Sedimentary structures of clastic rocks identified from electrical borehole images

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Abstract
High-resolution electrical borehole images are obtained from the Formation Micro Scanner * (FMS) by scanning the borehole wall with arrays of small electrodes pressed against the borehole surface. The images provide detailed sedimentary structural information from clastic rocks which is of great value for geological and reservoir modelling as illustrated by two case studies. Analysis of electrical borehole images through a Pennsylvanian deltaic cycle from Texas shows different types of bedding surfaces, including channel base scours, cross-bedding, slump folds and growth-faulting. The ensemble of the data constrains the subsurface geometry of the reservoir and helps to establish a depositional model. Thresholding may produce a segmentation of sand/shale images. In the aeolian Rotliegendes of the North Sea, electrical borehole images provide a vertical succession of facies, dip and azimuth measurements of interdune layers, bounding surfaces and foresets, as well as thickness measurements of the cross-bedded sets. Statistical modelling using the data indicates scale-invariant crescentic bedforms, and describes sizes, shapes and layering of the cross-bedded sandstones. Computer-aided design tools are used for forward-modelling of structures observed on borehole images in order to test possible interpretation hypotheses. The use of graphic workstations greatly facilitates the interpretation process.