Stress of Salt Diapir Reactivation in an Oblique Extensional System: A 3-D Geomechanical Modeling Approach

Abstract

Faulting associated with salt movement is a widespread phenomenon in salt provinces worldwide. The presence of salt bodies (e.g., diapirs, domes, canopies) strongly influences the in-situ stress field in surrounding sediments, controls the structural pattern, and has important implications for the oil industry in terms of well-bore stability issues. Oblique extension/rifting deformation is also a common feature that produces unique structural partitioning and en echelon fault arrays, which are usually controlled by oblique extension to a pre-existing weak zone. The Vulcan Sub-Basin of NW Australia provides an excellent natural example of integrating these processes together, with a salt diapir reactivated in an oblique extensional system. 3D seismic interpretation and scaled physical modeling have been utilized to study the geometry and kinematics of this multi-mechanical-component system. However, the stress status involved in this example is not fully understood. In this study, we attempt to reveal how pre-existing fabrics and salt diapir would affect the stress field and interact with each other by designing a series of 3D finite element geomechanical models. The weak zone is oblique to the extension direction (obliquity=37 °). A columnar salt diapir is placed in the center of the weak zone. A sedimentary cover is put on these models. We model the salt as a linear visco-elastic body. Elasto-plastic constitutive laws are applied to sedimentary rocks. Three individual models have been built for comparison (i.e., oblique system only, salt only, and both). The modeling results show that a pre-existing weak zone can introduce stress partitioning during oblique extension. We also demonstrate that stress fields close to the salt diapir are highly altered by salt body. Strong stress anisotropies are observed around salt diapir as a result of salt body's stress relaxation effect as well as the stress-concentration effect during extension. Shear stresses around salt diapir increase in areas perpendicular to the extension, and meanwhile, stresses decrease in areas parallel to extension direction. These are responsible for the normal fault arrays revealed in subsurface mapping and physical modeling results of Vulcan Sub-basin. The models presented not only help us better understand this unique geology phenomenon by providing insights into stresses in oblique extensional system and around salt diapirs, but also can provide valuable implications for hydrocarbon E&P activities around salt bodies in structurally active areas.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015