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Variation of seawater chemical composition during the Phanerozoic is still a contentious subject. Br concentration in primary marine basal halites was used to indicate variations in seawater composition during the past 550 Ma. Evaporation experiments and thermodynamic modelling suggests that the partition coefficient of Br in halite depends on the composition of the seawater. A combination of this correlation with the time‐dependent models for ocean chemistry of Hardie leads to a prediction of Br in basal halite during the Phanerozoic. This prediction is in good agreement with data from different ancient evaporite basins. The Br data give evidence for rapid and massive changes in seawater chemistry during the Phanerozoic and are thus a first independent test for the Hardie model.

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra-salt 'stringer' reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self-charging hydrocarbon system within partly highly overpressured carbonate bodies ('stringers'). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post-dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re-precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack-seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.

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