Borehole Micro-Imaging Applied to Siliceous Shale Reservoir Characterization

Scott J. Johnson and Maged Y. Fam

Electric and acoustic borehole micro-imaging logs greatly aid characterization of siliceous shale reservoirs. Borehole images yield reservoir data not attainable with conventional open-hole logs. While visual inspection of core remains indispensable, borehole images enhance visualization and facilitate orientation analysis of sedimentary, structural and diagenetic features. Style, density, and interconnectivity of natural fractures are assessed directly and fracture aperture is estimated under in-situ stress. Inspection of the borehole wall and cross-sectional geometry are used to constrain maximum horizontal in-situ stress orientation.

Borehole micro-imaging logs have been applied to characterize siliceous shale reservoirs in the Cymric Field of California. Significant variations in reservoir permeability due to natural fracturing are evident both vertically and laterally. Fracture-enhanced system permeability may impact depletion geometry to the extent that fractures are not induced during cyclic stimulation. Macroscopic reservoir heterogeneity is greatly enhanced by silica diagenesis resulting in improved image quality. Maximum horizontal in-situ stress orientations inferred from borehole images are comparable to geophysical stress measurements. Stress barriers are evident and may limit height growth of induced fractures. Reservoir characterization has improved well completion practices by allowing perforations to be strategically placed with respect to potential thief zones thereby reducing the possibility of bypassing hydrocarbons.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California