Structural geology in its simplest form comprises the collection of bedding plane orientations, from which structural tilt and its variations are established. Structural breaks may be caused by angular unconformities or by block rotation on in-well fault planes. An unconformity is well defined in time and space, subdividing older strata below from younger strata above. The rotation of the older rocks before the deposition of the younger strata can be accurately quantified.
Fault planes are defined in space, whereas the question of timing needs correlation with regional geology. Gradual change of structural tilt 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 symmetry axis.
In order to fully exploit the interpretation potential of a BHI data set, it is compared to regional geology established from seismic data, surface mapping and adjacent wells. This is a two-way process, as previous concepts will often be modified and upgraded by the integration of the meso-scale, sub-seismic structures, extracted from the BHI analysis.
Detailed observations of fault and fracture parameters may help to establish their history and their impact on reservoir performance. Genetically associated deformation of adjacent strata will provide insight on type of motion along fault zones as well as the potential, tectonic palaeo-stress systems involved, e.g. pure wrenching versus transpressional or transtensional.
The present-day in-situ stress system plays a crucial role when assessing the impact of faults and fractures on the reservoir. We evaluate in-situ stress by observing wellbore breakout and various types of induced fractures in borehole images (see Geomechanical Services).
Characterizing fractured reservoirs, borehole images can help us to establish whether fractures and faults act as conduits, baffles or barriers to fluid flow. We can produce qualitative and quantitative analyses implemented in a fracture reservoir model by categorizing fracture planes according to parameters such as orientation, apparent aperture, fracture length, planarity, spacing, cross-cutting relationships to other fractures and bedding planes, etc. We calibrate to cores wherever possible to verify our analyses/interpretation.
If a client needs a time-sensitive interpretation of fractures based on borehole images (for example, for completion, testing or hydraulic fracturing purposes), we can normally deliver results within 24-48 hours.
Structural multi-well studies of borehole images from entire fields or at regional scales provide a wealth of important details. When these studies are combined with a depositional interpretation (see Sedimentological Services), we arrive at a comprehensive tectono-stratigraphic model, balanced by results from seismic and core observations. These data sets are ideal input for subsequent reservoir modeling and exploration.