Integrating kinematic restoration and forward finite element simulations to constrain the evolution of salt diapirism and overburden deformation in evaporite basins

Abstract

In evaporite basins, salt deformation including inflation, diapirism, and salt canopy emplacement is inherently non-coaxial and ductile and thus it presents challenges for two-dimensional kinematic restorations that rely on line-length and area-balancing assumptions. Also, because salt flow and the resulting deformation of adjacent cover units can be driven by temporally and spatially transient salt pressure gradients, kinematic restorations are generally unable to predict the magnitude and distribution of subseismic deformation that results from a particular structural scenario. Here, we use a case study from the Atwater fold belt, Gulf of Mexico, to test a new workflow that involves comparison of kinematic restoration models with forward numerical (finite-element) models of structural evolution to examine the physical validity of solutions derived from the kinematic restorations and to determine the nature and spatial distribution of the resultant subseismic deformation. In the Atwater fold belt, which represents the downdip portion of a linked updip (landward) extensional-downdip (seaward) contractional system, our kinematic restorations indicated that major anticlines likely result from early short wavelength folding followed by (1) inflation of the autochthonous salt to drive failure of the overburden, (2) collapse of the updip limb of the major salt-cored anticline as the salt evacuates updip, and (3) rapid emplacement of the allochthonous salt canopy. In margin scale finite element models of the same system, progradation of the sedimentary wedge above the weak salt substrate leads to basinward migration of the salt and produces inflation of the major downdip salt-cored folds, as predicted by the kinematic model. However, in relatively strong overburden materials (equivalent friction angle = 32°), such salt flow only sustains inflation of the anticlines and is unable to reproduce the interpreted collapse of the anticlinal backlimb or emplacement of the salt canopy. Alternate model runs that include a significant reduction in material strength (equivalent friction angle = 18°) allow salt in the anticlinal crest to drive both reactive and active diapirism and ultimately lead to rapid emplacement of allochthonous canopies. In all of these models, diapirism drives substantial seismically-resolvable and subseismic deformation of wall rocks. Additionally, these models clearly show that the stress field, and particularly the K value (horizontal-vertical stress ratio) of the sediments adjacent to salt structures used for estimating stress magnitudes for drilling predictions, is fundamentally dependent on what point along the evolutionary path from autochthonous salt, to diapir, to salt sheet, that each structure resides. These results highlight the need to test complex kinematic restorations with physics-based techniques. Additionally, they demonstrate that integrating kinematic restorations with these finite element solutions can substantially increase our ability to predict both subseismic reservoir damage in sediments adjacent to salt structures and the K values used for forecasting drilling conditions, particularly in young basins filled with poorly consolidated sediments.