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A Geostatistical Approach for the Modelling of Earth’s Heat Flow

M. Dalla Rosa¹, D. Renard², and P. Ruffo¹
¹GEBA, Basin Geology Department, Eni S.p.a., E&P Division, Italy
²Centre de Géosciences, MINES ParisTech, France

Heat flow plays a major role in the context of Petroleum System Modeling and the understanding of its structural and thermal evolution represents one of the key factors in the assessment of the exploration risk associated to basin potential prospects. In fact, Earth's evolution reflects the history of heat transfer from the interior, due to plate tectonics and conduction through lithosphere. In particular, the role of heat flow is crucial for the determination of temperature history, which is one of the most influencing controlling factors in the maturation of organic compounds into fossil fuels. Hence, understanding Earth's thermal history is critical in the scope of an oil exploration strategy.

In order to reconstruct the thermal state of the basin, the modeling procedure needs three main information inputs: thermal properties of sediments and of water (conductivity and specific heat), the surface temperature and the heat-flow at the base of the sedimentary sequence. While sediments characterization can be derived thanks to petrophysical studies and surface temperature can be assigned on the basis of paleo-climate modeling, heat flow at the base of the sedimentary sequence is probably one of the most difficult input to provide in the modeling procedure (also one of the most critical for the results). It is defined by a heat flow map for each time step of the evolving basin model. As direct measurements of heat flow are seldom available, the usual approach is based on a trial-error scheme. In practice the idea is to make a hypothesis on the heat flow value at each known well of the basin. This is used as border condition for the temperature modeling. Once the simulation has been performed, the present-day temperatures can be computed along each well. It is therefore possible to compare the simulated temperatures with the measured ones to check if the initial hypothesis on the heat flow is correct and the chosen value is effectively significant for temperature evolution reconstruction. If the fitting is not satisfactory, another value for heat flow is proposed and the simulation is performed again. At the end of this procedure, we get the set of heat flow values at wells, which can better explain the temperature records. This is the first data source of present-day heat flow. The second source comes from thermo-tectonic modeling. The quantification of heat flow history during basin subsidence can be addressed by forward modeling of tectonic and thermal processes associated to the evolution of the lithosphere involved in extensional deformation. A numerical approach based on finite elements is devoted to this purpose (TECMOD2D by Geomodelling Solutions). It provides reconstructions of thermal history of sedimentary successions deposited in extensional basins along 2D sections. The inputs required by the procedure are present-day stratigraphy and thinning factors, respectively for crust and lithosphere. They are defined as the ratios between initial thickness and final thickness of crust and lithosphere. In this way it becomes possible to assess the heat flow and temperature evolution through time in frontier areas characterized by a lack of well and sub-surface data. Thermo-tectonic modeling is therefore useful in situation when measurements are scarce. The output of such numerical modelling constitutes the second data source.

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012