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