Use of Borehole Imaging to Improve the Understanding of Near-Wellbore Geological Features in an Exploration Well in Shallow Offshore Nigeria

Abstract Geological information gathering is critical in every exploration well. Such information helps with establishing subsurface models and deciding on the nature of appraisal to be carried out. Borehole imaging is a veritable tool to get information about the near-wellbore geological features of exploration wells. From structural to sedimentological information, borehole images provide important additional understanding of the geology transversed by a wellbore. The aim of this work is to show the geological features interpreted from high resolution borehole images while logging an exploration well in channelized turbidite reservoir in offshore Nigeria, and to show how these features support the depositional environment interpretation from regional studies. Like most logging processes, the procedure can be divided into three stages – pre-well planning, real-time logging, post-well interpretation. The pre-well planning includes the selection of borehole image logging tool and optimum logging frequency for the tool. The predominantly contrasting subsurface parameter and geological information being sought after helps in borehole image logging tool selection. In addition to tool functionality, the logging speed is a major check during the real-time stage. This helps to ensure the optimum vertical resolution is achieved. Post-well interpretation starts with data quality checking, data processing, geological feature orientation and identification, and other analyses. The recognition of a depositional environment can be achieved by field observations, formation evaluation of petrophysical logs and cuttings, fossil studies, mineralogical analysis, geochemical analysis, sedimentary facies and paleocurrent analysis. In this study, the main recognition tool is borehole image facies and the features identified include deformed beddings, thin beds, graded beddings, cross-beds, erosional boundaries, mud diapirs and injectites, faults, fractures, and other high-angled features. These are geological features that can be found in turbidites and associated coarse clastic deposits. Geological feature orientation and pattern recognition is the main ingredient of borehole image interpretation, and this work is a good documentation of features interpreted from borehole image logs acquired in offshore Niger Delta. Through a meticulous analysis of borehole images, this study aims to depict the geological intricacies present in the targeted shallow-water turbidites. Furthermore, it seeks to establish a correlation between the interpretations derived from borehole imaging and other data sources like seismic interpretation and field studies. This work will serve as a good reference for future detailed studies and comes handy for integrated interpretation.