Geological Interpretation Of Borehole Image Logs – An Integrated Approach Demonstrated From A North Sea Case Study

Abstract

Borehole image analysis involves identification and spatial distribution of geological features such as faults and fractures, bedding/lamination including cross-bedding, and image facies, which can be compared with seismic/core, or act as direct input for 3D reservoir modelling. The obtained dip data are utilised to evaluate structural tilt, faulting and fracturing as well as sediment transport directions and depositional architecture/environments. In addition, the stress field can be inverted from breakout/induced fracture in combination with sonic logs and other rock mechanical data. Based on a recent North Sea case study an integrated workflow is presented. The structural tilt, evaluated in stereographic displays and projection along the well trace, changes due to in-well faulting (block rotation) and across angular unconformities. Fault zones with significant displacement cause deformation and juxtaposition of different strata/facies, relevant for reservoir performance (baffle, barrier or conduit for fluid flow?). Fractures are often associated with fault zones and reflect the stress field at time of generation. Different orientation of open and closed (mineralised) fractures implies different stages of fracturing, as well as a change in stress field orientation. Relevant input parameters for the fracture model are provided as fracture orientation, density, aperture and porosity. The current stress field is best observed and evaluated by combining borehole image with dipole sonic tools. A thorough characterisation of the shear wave is vital for further interpretation. Drilling-induced fractures and breakouts are easily detected and represent reliable indicators for the stress field orientation at the time of logging. However, for slanted and highly deviated wells the full stress tensor including the stress magnitudes is necessary to precisely evaluate the stress field orientation. The full stress tensor then allows evaluation of the mud weight in order to optimise well paths. Other rock mechanical data are utilised from core measurements or are derived from empirical relations of elastic properties. After removal of the structural tilt from the dip data set, sedimentary features such as cross-beds or slumps indicate sediment transport directions. In combination with standard petrophysical logs the image facies and their stacking patterns are interpreted with respect to the depositional environment and included in a sequence stratigraphic framework. E.g. for the upper part of the Cook Fm a NW-SE bimodality reflects a tidal influence, calibrated by core observations. The resulting time lines aid palaeo-geographic reconstructions partly within an estuarine environment. The correlation with core provides a powerful, high-resolution data set spanning the scale from core to seismic, subsequently to be integrated with the reservoir models. Furthermore, resistivity variations of borehole images are on a considerably higher resolution compared to standard open- hole logs and connected to lithology and pore fluids. Hence high-resolution statistical evaluation of the image data enables also thin-bed analysis and detailed N/G estimates, as well as vuggy porosity calculations.

AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019