Development of Extensional Decollement Faults and Its Petroleum Implications in Wushi East Subsag, Beibuwan Basin
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摘要: 北部湾盆地乌石凹陷从古新世早期到渐新世末期经历了多幕次的伸展，在裂陷Ⅱ幕时期形成了一组近E-W向的伸展滑脱断裂系，但其形成机制及演化过程尚未明确. 基于油气勘探过程中获取的全新高精度三维地震资料，应用构造‒地层综合解释以及定量分析的技术方法，对乌石凹陷东洼伸展滑脱构造进行了深入的解析. 结果表明，乌石东洼新生代地层共发育7条近E-W或NE-SW向展布的伸展滑脱断层（Fa‒Fg），这些伸展滑脱断层与北倾的7号断层相向倾斜发育，共同控制了流沙港组一段到涠洲组一段的跷跷板式地层沉积和大型滚动背斜的形成. 从演化上来讲，在始新世早中期（T100-T83）盆地为由NE-SW向展布的7号断层和局部基底小规模正断层控制的地堑或半地堑盆地，早期局部沉积的流沙港组三段被晚期连续沉积的厚层流沙港组二段油页岩覆盖. 始新世晚期‒渐新世晚期（T83-T60），盆地沉积了由对倾发育的伸展滑脱断层和7号断层共同控制的流沙港组一段‒涠洲组一段，对倾断层的先后活动导致了沉降沉积中心的迁移，控制了复杂的“跷跷板式”滚动背斜及“包心菜”构造的发育. 新近纪以来（T60-海底），盆地的构造活动性减弱，转变为坳陷型盆地. 描述了乌石凹陷东洼裂陷Ⅱ幕时期形成的伸展滑脱构造的几何学特征，并解析了伸展滑脱构造的形成模式，提出了滑脱断裂发育有自下坡断层组向上坡断层组迁移的规律，对解决油气的运移、储存、保存等问题有重要的意义.Abstract: From the Early Paleocene to the end of the Oligocene, Wushi Sag of the Beibuwan Basin experienced multiple phases of extension. A group of extensional decollement faults formed during the 2nd phase of rifting, but their formation mechanism and evolution process are not clear. Based on the new high-quality 3-D seismic data obtained in the process of petroleum exploration, an in-depth analysis of the extensional decollement faults in the Wushi East subsag is conducted in this study. The results show that there are 7 extensional decollement faults (Fa‒Fg) in the Wushi East subsag, which are distributed in the near E-W or NE-SW direction. These decollement faults, together with north-dipping No.7 boundary fault, controlled sedimentation from the 1st Member of Liushagang Formation to the 1st Member of Weizhou Formation, which formed a huge rollover anticline. In terms of evolution, during the Early-Middle Eocene, the basin was a graben or half-graben controlled by the NE-SW No.7 boundary fault and other local small basement faults. The 3rd Member of Liushagang Formation strata was discretely distributed and was overlain by thick layer oil shale of the 2nd Member of Liushagang Formation. From the Late Eocene to the Late Oligocene, 1st Member of Liushagang Formation to the 1st Member of Weizhou Formation deposited in the basin, which was controlled by the oppositely dipping extensional decollement faults and the No.7 boundary fault. From the Early Miocene to the present, the basin changed into a sag basin. This study has depicted the geometry of extensional decollement structure which formed during the second episodic rift phase of Wushi East subsag, analyzed the growth and linkage pattern of decollement faults, and proposed the decollement faults upslope-ward migration model, which has a great significance for solving the problems of oil and gas migration, storage and preservation.
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图 2 北部湾盆地地层构造综合柱状图(据李春荣等(2012)修改)
Fig. 2. Summary of strata and tectonic event of Beibuwan Basin(modified from Li et al., 2012)
图 4 滑脱断层Fa‒Fe位移‒距离图
绿色线为T83‒T80累计古落差，粉色线代表T83‒T72的累计古落差，蓝色线代表T83‒T70的累计古落差，红色线代表T83‒T60的累计古落差
Fig. 4. Fa‒Fe faults throw-distance diagrams of extensional decollement faults
图 5 断层生长发育平面厚度图
a. 流一段（T83-T80），该段沉积时期Fa西段活动，Fb局部活动，Fc不活动，Fd全段活跃活动，Fe西段不活动，中西段、中段和东段活动；b. 涠三段（T80-T72），该段沉积时期Fa暂停活动，Fb西段暂停活动、东段活动，Fc中、西段活动，Fd全段和Fe中段、东段活动；c. 涠二段（T72-T70），该段沉积时期Fa全段、Fb全段、Fc全段、Fe中段及东段活动，Fd全段停止活动. d. 涠一段（T70-T60），该段沉积时期Fa、Fb、Fc和Fe断层局部微弱活动
Fig. 5. Plan view of fault growth and development
图 6 伸展滑脱断层系发育模式
滑脱Ⅰ幕时期下坡断层组发育，并发育相应的包心菜构造. 滑脱Ⅱ幕时期分为两种情况：（b1）下坡断层组发育停止（例如Fd），上坡断层组发育，并发育相应的包心菜构造；（b2）下坡断层组（例如Fe）和上坡断层组同时发育
Fig. 6. The evolution of the extensional decollement faults
表 1 研究区主要伸展滑脱断层的特征
Table 1. Characteristics of extensional decollement faults in the study area
断层名 走向 倾向 沿走向长度(km) 最大落差(m) 最大落差所处位置 Fa E-W S 2.9 1 500 西侧与6号断层连接的位置 Fb E-W S 9.0 810 断层中东部，Fd的正北方 Fc E-W S 8.9 1 000 东部 Fd NE-SW SE 2.7 750 中部 Fe E-W S 22.0 780 中部 Ff E-W S 6.3 380 东部 Fg E-W S 5.4 680 中部 
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