Variations in Fracture Characteristics Across the Stratigraphy of Outcropping Reservoirs: New Acquisition and Processing Techniques and Results

G. Bertotti¹, N. Hardebol², J. Taal-van Koppen³, S.M. Luthi³, J. Bruining³, and W.R. Rossen^3
1Vrije Universiteit, Amsterdam, Netherlands
²Vrije Universiteit - IFP, Amsterdam, Netherlands
³Delft University of Technology, Delft, Netherlands

Submitted to sufficient stresses, rocks forming reservoirs and seals break forming joints and faults, and change physical properties such as permeability. The variable characteristics of the sedimentary succession (bedding, grain-size, lithology, etc) will cause significant changes in the geometry and spatial distribution of fractures moving from one bedding unit to the other (fracture stratigraphy). Predictions on the fracture stratigraphy of reservoirs are essential to perform numerical simulations of flow and are often derived from outcrop analogs. Only partial information can be derived from boreholes as they sample small portions of space. Innovative techniques to acquire and process fracture data from outcrops efficiently provide a detailed reconstruction of the changes in fracture characteristics (intensities, orientations, spatial distribution, etc) across the stratigraphy.
As a case study, we have analyzed turbidite systems outcropping in the Tanqua-Karoo Basin (RSA). At the outcrop scale, fracture intensities change in a gradual rather than abrupt manner. In thick beds, fractures are more common close to the bedding surface rather than inside the bed. Layers in bundles of amalgamated turbidites show different fracture patterns despite the absence of mechanical interlayers. Patterns from outcrops closely resemble those derived from boreholes drilled in the vicinity. Fracture characteristics for major sedimentary unit and from structural domains (swarms, vicinity of folds and faults) are obtained by interpolating among outcrops of these domains. This information produces a realistic description of reservoir scale fracturing. Work on Devonian sands in N Africa and Oman provides information on changes in fracture patterns in gently folded areas.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece