Comparison of Fracture Patterns from Outcrops and Shallow Boreholes: The Quest for 1-D to 3-D Extrapolation

Jose K.J. Taal-van Koppen¹, Stefan M. Luthi², and Giovanni Bertotti^3
1 Delft Universtiy of Technology, Delft, Netherlands
² Department of Applied Earth Sciences, Delft University of Technology, 2628 RX Delft, Netherlands
³ Vrije Universiteit, Amsterdam, Netherlands

Extrapolating fracture patterns from borehole measurements to a reservoir-scale three-dimensional fracture network is a great challenge because the geometries of the different sets and their mutual relations are difficult to study on such a small scale. We approach this problem with a field study in South Africa where we have access to outcrops of Permian turbidites that show excellent fracturing along extensive cliff faces as well as on the adjacent plateaus. In addition, seven research wells drilled at various distances behind the outcrops were fully logged and cored. Aerial photography shows two dominant lineaments oriented E-W and NW-SE. Outcrop studies show the E-W lineaments to be related to small-scale faulting and folding caused by N-S compression, while the NW-SE lineaments are related to fracture swarms attributed to strike-slip movements.

Analysis of the borehole data focussed on electrical borehole images, which were validated by cores. The same general trends could be observed on the outcrops, although with some significant differences. Thinner sandstone beds show a higher degree of fracturing at the surface than in the wells, while in the subsurface the thicker, massive beds are predominantly fractured. Modeling of the near- and far-wellbore fracture field was successfully undertaken by constraining the model with the observations in the boreholes, and by stochastically filling in the surrounding volume with fractures whose properties follow the statistics derived from the outcrops.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005