Fluid flow and heat transfer in sedimentary basins

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Abstract One of the major assumptions underlying the science of geothermics over the last 100 years has been that heat is transported through the Earth’s crust only by conduction, except in the immediate vicinity of active volcanoes. In old shields this assumption may be justifiable, but in sedimentary basins the ability of water to move through permeable aquifers provides a means of heat transport that may under certain conditions be as effective as conduction. The thermal history of a basin depends on its tectonic origin and the circumstances of its development. The dissipation of excess heat associated with basin formation and the transfer of the continuous heat supply from the basement depend on the thermal properties of the strata and their water content. In some basins widespread contrasts of heat flow are observed, both laterally and vertically, suggesting forced convection by water migration. In basins where part of the surface is above sea-level a hydrological drive may be inferred from the topographic surface. In sub-sea basins this drive is not available in the same form. Nevertheless, patterns of heat-flow variation have been observed for which some form of water flow seems to be the most probable explanation. A complete quantitative description of the present thermal state has not yet been developed for any basin. Some two-dimensional numerical models have been developed, but, since flow directions may vary between aquifers, three-dimensional models must be the ultimate objective. There is scope to extend such models to the geological past by using palaeotemperature indicators such as vitrinite reflectance and fission-track measurements. So far we have been able to establish only an understanding of the principles and processes that control the thermal state of basins, so that detailed description of small parts or two-dimensional models may be developed.