Jeffrey A. Nunn¹

(1) Louisiana State University, Baton Rouge, LA

ABSTRACT: Flexure, Bending Stresses, and Fluid Migration in Foreland Basins

Foreland basins develop adjacent to mountain belts as a result of flexural downwarping and rapid sedimentation associated with uplift, thrusting, and erosion during orogeneis. Flexural compensation of surface loads in mountain belts and adjacent foreland basins has several effects that are potentially important in understanding hydrocarbon migration. First, flexural compensation acts as a buffer in development of topography. During accumulation/removal of surface masses, flexural compensation causes the surface to move downward/upward, thus reducing the net increase/decrease in topography. This effect has been ignored in some fluid migration models of foreland basins and thus paleotopography (and topography-driven fluid migration) has been overestimated. Instead, flexural compensation causes strata to tilt towards the fold-thrust belt during orogenesis. Subsequent erosion causes the tilt angle to decrease. These changes in dip angle with time can effect buoyancy-driven hydrocarbon migration. Flexural compensation also produces a bulge or upwarp at the edge of the foreland basin. A basement upwarp may act as structural trap and/or focus fluids upwards into a trap. Finally, flexure of the elastic portion of the lithosphere can generate tensional bending stresses in the vicinity of the flexural bulge. These stresses can exceed the tensile strength of rocks and should produce fractures and faults, which may act as pathways for fluid migration. Moreover, upwards movement of hot fluids out of the underlying basement may generate additional heating of overlying sediments and thus alter the extent or timing of thermal maturation. Using a two-dimensional profile across the Arkoma basin, where the thickness of eroded sediments can be estimated from coal rank, these four effects are quantitatively evaluated.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado