--> --> A New Look at Inverting Subsidence to Heat Flow in Rift-Related Basins – Deconvolution of Processes and Phases

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A New Look at Inverting Subsidence to Heat Flow in Rift-Related Basins – Deconvolution of Processes and Phases


We present a new methodology to derive the heat flow history in rift-related basins from the subsidence history with significantly better results that are consistent with geological observations and deliver more precise prediction for petroleum systems. Extension related basins generally experienced a syn-rift phase during whith lithospheric stretching and a post-rift phase with mantle cooling, both generating tectonic subsidence. In petroleum systems modeling (PSM) the heat flow through geological time is required to reconstruct the thermal history of the basin. The standard multi 1D workflow to estimate the heat flow evolution in rift basins or passive margins is to invert the basin's subsidence by fitting the observed tectonic subsidence by thickness changes within the lithospheric layers. The resulting stretching factors are used to calculate the heat flow at the base of the sediments throughout the basin evolution. The new inversion method presented in this paper eliminates most limitations of the multi 1D approach. Brittle deformation of the upper crust tends to generate syn-rift highs (horsts) and lows (graben/half-graben). Post-rift subsidence however may just be as high over a high as it is over the low next to it. The post-rift subsidence is controlled by cooling processes within the upper mantle, while the syn-rift subsidence is controlled by processes within the mantle and the crust. Taking these processes into consideration we apply a two-step process: 1) fit the upper mantle stretching to the post-rift subsidence and 2) use the results together with the syn-rift subsidence for the fitting of the crustal stretching. This way we deconvoluted the stretching in both lithospheric layers and gained geologically reasonable inversion results for the respective b-factors. In the Browse Basin case study we compare the new methodology with the traditional one and illustrate significant enhancements to lithospheric modeling results. As the thickness of the upper mantle through geological time defines the distance of its base (1330°C isotherm) to the base of the sediments, our enhanced predictions for mantle stretching and cooling deliver a better correlation between the observed subsidence and the final thermal history for PSM. We illustrate how the new inversion method delivers more confidence in the thermal history and how the timing and location of hydrocarbon generation, migration and trapping can be predicted with higher accuracy.