The Stratigraphic Record of Minibasin Subsidence, Precaspian Basin, Kazakhstan

Abstract

Minibasins are fundamental components of many salt-bearing sedimentary basins, where they may host large volumes of hydrocarbons. Although we understand the basic mechanics governing their subsidence, we know surprisingly little of how minibasins subside in three-dimensions over geological timescales, or what controls such variability. Such knowledge would improve our ability to constrain initial salt volumes in sedimentary basins, the timing of salt welding, and the distribution and likely charging histories of suprasalt hydrocarbon reservoirs. We use 3D seismic reflection data from the Precaspian Basin, onshore Kazakhstan to reveal the subsidence histories of 16, Upper Permian-to-Triassic, suprasalt minibasins. These minibasins subsided into a Lower-to-Middle Permian salt layer that contained numerous relatively strong, clastic-dominated minibasins encased during an earlier, latest Permian phase of diapirism; because of this, the salt varied in thickness. Suprasalt minibasins contain a stratigraphic record of symmetric (bowl-shaped units) and then asymmetric (wedge-shaped units) subsidence, with this change in style seemingly occurring at different times in different minibasins, and most likely prior to welding. We complement our observations from natural minibasins in the Precaspian Basin with results arising from new physical sandbox models; this allows us to explore the potential controls on minibasin subsidence patterns, before assessing which of these might be applicable to our natural example. We conclude that due to uncertainties in the original spatial relationships between encased and suprasalt minibasins, and the timing of changes in style of subsidence between individual minibasins, it is unclear why such complex temporal and spatial variations in subsidence occur in the Precaspian Basin. Regardless of what controls the observed variability, we argue that vertical changes in minibasin stratigraphic architecture may not record the initial (depositional) thickness of underlying salt or the timing of salt welding; this latter point is critical when attempting to constrain the timing of potential hydraulic communication between sub-salt source rocks and suprasalt reservoirs. Furthermore, temporal changes in minibasin subsidence style will likely control suprasalt reservoir distribution and trapping style.

The competition for salt and kinematic interactions between minibasins during density-driven subsidence: observations from numerical models

“…The bowl-to-wedge transitions in the stratal geometries of minibasins has previously been linked to the basal welding of minibasins (Rowan and Weimer, 1998). Our numerical models illustrate that this interpretation may not be appropriate in all cases, as pre-welding tilting of minibasins can occur due to the kinematic interactions between minibasins (see also Jackson et al, 2019).…”

“…As a result of the strong coupling between minibasin subsidence and sedimentation, changes in subsidence style are recorded by synkinematic stratal packages within minibasins (e.g. Giles and Lawton, 2002;Prather, 2003;Giles and Rowan, 2012;Sylvester et al, 2015;Jackson et al, 2019).…”

Four‐dimensional Variability of Composite Halokinetic Sequences

“…During the Upper Permian, sedimentation was dominated by westward progradation of a non‐marine clastic wedge comprising material shed off the rising Ural Mountains. This wedge loaded and expelled salt westward and up into rising diapirs (Volozh et al, 2003); and resulted in development of broadly margin‐parallel, N‐oriented salt walls and expulsion rollovers near the eastern basin margin (Duffy et al, 2017; Jackson et al, 2019).…”

“…Several Late Permian minibasins up to 1‐km‐thick containing evaporites and non‐marine clastics subsided into and are now being fully or partially encased in Lower‐Middle Permian salt (Figure 2c; Fernandez et al, 2017). A subsequent generation of (supra‐salt) minibasins formed by load‐driven subsidence and passive diapirism during the Late Permian to Triassic (Duffy et al, 2017; Jackson et al, 2019). These minibasins are up to 10 km in diameter and up to 5.5‐km deep, being typically welded to the pre‐salt interval and/or to the encased minibasins (Duffy et al, 2017; Jackson et al, 2019).…”

Mass-transport complexes (MTCs) document minibasin subsidence patterns and diapir evolution in the northern Gulf of Mexico

“…which mainly record post-welding aggradation of 267 sediment above Minibasin 5 and its flanking diapirs) during Stage 3 reflects the high sediment 268accumulation (and possibly supply) rate at this time Rowan and Weimer (1998). also269 interpreted that layer-shaped packages reflect relatively long-wavelength subsidence across 270 now-welded minibasins (see also the layer-shaped package fromJackson et al, 2019). 119 km 2 and 25 km 3 ) is laterally and frontally confined by salt diapirs(Figure 10a, b).275 It is 160-190 m thick, and its NW-SE-striking, south-western lateral margin defines a sharp 276 erosional contact between remobilised sediments (SFc3) and undeformed slope sediments 277 (SFs1 and SFs2)(Fig.…”