A method for estimating the in situ thermal conductivity at basin scale from various well logs is presented. This method is based on the use of the geometric mean model for estimating the thermal conductivity of the different lithofacies of a given well; the various well logs are interpreted in terms of lithologic content and porosity; a mineralogic composition varying with the stratigraphy is assumed for each lithologic unit and a procedure for computing the in situ thermal conductivity profile is proposed. From there, the variations of the average thermal conductivity in each formation - obtained as an harmonic average over the depth - can be mapped at basin scale. The procedure is applied to a number of wells in the Viking graben (North Sea) and the resulting conductivity structure is compared to the present thermal structure.
Thermal conductivity of sedimentary rocks is a very important parameter for basin modelling: its value directly controls the vertical distribution of temperature. Unfortunately, in oil exploration boreholes, core samples are seldom and laboratory measurements of conductivity difficult to perform. Therefore, the thermal conductivity has to be estimated from the available, routinely recorded information such as well logs. The possibility of deducing thermal conductivity from well logs has been studied by various authors. A first approach is to define an empirical relationship between thermal conductivity and one or several well logs (Houbolt & Wells, 1980, Vacquier et al, 1988). However such an empirical relation has no physical basis and should be calibrated for each situation (Brigaud et al, 1989). An alternative approach, proposed by Brigaud et al (1989) and developped here, makes use of a composition model - the geometric mean model expressing the thermal conductivity of a multicomponent sedimentary rock as a function of its composition and structure. In this later case, the well logs are first interpreted in terms of lithologic composition and porosity; the available mineralogic information is also used for estimating the conductivity of the various lithologic end-terms. A vertical profile of thermal conductivity can then be computed using the relevant composition law, i.e. the geometric mean model. Although this later method is somewhat more laborious than a direct regression vs. well logs, it has the advantage of taking into account the actual mineralogic composition which is difficult to approach using well logs data alone. The results of this method have been found to yield consistent thermal conductivity estimates when compared with laboratory measurements (Brigaud et al, 1989) or to detailed temperature loggings. The purpose of this paper is to present the various aspects of a method designed for obtaining the detailed in situ thermal conductivity structure at basin scale. Then the procedure for estimating in situ thermal conductivity at the formation scale, using well logs as well as complementary information, is described in details. Finally, this technique is applied to a data set in the Viking graben in the North Sea; the conductive structure is described and compared with the thermal structure.