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)

Abstract

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 SHmax orientation 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.