Geomechanical Forward Modeling as a Trap-Seal Risking Tool in Rifted Margin Salt Tectonics: Applications in a Layered Evaporite Sequence (LES), Red Sea

Abstract

Layered Evaporite Sequences (LES) in petroliferous salt basins offer unique challenges in structural interpretation, assessment of trap/seal timing and drilling hazards. In the Red Sea, the LES comprises alternating layers of salt which deforms by creep, and anhydrite and other sediments which deform predominantly by frictional-plastic processes. In an LES, the seal risk is low primarily due to the presence of salt. However significant uncertainties exist in the assessment of trap timing and reservoir integrity within the intercalated sedimentary layers of the LES. Traditional kinematic techniques and restoration algorithms, which are better suited for salt structures composed of massive halite, cannot be directly applied to LES. Geomechanical forward models (GFMs) can complement existing kinematic techniques and strengthen understanding of the impact of mechanical heterogeneity on deformation in an LES. We present a 2D plane-strain, prospect-scale GFM of a turtle structure in an idealized LES. The models use viscoplastic rheology for salt and a cap-plasticity-based rheology to model compaction and dilational failure in sediments. Gravity, sediment and thermal loading are imposed on a pre-existing basal halite and sediment layer and the impact of competition among these on the structural style is systematically investigated. Model results suggest that the initiation of diapirism in the basal halite layer and the eventual formation of the turtle structure are primarily dependent on the relative proportions of salt and sediments in the LES, salt mobility, sediment loading and base-salt slope. In the LES, deformation within the sediment layers decreases with increasing salt as the latter preferentially accommodates the strain from prospect-scale down-slope gravitational failure of the system. Deformation within the LES sediments is highest near the salt diapir, where significant folding can occur, followed by the dipping portions of the reservoir units. The orientations and relative magnitudes of the principal stresses suggest that the local stress field in the turtle structure may not be reconciled with standard assumptions of regional stress fields used pre-drill in rifted margin salt basins. GFMs can provide a physics-based assessment of potential trap/seal timing in an LES environment and can reduce capital costs in offshore wells by providing directional guidance on the reservoir integrity and in-situ stresses as a function of the structural position.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017