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The Gulf of Lions (northwestern Mediterranean) is a young Oligocene-Miocene rifted basin characterized by the presence of a thick evaporitic sequence. Observed data (surface heat flow, well temperatures, geopressures below the evaporites, crustal thinning) are used to model (1) the basin-forming mechanisms, and (2) the intrasedimentary processes that influence the temperature history, and thus, the maturation of source rocks.
A two-dimensional finite-difference basin model, Temispack, simulates the sedimentation, compaction, and overpressures related to the low permeability of the post-rift sequence, the heat transfer from the rifted lithosphere into the sediments, and the level of maturation by using a kinetic approach.
The classic passive margin model, which relates the history of heat flow and the rate of subsidence to a uniform thinning factor, yields subsidence and heat flow predictions that agree with observations. The recently accelerated subsidence observed in the deep margin is probably an effect of recent regional compression rather than the consequence of alternative rifting mechanisms. The history of maturation of organic matter was influenced by the variation of the regional heat flow due to the rifting, the effect of sedimentation (or blanketing), and the variability of the lithologies and porosities. The undercompaction of sediments underlying the evaporites is accounted for by the model, which suggests that hydraulic fracturing is taking place at present in the overpressured sediments. Fluid flow associated with the compaction disequilibrium has negligible thermal effect. Undercompacted shales below the evaporites have lower thermal diffusivity and thus higher temperatures, which has a significant effect on maturation.
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