Fracture characterization and detection in the deep Cambrian dolostones in the Tarim Basin, China: Insights from borehole image and sonic logs

Characterization and detection of natural fractures using well logs in ancient deeply buried dolostones with low matrix porosity are very difficult. Core description, thin section analysis, conventional well logs, borehole image and sonic logs are used to document distribution of fractures in deep Cambrian dolostones in the Tarim Basin, China. The SHmax (maximum horizontal stress) direction is determined from drilling induced fractures, borehole breakouts and shear wave birefringence. Natural fractures with various attributes (open to partly open to closed) and orientation (including dip angles) are recognized. Natural fractures are classified into closed fractures, partly closed fractures, vuggy fractures and open fractures, and they can be further classified into high angle fractures, low-medium dip angle fractures, and low angle or horizontal fractures. Natural fractures are mainly associated with the more brittle rocks (dolograinstone, fine-medium-coarse crystalline dolostones), while fractures in dolomicrites and evaporitic gypsums-bearing dolostones are rare. Open to closed fractures are recognized as dark to bright sine waves on image logs, and the presences of open fractures cause minor deviations on the DEN logs, but significant deviations in resistivity values, while AC will be significantly increased in front of fractures. The energy of sonic full-waveforms will be strongly attenuated in front of open fractures. Fractures with strike parallel to the SHmax tend to have large aperture and good connectivity, and are suggested to enhance hydrocarbon productivity. Dolostones adjacent to the deep faults are heavily fractured, and fractures act as the hydrothermal fluid flow pathways, facilitating dissolution process. The abundance of vuggy fractures, and enlargement of the pre-existing microfractures support that fractures had enhanced dissolution. The dissolution along fracture surfaces helps increase fracture porosity. The results of this study about subsurface fracture patterns and their distributions are important for the hydrocarbon exploration and production in the Cambrian dolostones in the Tarim Basin.