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With current exploration efforts in areas of overthrust tectonics, it is important to evaluate efficiently and effectively the effects of thrust-related thermal perturbations on hydrocarbon maturation. Because of the transient nature of the temperature field during and after thrusting, the simple assumption of constant geothermal gradients is inappropriate in most cases. A simple graphical technique that combines Lopatin diagrams with thermal models for thrusting allows the explorationist to develop relatively detailed models for the timing of hydrocarbon maturation in overthrust terrains. The thermal regime that exists after thrusting may be calculated by solving the equation for the one-dimensional conduction of heat. Geothermal gradients after thrusting can then be der ved from solutions to the heat conduction equations that have been plotted on depth versus temperature diagrams. By superimposing the appropriate geothermal gradients on the Lopatin diagram for a given sedimentary unit, the theoretical vitrinite reflectance can be calculated at any point along the burial history of that sediment.
It is critical in these models to modify the geothermal gradients used in the Lopatin diagrams according to the perturbations in the normal gradient caused by the thermal effects of thrust faulting. The effect of thrusting is to cool the overriding sheet and heat the sediments being overridden. The sheet being thrust from deep within the earth's crust to a shallower level will cool and lose heat to the sediments below. Consequently, the overridden sediments will become warmer as they attain equilibrium with the new thermal conditions imposed by burial beneath the thrust plate. The thermal effects due to thrusting can be quite pronounced. A calculated example demonstrates that the same stratigraphic unit located in the hanging wall of a 21,000 ft (6,400 m) thick thrust sheet in the Ove thrust belt is still in the liquid window while the equivalent unit in the footwall of the thrust is overmature.
The graphical approach developed here is applicable not only to cases of simple overthrusting, but can also be modified to include the effects of multiple thrusting events, subsurface thrust planes, and post-thrust erosion. All of these models can provide critical constraints on the timing of maturation and migration as well as information on the degree of maturity of potential source rocks. Integration of maturation data generated from the Lopatin diagrams with the structural history of the region can help predict prior to drilling whether a prospective structure may contain hydrocarbons or if it is more likely to be a dry hole.
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