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Erslev, Eric A.1, Peter H. Hennings2
(1) Colorado State University, Fort Collins, CO
(2) ConocoPhillips, Houston, TX

ABSTRACT: Variations in Predicted Reservoir Heterogeneity Due to Multiples Modes of Folding in Basement-Involved Foreland Anticlines

Petroliferous anticlines in basement-involved foreland systems can be explained as second-order accommodation features formed during first-order basement arching above master, listric thrust faults. These anticlines can be classified by their mode of occurrence as backlimb tightening, backthrust tip, forelimb wedge, and subthrust splay anticlines.
Although many of these anticlinal geometries can be replicated by shear-based fault-propagation-folding algorithms, recent field observations and kinematic analyses show that an additional component of flexural slip is commonly required to produce their geometries and deformation patterns, particularly in shale-rich basins. Out-of-the-basin, layer-parallel shear is required by the geometry of back-limb tightening, backthrust tip, and forelimb wedge anticlines. For subthrust splay anticlines, into-the-basin, layer-parallel shear can be driven by gravitational spreading of basement arches during movement on their master thrust faults.
The mode of folding controls fault-generated reservoir heterogeneity. In competent strata near the basement interface, triangular shear zones form arcuate folds as forelimb strata are thinned and thickened by distributed faulting. The resulting reservoir-scale faulting and heterogeneity is proportional to the dip of units within the shear zones. In contrast, flexural slip folding concentrates layer-oblique faulting at fold hinges, particularly in the upper and lower layers of more competent stratal packages underlain by less competent strata. The degree of reservoir-scale deformation and heterogeneity will be dependent on the local curvature of layers, not their dip, and their position relative to other strong and weak strata. As a result, determining the distribution of fold mechanisms is essential to the prediction of reservoir heterogeneity in basement-involved anticlines.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.