--> Abstract: Physical Models of Sills and Cone-sheets Formed by Sand Injection; #90063 (2007)

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Physical Models of Sills and Cone-sheets Formed by Sand Injection

 

Rodrigues, Nuno E. T.1, Peter R. Cobbold2, Helge Loseth3 (1) Université de Rennes 1, Rennes, France (2) Rennes University, 35042 Rennes, France (3) Statoil Research Centre, 7005 Trondheim, Norway

 

In sedimentary basins, injected sands form bodies of various shapes, depending on the tectonic context. In passive basins, sand bodies are commonly flat-lying sills or cone-sheets. The latter are responsible for characteristic “v-brights” on seismic data. Their host rocks are commonly mudstones of low permeability.

 

To understand how such sills and cone-sheets may form, we have done experiments on scaled physical models. The model materials were granular and the permeability increased with grain size. The materials yielded according to a Mohr-Coulomb criterion and were capable of tensile failure, if cohesive. Compressed air served as a pore fluid in the experiments. Sills and cone-sheets formed readily in models consisting of three layers, (1) a substrate of quartz sand, (2) an overlying thin layer of cohesionless glass microspheres, and (3) an uppermost layer of fine cohesive silica. On applying a fluid overpressure beneath the substrate, air flowed upwards through the layered model. Fluid pressure first reached and exceeded the weight of overburden beneath the silica, which was the least permeable layer. Horizontal and conical hydrofractures then formed inside this layer. Glass microspheres fluidized and infilled the fractures, forming sills and cone-sheets. We infer that the least effective stress in the silica was vertical and explain this in terms of seepage forces.

 

On comparing the natural examples of injected sands from the North Sea to the physical models, we infer that seepage forces and hydraulic fracturing were probably the main agents in their formation.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California