AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Visualization and Quantification of Fracture-Related Karst in Barremian Limestones, Cassis, France
(1) RSMAS - University of Miami, Miami, FL.
(2) Géologie des Systèmes et des Réservoirs Carbonatés, Université de Provence, Miami, France.
High-resolution 3D Ground Penetrating Radar (GPR) images the fracture network in low porosity (1.8%) Barremian rudist bearing strata in a large quarry near Cassis, France. About half of the fractures are solution enhanced to form small cavities along the fracture planes and on fracture intersections. Attribute analysis and extraction techniques of the migrated 200 MHz 3D GPR cubes visualize the karst and the fractures, and allow for an unprecedented quantitative assessment of the distribution and volume of fractured-related karst.
Innovative acquisition and processing techniques of three 200 MHz 3D GPR cubes acquired on the quarry floor image the fractures and karst cavities to a depth of up to 12.5 m. Signal attribute, volume rendering and automated geoanomalies extraction techniques are applied to the migrated 200 MHz 3D GPR cubes in order to enhance the visibility of karst and fractures. A comparison of the fracture network within the GPR cubes with the outcrop-based fracture analysis reveals one more fracture family that runs parallel to the quarry wall and is not captured using the scanline method performed in the outcrop. Karst of various size and shape are observed along fracture planes, and areas with plenty of karst intersections tend to have large and extensive karst cavities. Automated geoanomalies extraction of the fracture-related karst from the 3D GPR volume allows to quantitatively measure the volume of individual karst bodies. Based on the extraction from all 3D GPR cubes, karst cavities are up to 3.8% of the total volume of the strata in the Solvay quarry. The size-frequency distribution of karst in the quarry follows a power law distribution, indicating that small karst bodies are more common than large ones.
The ability of 3D GPR to image decimeter to meter scale karst and quantify their volume has implications for estimating karst-related porosity in carbonate reservoirs. In reflection seismic data these karst features will be below resolution. Therefore, calculation of the actual total porosity is likely underestimated when only matrix porosity and large-scale karst are taken into account. Results of this and similar studies will help assess the volume of fracture-related karst.