Elastic Models
of Deformation in Nature: Why Shouldn’t We Use The Present Day Fault Geometry?
Dee, Stephen J.1, Brett
Freeman2, Graham Yielding2, Peter Bretan2 (1)
Badley Geoscience Limited, Hundleby, Lincolnshire, United Kingdom (2) Badley
Geoscience Ltd, Lincolnshire, United Kingdom
Elastic dislocation (ED) models of
deformation associated with faults are used for the prediction of small-scale
faults and fractures. Typically, a model will use a definition of fault
displacement on the present day fault geometry (i.e. in the geologically
deformed state) and forward model the strains on some observation surface. This
approximation seems to be valid when the fault displacements are small. In
reality, because the material bounding a dislocation deforms, the shape of the
dislocation itself is also changed during deformation. Most boundary element
and ED methods ignore this effect. As a consequence, during forward
deformation, material points apparently move towards and can move through the
fault plane which leads to problems in defining the strain field in close
proximity to the fault.
Intuitively we argue that the fault
geometry should be defined in the undeformed state i.e. the faults should have
their geometry restored by applying the reverse of the elastic deformation
prior to forward modelling. Unfortunately, this poses a new set of problems
since a direct restoration, at the fault plane, is not possible mathematically.
Instead, we define a bounding polyhedral surface for each fault which can be
collapsed to the fault plane itself. Restoration of the bounding surface and
subsequent collapse to a plane effectively yields a set of faults in their
undeformed state.
We demonstrate that this is a more robust
approach to locate predicted fracture intensities and we illustrate why this is
important in determining fracture sweet spots in sub-surface models.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California