PRESTHOLM, E., and W. FJELDSKAAR, Rogaland Research, Stavanger, Norway

ABSTRACT: The Effect of Thermal Conductivity Anisotrophy on Petroleum Potential Evaluation in Sedimentary Rocks

Temperature gradients in individual layers of a sedimentary basin are controlled by thermal conductivity of the rock sequence. Detailed knowledge of the thermal conductivity of the rock sequence thus is necessary for a successful reconstruction of the time-temperature history of the paths of heat transport in a sedimentary basin.

A database of measurements on sedimentary rock has been established and contains about 300 rock samples, mainly from the Norwegian Continental Shelf. The vertical and horizontal thermal

conductivities have been measured by a needle probe technique. The conductivity values measured differ from the standard values given in the literature. The estimated horizontal thermal conductivity is somewhat higher than the isotropic value from the literature, whereas the vertical is much lower. The resulting conductivities are anisotropic, with increasing values for decreasing porosity, up to 1.8 in shales.

The measured conductivity values result in temperature gradients that are higher than those obtained from simulation using the standard conductivity values from the literature. Calibration with well data may reduce the errors of the estimated temperature regime introduced by the use of literature values of thermal conductivity. However, calculation shows that even with a well control at 3 km depth, the difference in the estimated temperature at 5-6 km is 12-15 degrees C, which is significant considering the following facts. Transformation from kerogen to oil in a good source rock takes place over a depth range of approximately 1 km. The expulsion efficiency during the early phase of petroleum generation is mainly controlled by diffusion in kerogen matrix and probably is much lower tha the expulsion efficiency encountered at the stages where a significant degree of oil saturation has occurred and the expulsion is a pressure-driven petroleum phase movement. Hence, the depth range of extensive petroleum expulsion is approximately 500 m or in a temperature range of 15 degrees C. Erroneous temperature predictions by 10-15 degrees C thus may be fatal to the evaluation of the petroleum potential of a particular prospect.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.