Geological and geophysical expression of a primary salt weld: An example from the Santos Basin, Brazil

Abstract

Primary salt welds form at the base of minibasins in response to complete evacuation of autochthonous salt. Analytical and numerical models suggest it is difficult to completely remove salt from a weld by viscous flow alone, which is especially true in multilayered evaporites, within which flow is likely heterogeneous due to lithologically controlled viscosity variations. Welds are important in the hydrocarbon industry because they may provide a hydrodynamic seal and trap hydrocarbons, or may allow transmission of fluids from source to reservoir rocks. Few papers document the subsurface expression of welds, principally because they have not been penetrated by wells or because the associated data are proprietary. We use 3D seismic and borehole data from the Santos Basin, offshore Brazil to characterize the geological and geophysical expression of a primary weld associated with flow of Aptian salt. The seismic data that we evaluated suggested that, locally, presalt and postsalt rocks are in contact at the base of an Upper Cretaceous minibasin, implying that several apparent welds, separated by low-relief salt pillows, are present. However, borehole data indicated that 22 m of anhydrite, carbonate, and sandstone are present in one of the welds, indicating that this and other welds may be incomplete. We find that seismic data may be unable to discriminate between a complete and incomplete weld, and we suggested that, during the subsurface analysis of welds, the term apparent weld is used until borehole data unequivocally proves the absence of salt. Furthermore, we speculate that preferential expulsion of halite and potash salt from the autochthonous layer during viscous flow and welding resulted in the formation of an incomplete weld, which, when compared with the initial autochthonous layer, is volumetrically enriched in nonevaporite lithologies and relatively viscous evaporite lithologies (anhydrite). The composition and stratigraphy of the autochthonous layer may thus dictate weld thickness and seal potential.

Subsurface expression of a tertiary salt weld, Gulf of Mexico

“…Our data, and that from the Campos (Wagner and Jackson, 2011) and Santos basins (Jackson et al, 2014), thus support the hypothesis of Wagner and Jackson (2011) that viscous flow is a good analytical approximation of the physical processes occurring during salt thinning and welding, but that viscous flow alone is unlikely to result in complete evacuation of a salt layer. However, it is important to note that a borehole, irrespective of the quantity and quality of data it provides, is only a 1D sample point; it is of course possible that, away from this sample point, the weld may be locally complete.…”

“…For example, low viscosity, more mobile lithologies, such as halite and potash salt, occurring in thick autochthonous salt in sufficiently large quantities, may be 11 preferentially expelled from thinning salt before relatively high viscosity, less mobile lithologies, such as carbonate, anhydrite and sandstone. As such, during welding, salt becomes relatively enriched in these less mobile, non-halite/potash salt lithologies, which, due to the effects of boundary drag along the upper and lower salt contacts, becomes trapped in the weld as the salt thinned (compare 'differential purification by movement'; Kupfer, 1968;see also Wagner andJackson, 2011 andJackson et al, 2014).…”

“…Jackson et al, 2014). We used a combination of logs to broadly differentiate between: (i) relatively coarse-grained, likely sandstone-prone lithologies; (ii) relatively fine-grained, likely mudstone-dominated lithologies; and (iii) non-clastic lithologies (e.g.…”

Geomechanical analysis of a welding salt layer and its effects on adjacent sediments

“…1a). Several studies have documented geologic attributes of these welds and their importance to hydrocarbon exploration (Hoetz et al, 2011;Jackson et al, 2014;Maione, 2001;Peel, 2014;Rowan et al, 2012;Wagner, 2010). For instance, Hoetz et al (2011) studied the data from wells drilled through horizontal welds and found a consistent reduction in the porosity of sediments near the welds.…”

Salt thickness and composition influence rift structural style, northern North Sea, offshore Norway

“…Zechstein Supergroup control the structural styles that develop during Middle Jurassic-to-Early Cretaceous 492 rifting (see also Lewis et al, 2013;Jackson and Lewis, 2016). Diapirism is common in hangingwall basins, 493 where autochthonous salt was thick and halite-rich (e.g.…”