Structural Geology Services

Structural geology it its simplest form comprises the collection of bedding plane orientations and from these establish the structural tilt of strata and the variation hereof. Variation may occur as gradual change or more abruptly.

Structural breaks may be caused by angular unconformities or by block rotation on in-well fault planes. The unconformity is well defined in time and space, being younger than the strata below and older than those above, and the rotation involved of older rocks can be accurately quantified.

Fault planes can be defined in space, whereas the question of timing needs correlation with known characteristics of the regional, structural geology. Gradual change of structural tilt is also important and may be attributed to basin subsidence, folding due to tectonic activity or differential compaction to name a few. The change of tilt can be described by its axial trend.

Again it is important to compare with regional geology as established by seismic data, surface mapping and results from previous wells to fully exploit the interpretation potential. This is a two-way process, as previous concepts of the picture will often be modified and upgraded by the integration of the meso-scale, sub-seismic structures that can be extracted from the borehole image or dipmeter interpretation.

Detailed observations of fault and fracture parameters may help to establish their history and impact on reservoir performance. Associated deformation of adjacent strata will provide insight on type of motion along fault zones and the potential, tectonic palaeo-stress systems involved, e.g. pure wrenching versus transpressional or transtensional.

The current, in-situ stress system plays a very crucial role when assessing the reservoir impact of faults and fractures. In-situ stress may be evaluated by borehole image observations of wellbore breakout and various types of induced fractures (see Geomechanical Services).

For fractured reservoirs it is important to establish whether fractures and faults act as conduits, baffles or barriers to fluid flow. Borehole images will provide this insight (if of sufficient resolution), and by categorizing individual fracture planes according to parameters such as orientation, apparent aperture, fracture length, planarity, spacing, cross-cutting relationships to other fractures and bedding planes, etc., it is possible to produce qualitative and quantitative analyses leading to fracture reservoir models. Obviously such analyses should be accompanied by core calibration exercises, whenever possible.

Core goniometry may be carried out by treating autocar 360 deg photos of the core as borehole images by inverting and merging them and adding hole orientation data. Another approach is to trace visible features in the core on acetate overlays, then invert and scan these and treat as any borehole image log. This approach can also be used on half and 3/4 cuts of slabbed core. The core goniometry is part of the core description and delivers the ultimate core to image calibration.

Rush interpretation service of fracturing in a well based on borehole images is sometimes needed for completion, testing or hydraulic fracturing purposes. Delivery is normally within 24-48 hrs.

Structural geological multi-well studies of borehole images from entire fields or regional scale areas provide a wealth of important details, which, when linked to the depositional image interpretation (see Sedimentological Services), will lead to a comprehensive tectono-stratigraphic model, balanced by results from seismic and core observations. Such result data sets provide the ideal input for subsequent reservoir earth modeling exercises or further exploration efforts.