BOREHOLE IMAGE LOGS: A POWERFUL TOOL FOR IMPROVED SUBSURFACE GEOLOGICAL INTERPRETATION

Electrical borehole image (BHI) logs are the outcrop analog of the subsurface and can provide a wealth of geological information for structural and sedimentary interpretations. Dip data (dip/azimuth) of different geological planes (bedding, faults, fractures, etc.) can be categorized during manual dip picking and lithofacies can be identified using both the image texture and supplementary triple-combo, open hole petrophysical data. Dip patterns over an entire logged interval can be analyzed stereographically. Statistical eigenvector calculations over a given depth interval can provide the mean dip/azimuth of strata for a cluster distribution of poles, whereas a girdle distribution of poles gives the trend/plunge of an axis. Gradual and abrupt changes in dip data may reveal subsurface folding, faulting, or an unconformity, and detailed examination of the image log where the dip pattern changes may reveal its geological significance. The structural tilt of the strata can be removed using vector subtraction for a sedimentological analysis, and paleo-horizontal dip data can be correlated with electronic lithofacies to identify different stacking trends. Axial symmetry trends of paleo-horizontal dip data may reveal common symmetries for different lithofacies, which may in turn assist in determining sedimentary paleo-transport directions. If depth matched properly, these data can be bolstered with and correlated to detailed core analyses as well as directional measurements from 360° core scans. This presentation will demonstrate how BHI logs can lead to improved interpretations of subsurface geology and detailed characterization for a variety of reservoirs (e.g. oil and gas, geothermal, etc.), particularly if geophysical data are poorly resolved (i.e. adjacent to and/or beneath salt bodies).