AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Geometry of Upper Jurassic Sponge/Microbial Bioherms: 3-D Georadar Analysis and Modelling (Swabian Alb, SW-Germany)
(1) Sedimentary Geology, University of Tübingen, Tübingen, Germany.
The aim of this research project is the three- dimensional reconstruction and visualisation of Upper Jurassic sponge/microbial bioherms of the Swabian Alb. Knowledge on three-dimensional shapes of these carbonate buildups can be useful in hydrocarbon exploration and reservoir characterisation.
Georadar (Ground Penetrating Radar) measurements provide high resolution images of depositional geometries. GPR measurements are carried out at two locations:
1) The quarry of Gerhausen near Blaubeuren covers stratigraphically the “Obere Felsenkalke” (ε) to the “Liegende Bankkalke” (ζ1) (Pawellek, 2001).
2) The quarry of Genkingen is located in the “Untere Felsenkalke” (δ3 and δ4) (Rathgeber, 2002).
For the data acquisition a 200MHz GPR measurement tool was used. In the quarry of Gerhausen there only 2D sections were acquired, in the quarry of Genkingen however the data acquisition covers a 3D Georadar grid consisting of 138 2D sections 2,5 metres away from each other. The depth of penetration (DOP) of the GPR is dependant on the pureness of the limestone. Due to the Glauconite bank in the quarry of Genkingen the DOP is only up to 7 metres whereas the DOP in the quarry of Gerhausen with very pure limestone (Pawellek and Aigner, 2004), reaches 17 metres. To interpret the GPR data a Georadar facies atlas was developed, defining the main three carbonate facies: the massive (reefal) facies, the bedded (basinal) facies and the transition zone (reef margin) facies. Each of them shows a characteristic signature both in the radargram and in the single traces. The massive facies is defined by well developed hyperbole reflexes exhibiting the characteristic width of electromagnetical wave propagation in limestone. The bedded facies displays strong and well traceable. The processed GPR data was interpreted in the seismic interpretation module of the software Petrel. First the strong reflectors of the small basins located between the massive facies were mapped. This resulted in the reconstruction of elongated basins in SW-NE direction. Some neighbouring surface shapes, covered with vegetation, possibly also representing sponge/microbial bioherms shows an elongation in the same direction. Secondly, deterministic modelling was carried out in Petrel, resulting in a 3-dimensional model of the sponge/microbial bioherms.