Salt flows downslope, irrespective of overburden. In salt basins on passive margins, the salt will tilt and flow towards the ocean immediately after continental rifting has ended due to thermal subsidence. Using real examples, as well as physical and numerical models, tilting is shown to be relatively rapid, enhanced by isostatic rebound updip and loading downdip where salt pools and inflates behind an outer high. In the Santos, Campos and Kwanza basins, this outer high is represented by an embryonic mid-Atlantic ridge, amplified in height by the differential weight of the inflating salt. Widespread extension and translation of overburden, utilizing both seaward- and landward-dipping normal faults, characterizes the early evolution of the inboard region. Inflation and contraction occur outboard, the effects of which tend to expand in a landward direction over time. Rapid accumulation of salt implies wholesale dewatering of pre-salt sediments, the water possibly permeating the salt once it has reached a burial depth of c. 3 km. The process of thermal subsidence, salt drainage and isostatic amplification is an efficient mechanism for moving sediment on passive margins tens of kilometres seaward during a relatively short period and helps explain why great thicknesses of salt can accumulate there in the first place.

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.

No abstract