Sedimentological interpretations of siliciclastics and carbonates can be based on various data sources, such as borehole images in combination with wireline logs, seismic cross-sections, correlation charts, cores and their logs or outcrops. The combination of borehole images and cores is probably the most powerful tool for interpreting high-resolution sedimentology, and can also significantly influence seismic-scale reservoir models.

Interpretations Based on Borehole Images

Information from borehole images ranges in scale between seismic-scale and core-derived data. Depending on the quality of the image and its type (water or oil based mud), images offer resolutions down to millimetre-scale, suitable for detailed facies interpretation (e.g. showing rootlets or bioturbation). Where available, core calibration increases the reliability of results, without being a basic necessity for the interpretation.

Based on typical and systematic dip patterns as well as resistivity variations within borehole images, the interval can be subdivided into individual facies types. Correlated to wireline logs, these facies types can be calibrated to cores (see below) and attributed to specific depositional systems. In this way facies characterisation can also be extended to uncored intervals, showing a continuous record of facies evolution in time.

As all measured planes are oriented in space, directional results from borehole images are used to show the evolution of palaeocurrent directions (e.g. from cross bedded sandstones), slope orientation derived from slump axes, angularity of unconformities as well as spatial geometry and architecture of sedimentary bodies and their depositional agents from axial trend changes (see Structural Geology Services).

By recognizing sequence boundaries and flooding surfaces, sedimentary packages can be subdivided into, for example TR cycles (transgressive-regressive), which are indispensable for sequence stratigraphic correlations along specific transects. The sedimentary packages can be split into chronostratigraphic time-slices, which enable palaeogeographic reconstructions to be created for each time step. Thus the history of a depositional area can be displayed in a series of palaeogeographic maps, for example, showing the migration of facies belts or changes in palaeocurrent directions. These tectono-stratigraphic models (see Structural Geology Services) are an important source of data for regional, seismic-scale studies and are used as input for geocellular reservoir models.

Resistivity variations in borehole images have a characteristic response to individual facies types as well as their pore fluids. This fact can be used to obtain high resolution thin-bed analysis or vuggy porosity, resulting in detailed N/G estimations.

Core Logging

Cores are basically interpreted in terms of their depositional environment. As small-scale sedimentary structures can not always be identified in borehole images, cores may serve as an important indicator and calibration for facies analysis. Recognising flooding surfaces in cores, for example, facilitates the building of sequence stratigraphic models and correlating systems tracts. Trace fossils are important palaeo-environmental indicators and can be attributed to an ichnofacies type, providing crucial information about the depositional milieu. Core fracture patterns are mapped by using circumferential transparencies from which dips can be computed (core goniometry). The resulting data can be used in detailed analyses of both palaeo- and current-stress frameworks.