Foreland Basement-Involved Structures

Abstract

Basement-involved structures commonly occur as long, irregular chains of uplifts in foreland basins. These structures commonly contain significant hydrocarbon accumulations, with most major fields located on the broad crests of these structures. The search for complex traps in deeper targets and in subthrust structures requires an improved understanding of the geometry and evolution of these structures. Characteristic features of basement structures include deformation zones within the sedimentary cover that dissipate significant fault slip, and gently dipping frontlimbs and backlimbs. Fault slip in the basement is usually accommodated in the cover by a triangular, widening-upward deformation zone on the forelimb, with the nature of deformation controlled primarily by the mechanical stratigraphy. If the cover contains interlayered competent and incompetent units, the incompetent units are characterized by significant penetrative deformation, whereas the competent units are faulted after a relatively small amount of penetrative deformation. Depending on the competency contrast between the basement and cover, the nature of basement, and the physical conditions of deformation, the deformation zone may also propagate downward into the basement. Gently dipping backlimb and frontlimb panels are related to movement of the hanging wall over synclinal and anticlinal bends in the major fault, respectively. Many basement faults are characterized by a number of synclinal fault bends within the basement, which result in long and gently dipping backlimbs. Forelimb panels are related to anticlinal bends that typically occur at the basement-cover interface, as well as at one or more locations in the sedimentary cover. Case studies of well-constrained examples of structures from the Bighorn and Uinta basins and the Central Basin platform, demonstrate the development of these characteristic features and their strong dependence on the mechanical stratigraphy. These models and case studies will be useful in interpreting foreland basement structures in areas with poor or limited data.