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Numerical Simulation of Reservoir Structures, Part III: Folding of a Layered Rock Sequence in Flat-Ramp-Flat Thrust System

Heesakkers, Vincent 1; Busetti, Seth 1; Reches, Zeev 1
1 Oklahoma University, Norman, OK.

We present a 2D numerical model of a flat-ramp-flat thrust system that includes mechanical stratigraphy and inter-layer friction to investigate the effects of rheology, friction and ramp angle on the geometry of ramp associated folds. This work is part of our study on structural processes in reservoir rocks by numerical simulations with Abaqus finite element code. Part I discusses rock rheology and benchmark simulations, and Part II investigates fracture propagation into a sequence of damaged rock layers (Busetti et al., this meeting).

Analysis and restoration of fault-fold systems requires understanding of the mechanical processes associated with their development. In our calculations we use in-situ conditions including relevant dimensions, gravity, deformation rates, inter-layer friction, and rock properties of Berea sandstone, as a realistic elastic-plastic-damage rheology (Part I, Busetti et al, this session). The thrust fault-zone is modeled by a weak layer (representing salt or clay) with a visco-plastic rheology calibrated with experimental results of rock salt (Carter et al., 1993).

In a typical model, we consider a system of a flat thrust, 60-80 km long, with a 5-45 degrees ramp in its center. The sedimentary hanging wall is up to 14 km thick with individual alternating layers 1-2.5 km thick each. The footwall consists of a 5 km thick competent, elastic basement. In the simulations, the hanging wall is transported horizontally up to 16 km over the basement ramp in a quasi-static mode.

The structural geometry and the associated patterns of stress, strain and damage are explored as a function of basal friction (of the thrust fault-zone), inter-layer friction and fault ramp angle. The main results show that (1) large inter-layer friction significantly reduces the fold amplitude leading to a flat crest above the ramp-flat transition; (2) high basal friction leads to asymmetric folding, whereas low basal friction leads to symmetric folding; (3) high amplitude folds occur for ramp angles that are 10 to 30 degrees; (4) steep front limbs develop for ramp angle > 30 degrees.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009