Determination of Foliation in Basement Rocks Using Borehole Image Log Data and Its Influence on the Fracture System
Rambousek, Christian *1; Decker, Kurt 2
(1) Fronterra Geosciences, Vienna, Austria. (2) University of Vienna, Vienna, Austria.
A factor that can have an influence on the natural fracture system is foliation. Until recently this feature was rather under estimated within the industry.
Foliation does not occur as individual feature such as fractures but rather affects the entire host rock, usually as a preferred shape and/or crystallographic orientation of mineral grains and/or aggregates of mineral grains (Milnes et al., 2006).
The determination of foliation using borehole image log data requires special recognition criteria that differ from regular fracture characterization. An often used approach to define foliation is to identify lithological boundaries and metamorphic layering first. The lithological boundaries are most commonly not imaged by the acoustic imaging instrument. The fact that foliation is not visible on acoustic borehole image data is another determination criteria.
In general it is assumed that an intact, non-foliated rock is isotropic and therefore will have the same stiffness (Young’s modulus) regardless of directional stress. Since foliation adds anisotropy to the rock, the Young’s modulus and strength values will change depending on the direction of stress relative to the foliation. Such a differentiated behavior under loading has severe influence on the mechanical parameters of the rock and needs to be taken into account when creating a geomechanical model of the reservoir.
The usage of borehole image log data is the only way to clearly identify sub-seismic features such as foliation and fractures within the sub-surface. The discrimination of natural fractures and foliation during analysis of borehole image log data is critical and needs profound knowledge of the environment. The anisotropy effects caused by foliation do have an effect on the later formation of natural fractures in the intact rock and therefore also influence the mechanical properties of the rock mass.
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AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain