Investigating controls on salt movement in extensional settings using finite-element modelling

Abstract

Salt structures present numerous challenges for targeting reservoirs. Salt movement within the subsurface can follow complex pathways, producing deformation patterns in surrounding strata which are often difficult to decipher. Consequently, the relative role of key salt flow drivers and geological sensitivities on salt structure evolution are often poorly understood. To address this, we have developed 2D geomechanical models using the finite element method to simulate salt diapir and pillow development in two extensional tectonic settings. We conducted model sensitivity analyses to examine the influence of geological parameters on field-scale salt structures and their corresponding deformation pattern. Modelled diapirs developing in thinskinned extensional settings closely resemble published analogue experiments, however active and passive stages of diapir growth are seldom or never reached, respectively, thus challenging existing ideas that diapir evolution is dominated by passive growth. In all modelled cases, highly strained domains bound the diapir flanks where extensive small-scale faulting and fracturing can be expected. Asymmetric diapirs are prone to flank collapse and observed in models with fast extension or sedimentation rates, thin roof sections or salt layers, or initially short or triangular shaped diapirs. In modelled thickskinned extensional settings, salt pillows and suprasalt overburden faults can be laterally offset (decoupled) from a reactivating basement fault. This decoupling increases with increased salt layer thickness, overburden thickness, sedimentation rate and fault angle, and decreased fault slip rates. Contrary to existing consensus, overburden grounding onto the basement fault scarp does not appear to halt development of salt structures above the footwall basement block.

Geologically constrained evolutionary geomechanical modelling of diapir and basin evolution: A case study from the Tarfaya basin, West African coast

“…Despite its low value, this shear stress is able to mobilize the salt, given the salt's average viscosity. The relatively low strain rates and low upper crustal temperatures used in this study yield an average salt viscosity between 10 17 and 10 19 Pa·s, consistent with typical values reported for salt rocks (Hamilton‐Wright et al., 2019; Marketos et al. 2016; Rowan et al.…”

“…Evolutionary geomechanical (forward) modelling can simulate time-dependent processes, such as deposition, tectonic loading, salt flow and porous fluid flow. Hence, it couples the deformation and strength of sediments with the development of geologic systems (Goteti et al 2012;Hamilton-Wright et al 2019;Thigpen et al 2019). The principal limitation of the evolutionary models is the difficulty in producing the observed present-day geometry (Nikolinakou et al 2014).…”

“…2012; Hamilton‐Wright et al. 2019; Nikolinakou et al. 2018; Thigpen et al.…”

External signal preservation in halokinetic stratigraphy: A discrete element modeling approach

“…Finite-element modeling (FEM; models based on continuum methods) has enhanced our understanding of salt flow (Gemmer et al, 2005;Hamilton-Wright et al, 2019), minibasin dynamics (Fuchs et al, 2011;Fernandez et al, 2020), and stress evolution (Nikolinakou et al, 2018). FEM treats the overburden as continuous frictional-or viscous-plastic material, which is useful for studying ductile deformation, but it prevents the development of brittle structures.…”

Halokinetic modulation of sedimentary thickness and architecture: A numerical modelling approach

“…Most recent advances in the numerical modelling of salt‐related deformation use finite element models (FEM), which are based on continuum methods. Such studies have focused on the physical conditions required for the initiation and development of diapirism (Chemia et al., 2008; Fernandez & Kaus, 2015; Fuchs et al., 2011; Gemmer et al., 2004, 2005; Hamilton‐Wright et al., 2019; Nikolinakou et al., 2017; Peel et al., 2020; Poliakov et al., 1993), the stratigraphic architecture of subsiding minibasins (Fernandez et al., 2020; Sylvester et al., 2015; Wang et al., 2017), reconstructing the evolutionary history of salt‐affected stratigraphy (Ismail‐Zadeh et al., 2001, 2004; Pichel et al., 2017, 2019), and salt‐related stress (and strain) analysis (Heidari et al., 2017; Luo et al., 2012, 2017; Nikolinakou et al., 2012, 2014a, 2014b, 2018). FEM treats the overburden as a continuous frictional‐plastic or viscous‐plastic material, which prevents the development of realistic brittle deformation, for example, fracturing and faulting, in overburden stratigraphy (Figure 1).…”