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Salt pillows and localization of early structures: case study in the Ucayali Basin (Peru)

Abstract In many compressive zones, there is a risk of undercharged hydrocarbon prospects as a result of timing, that is, the growth of the structure is younger than the main fluid migration phase. The North Ucayali Basin represents a setting of this type, where locating the earliest structures is crucial for well placement. In the North Ucayali Basin, the variable amount of erosion at the top of the structures shows that they are not uniformly recent. Although the growth of early structures may be explained by the reactivation of inherited features during shortening, evaporitic pillows may represent an alternative factor controlling the localization of deformation in the studied area. Indeed, subsurface data reveal the presence of flat salt domes that have an influence on thrust localization. The existence and tips of such efficient gliding surfaces concentrate the strain and, as a result, localize the early zones of deformation. Analogue models designed to study these phenomena highlight the crucial role of salt pillows as potential weak zones localizing deformation ahead of the propagating wedge. These models also emphasize the need to constrain certain parameters that have been previously disregarded, including the thickness of the brittle layers between the main décollement level (if present) and the pillows, as well as their burial depth.