国西部盆地台盆区高角度断层的形成主要受压扭性应力场控制, 同时与脆性地层的变形特点有关.断层面静封闭压力由岩石泊桑比、上覆地层容重、断层埋藏深度和倾角、最大和最小主应力以及断层走向与最大主应力方向的夹角等计算, 作为断层开启性评价的一个指标, 断层面静封闭压力的计算表明, 断层的开启性随断层倾角的增加而增加, 同时, 沿断层面运移的油气所受浮力在平行断层面方向上的分力也随着断层倾角的增加而增加, 这就决定了高角度断层更有利于油气的垂向运移.结合西部盆地的具体分析, 认为中国西部盆地区域性高角度断层是沟通深部成藏动力学系统与中上部成藏动力学系统的重要通道, 是中上部系统成藏的关键, 直接决定着中上部系统中油气的分布, 在每一系统内部发育的高角度断层具有使油气在断层断开的最新层位中优先充注成藏的基本规律.这些认识对指导油气勘探具有重要意义, 而且, 这些认识同样可推广到其他存在高角度断层的盆地中.Abstract: High-obliquity faults (superior to 45°) are very commonly encountered in the platform areas in Chinese western basins. Geomechanical analysis demonstrates that high-obliquity faults are produced by the compression-torsion stress field existing in Chinese western basins, and they may also be related to the stratum's brittle deformation. Static sealing pressure, which can be calculated from Poisson's coefficient, burial depth, obliquity degree of the fault, and maximum and minimum principal stresses, is used as an index of the sealing capacity or openness degree of a fault for petroleum migration. The calculation of static sealing pressure indicates that the openness degree of a fault increases with the fault's obliquity degree. The floating force decomposition illustrates that the decomposed force along the fault surface also increases with the fault's obliquity degree. Therefore, vertical, cross-formational petroleum migration via a high-obliquity fault is easier than via a low-obliquity fault. This conclusion is applied to interpret the control of high-obliquity faults on petroleum migration and occurrence in the Tarim and Junggar basins of western China.Conclusionsare: (1) Regional high-obliquity faults crossing different dynamic fluid systems control the petroleum distribution in different systems, especially as petroleum migration pass-ways for the middle-upper systems, as they are essential for pool formation in the latter; (2) Inside each dynamic fluid system, petroleum migrating along high-obliquity faults is most likely tofill the newest layer cut by the faults. The conclusions obtained have clear implications for petroleum exploration and these conclusions can be extended to other basins where high-obliquity faults exist.
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图 1 塔里木盆地轮南断层与轮南南断层的“Y”字型组合
Fig. 1. "Y" type combination between Lunnan fault and southern fault in Lunnan uplift of Tarim basin
图 3 滴西1井区TJ3反射层断层分布及左行走滑图(据新疆油田公司勘探开发研究院改编, 2000)
Fig. 3. Fault distribution and levorotatory sliding TJ3 reflecting layer in the Dixi 1 zone
图 4 准噶尔盆地陆东地区燕山晚期不同方向断层在3 000 m深度处的静封闭压力
Fig. 4. Static sealing pressure at the depth of 3 000 m on the faults of different directions during the late Yanshan period in Ludong area, Junggar basin
图 6 塔里木盆地轮南凸起下三叠统油气分布与断层关系
1.油层; 2.油气层; 3.油水同层; 4.含油水层; 5.含气水层
Fig. 6. Relation between fault and distribution of oil & gas of Lower Triassic in Lunnan uplift of Tarim basin
图 8 准噶尔盆地陆东地区成藏模式
Fig. 8. Petroleum migration-accumulation mode of Ludong area of Junggar basin
图 9 塔里木盆地轮南2井油藏油气运聚和调整剖面(顾家裕和周兴熙, 2001)
1.油层; 2.残余油显示; 3.断层
Fig. 9. Profile of petroleum migration-accumulation and adjustment of well Lunnan 2 reservoir in Tarim basin
表 1 西部叠合盆地部分断层倾角统计
Table 1. Statistics of some faults' obliquity in some western basins
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