Recognition of Sedimentary Facies and Deformation from Borehole Images in Deep-water Environments - Applications from Offshore Nigeria

Borehole image logs in deep-water turbidite reservoirs have often been used with limited success in sedimentary facies recognition, since many wells are drilled with oil-based muds preventing the use of high-resolution resistivity imaging tools. This paper presents the successful application of acoustic images (UBI¹), oil-based dipmeters (OBDT²) and more recently oil-based micro-resistivity images (OBMI³) in enhancing deep-water reservoir evaluation with examples from offshore Nigeria.

Specific aspects highlighted in this evaluation are lithofacies discrimination, syn-sedimentary deformation and structural applications. Continuous ‘image facies’ interpretations, calibrated initially with core, were generated over the entire reservoir-bearing intervals and compared with conventional ‘electrofacies’ identification from wireline logs. A variety of syn-depositional deformation features are recognised, including slump folding, slide blocks and sandstone injection. Careful calibration with seismic, including generation of scaled seismic overlays of dip data, significantly enhances well to seismic calibration and highlights sub-seismic faults that may have been overlooked in reservoir model construction.

This paper highlights the value of borehole image logs in deep-water reservoir analysis, particularly when fully integrated with core and seismic data. Some implications for reservoir model construction are also emphasised.