Numerical and Experimental Evidence for Polygonal Faults in Carbonate Rocks
Bruhat, Lucile 1; Gauthier, Bertrand D.*1; Leroy, Yves M.2
(1) Total Exploration et Production, Total S.A., Paris, France. (2) Laboratoire de Géologie, Ecole Normale Supérieure, Paris, France.
Polygonal fracture patterns in clastic environments have been first observed in the early 90’s, thanks to 3D seismic. In carbonate rocks only few examples, e.g. Abu-Dhabi grainstone reservoirs, are observed using seismic attribute techniques. The lack of fossil analogues and the absence of resolvable displacement results in a debate about their origin: differential compaction under low differential isotropic extensional stress or seismic artifact. Using numerical and sandbox modeling, we test the geological origin of polygonal fracturing in carbonates. Numerically, a 3D mesh reproduces the isotropic extension due to the gravity of an elastoplastic layer over a thicker viscous one. The plasticity criterion is defined by the Coulomb criteria with a maximum tensile strength and the plastic strain is calculated using a double-gliding model. The model shows that strain localization is radial, forming concentric grabens. In map view, polygonal shapes develop after the first radial opening. If we change cohesion properties for specific meshes, hence simulating differential compaction, the plastic strain first localizes at the edges of the meshes. A second order polygonal pattern develops only if the cohesion contrast is small. Analogue experiments model the evolution of sand-plaster mixture, i.e. cohesive, layers over a silicone, i.e. viscous, layer. Different types of sand-plaster combination are considered to generate cohesion contrasts both vertically and laterally. We therefore mimic differential compaction, by changing the cohesion of the layers and by reproducing known geometries of sedimentary heritage like mudpools or clinoforms. Under gravity related isotropic extension a polygonal pattern develops. The surface pattern is extremely influenced by the geometry of the layer underneath and its cohesion contrast. We show that the main basis for polygonal fracturing is an isotropic horizontal extensional stress field, probably under low compaction. Differential compaction related to varying lithologies influences the shape and the dimensions of the pattern but we believe that it is not the sole mechanical cause. Combining established methods like numerical and analogical modeling proved to be an innovative way to better understand the genesis of below seismic scale fractures and to better constrain the conditions under which polygonal fracturing can occur.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain