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Radial Fractures and Ring Faults in Sediments Overlying Layered Evaporite Sequences During Active Salt Diapirism: Insights From Geomechanical Forward Modeling

Abstract

Subsurface faults and fractures in sediments adjacent salt bodies are ubiquitous and a comprehensive understanding of these structural heterogeneities is important to assess their critical role in the entrapment and migration of fluids. The controls of hydraulically conductive faults are particularly evidenced in salt basins where the deposition and flow of thick layered evaporite sequences (LES) have affected the structural styles of overlying sediments. These include the development of radial and ring faults that could compartmentalize hydrocarbon reservoirs and either breach seals or promote the migration of fluids through reservoirs and seal units. However, understanding the dynamics forming these faults and the control of their geometry during active diapirism remains unclear due to rare surface exposures, limitations of 2D physical modeling techniques to define 3D fault patterns, and uncertainties in 3D seismic velocities when imaging these intensely deformed zones. To better understand the mechanism driving the formation of these faults, a forward numerical modeling approach based on 3D finite element analysis is utilized. The modeling approach uses a Lagrangian framework for accurate description of material interfaces to simulate the emplacement of a salt stock triggered by upbuilding or gravitational loading. The LES are modeled as visco-elastic and overlying sedimentary overburden as elasto-plastic rheologies respectively. The numerical results show radial faults formed by outer arc extension (roof stretching) in sedimentary roofs above rising salt stocks. Circumferential/hoop strain due to the expanding salt stock (bulb widening) tend to propagate the initiated radial fractures. Concentric ring faults tend to form due to salt withdrawal to higher structural levels as the diapir stem constricts. Understanding the mechanisms generating these structural compartments, fracture occurrence, orientation and density in sediments adjacent to salt structures could enhance the economic potential and feasibility of hydrocarbon prospects or plays beneath allochthonous and autochthonous salt structures.