Abstract:

The borehole of WFSD-3 is a main hole of the Wenchuan Earthquake Fault Zone Scientific Drilling. The continuous coring(total length of 1,548.44 m) and logging data of borehole  WFSD-3 have been acquired. The image logging data of borehole WFSD-3 contain a mass of in-situ geological feature information, which plays an important role in the identification of fractures, fracture zones and in the analysis of tectonic stress field. Using the resistivity and acoustic image logging data in conjunction with core data, the identification and analysis of fracture characterization were carried out. The results show that in the strata of borehole WFSD-3, high-angle oblique crossing fractures develop dominantly, followed by low-angle oblique crossing fractures. Vertical and horizontal fractures are rare. The fractures particularly develop at intervals of 25 m to 200 m and 900 m to 1,000 m in borehole WFSD-3. The main dip directions of fractures at different depths are obviously different: (1) above 410 m the main dip directions of fractures are between 260° and 290°; (2) from 410 m to 730 m, the main dip directions are between 330° and 360°; (3) from 730 m to 960 m, the main dip directions are between 210° and 240°; (4) from 960 m to 1,185 m, the main dip directions are basically consistent with those above 410 m;(5) from 1,410 m to 1,450 m, the fractures are different from those of the depths above with the main dip directions between 180° and 200°. Fractures are closely related to fracture zones and beddings, and the macro distribution of fractures is controlled by tectonics.

Key words: Wenchuan Earthquake Fault Zone, Scientific Drilling Project；image logging；fracture characterization