Fracture and Deformation Band Visualization Using Full-Resolution 3-D Ground Penetrating Radar (GPR) in Cretaceous Carbonates
Fractures are important conduits in many reservoirs. Yet, assessing fracture patterns is not an easy task as most fracture analyses rely on cores/bore walls or two-dimensional outcrop analogs. A newly developed acquisition system of full-resolution 3D Ground Penetrating Radar (GPR) and subsequent migration of the data allow, for the first time, to image fracture and deformation band networks in three dimensions.
Some of the crucial elements of the full-resolution 3D GPR acquisition are a) a rotary laser positioning system for centimeter precise positioning of the antenna, b) the simultaneous acquisition of 100 and 200 MHz antenna, and c) a regular and dense grid acquisition (5 and 20 cm) of GPR profiles for the high resolution characterization of fractured carbonates. The datasets are processed using the following flow: dewow, time zero correction, gain application, background removal, and 3D migration. Faults and fractures are visible on GPR data as zones of higher amplitude due to the generation of diffractions along the fractures.
Several 3D cubes were acquired at two sites. In Cassis (SE France), a karst-enhanced fracture system in Barremian rudist-bearing strata is imaged with great clarity to a depth of 25 m. A simultaneously performed outcrop-based fracture analysis recognizes fractures of all sizes but the 3D GPR data adds important information in regards to the vertical fault terminations and fault systems that are parallel to the outcrop. Furthermore, solution-enhanced faults and randomly distributed karst caves are visible on 3D data.
Similarly, in the Madonna della Mazza quarry, Maiella Mountains Italy, near- vertical deformation bands and the cross-cutting relationships are clearly visible in the GPR data to a depth of 13 m in Maastrichtian rudist grainstones. The 3D GPR data reveal bedding and even sedimentary structures that are not visible on the quarry wall. Likewise, the six sets of deformation bands observed in the quarry are imaged by the 3D GPR with great clarity. In particular, the near surface time slices display a very similar pattern as described by Tondi et al., (2006) on the quarry floor. In addition, when the semblance algorithm attribute is applied these deformation bands can be followed for more than 10 meters depth and allow the determination of crosscutting relationships with other fractures and dipping layers in order to realize kinematic studies and more precise flow modeling.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009