Fracture analysis and determination of in-situ stress direction from resistivity and acoustic image logs and core data in the Wenchuan Earthquake Fault Scientific Drilling Borehole-2 (50–1370 m)
Fracture analysis and determination of in-situ stress direction from resistivity and acoustic image logs and core data in the Wenchuan Earthquake Fault Scientific Drilling Borehole-2 (50–1370 m)
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DOI：
10.1016/j.tecto.2013.03.005
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发表时间：
2013-05
期刊：
影响因子：
2.9
通讯作者：
X. Nie;C. Zou;L. Pan;Zhaohui Huang;Dongming Liu
中科院分区：
文献类型：
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作者：
X. Nie;C. Zou;L. Pan;Zhaohui Huang;Dongming Liu
2008年5月12日汶川大地震发生后，为了研究断裂带的结构和地震机制，启动了汶川地震断裂科学钻探工程（WFSD）。WFSD包括4个钻孔（WFSD-1、WFSD-2、WFSD-3和WFSD-4），位于映秀-北川断裂带和冠县-安县断裂带沿着最大位移位置，WFSD-2是第二个钻孔，目前仍在钻探中。采集了50 ~ 1370 m的岩心样品、电阻率和声波成像测井数据。从岩心和成像测井资料中识别出天然裂缝、井漏、钻井诱发裂缝和钻井强化天然裂缝，并进行统计分析。天然裂缝的走向有系统的变化，按深度可分为四组：（1）637 m以上，主要走向为ENE-WSW，（2）637- 932.6 m段，走向为NNE-SSW，（3）932.6- 1200 m段，走向为ENE-WSW，然后走向为WNW-ESE，1030 - 1150 m段，走向为NE-SW;（4）1200 ~ 1370 m，维持NW-ESE走向。从50米到637米的天然裂缝似乎是逆断层，走向大约与主断层垂直。1002.4m左右的两组共轭裂缝，指示近垂直的最大主古应力方向，可能是局部应力场引起的主断层的附属构造，揭示了映秀-北川断裂带裂缝形成时复杂的应力场。在WFSD-2中，从960 m到1370 m共解释了12个BO、2套DIF和1套DEF，总长度为30.4 m。WFSD-2（960- 1370 m）解译的平均SHmax方位为120.7°-300.7 ° N（即WNW-ESE），标准差为9.2°，与2008年汶川地震的主要断裂带之一映秀-北川断裂带的应力状态一致。
After the Wenchuan Earthquake on May 12th, 2008, the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD) was initiated in order to investigate the structure of the fault zones and the mechanism of the earthquake. The WFSD contains four boreholes (WFSD-1, WFSD-2, WFSD-3 and WFSD-4) lying at the maximum displacement locations along the Yingxiu-Beichuan fault zone and the Guanxian-Anxian fault zone, and WFSD-2 is the second borehole and is still being drilled. Core samples, resistivity and acoustic image logging data were acquired from 50 to 1370m. The natural fractures, borehole breakouts, drilling-induced fractures and drilling-enhanced natural fractures were identified from the cores and the image logs and were statistically analyzed. The strikes of the natural fractures systematically vary and can be sorted into four groups according to depth: (1) above 637m, mainly striking ENE–WSW; (2) in the interval of 637–932.6m, striking NNE–SSW; (3) in the interval of 932.6–1200m, directed ENE–WSW then to WNW–ESE, while striking NE–SW from 1030m to 1150m; (4) from 1200m to 1370m, maintaining a strike of WNW–ESE. The natural fractures from 50m to 637m seem to be reverse faults which strike approximately parallelly to the main fault. Two sets of conjugate fractures around 1002.4m indicating subvertical maximum principal paleo-stress direction may be a subordinate structure of the main fault caused by a local stress field, and it reveals the complex stress field of Yingxiu-Beichuan fault zone when the fractures formed. A total of 12 BOs, 2 sets of DIFs and one set of DEFs with an overall length of 30.4m were interpreted from 960m to 1370m in WFSD-2. The average SHmaxorientation interpreted for WFSD-2 (960–1370m) is 120.7°–300.7°N (i.e. WNW–ESE) with the standard deviation of 9.2° and it is consistent with the stress status of Yingxiu-Beichuan fault zone which is one of the main fault zones in the 2008 Wenchuan Earthquake.