Thermal Anomalies Around Evolving Salt Sheets

ABSTRACT The thermal conductivity of salt is about a factor three larger than that of sediments at sediment surface temperatures. The increase of sedimentary thermal conductivities with burial and compaction, and the rapid decrease of salt thermal conductivity with increasing temperature, both imply that the focusing and defocusing of heat a round salt sheets depends on (i) sub-surface depth of salt; (N) the shape of a salt body and (iii) regional temperature gradient and mud-line temperature. Examples are given illustrating the impact of all of the above factors for different salt shapes and burial depths, with emphasis on the thermal conditions under mobile sheets connected to a feeder salt stem. Generally, thermal anomalies provide a cooler regime under salt sheets relative to a regional picture (with about 30°C anomaly possible under extreme conditions)provided the salt top is shallower than about 5 km and/or the salt sheet is thinner than about 5 km. For deeper buried salt sheets and/or thicker sheets, the sediment thermal conductivity is larger than that of salt, thereby reversing the trend of salt as a thermal focusing region. The sub-salt thermal regime is then warmer than regionally. This variation of the thermal regime under evolving salt sheets has impacts on hydrocarbon generation and on retention in the oil phase rather than to gas conversion, both of which are discussed. INTRODUCTION The temperature distribution in the subsurface through time has an impact on the generation of hydrocarbons. Shallow, present-day temperature anomalies are important for exploitation of geothermal energy. Therefore, it is of interest to model the variation of the temperature from the regional picture and to discuss the causes of the variation. The thermal conductivity of salt is a factor of two to three times higher than that of typical sediments. Salt structures often display large vertical relief and so provide a path of low thermal resistance for the conduction of heat from depth to the surface. Thus heat tends to be focused through an uprising salt structure at the expense ofsurrounding basal sediments. The focused heat re-enters the sediments near the apex of the salt structure so that sediments close to the apex are warmer than sediments far from the salt (the regional domain), while sediments close to the salt base and in the rim syncline are cooler than the regional domain. Several examples of temperature calculations and observations around simple geometrical shapes are found in the literature (Selig and Wallick, 1966; Geertsma, 1971; Von Herzen et aI., 1972; Vizgirda et aI., 1985; Jensen, 1983, 1990) where different numerical methods have been used to calculate the temperature field around axis symmetric salt structures. An analytic approach was developed by O'Brien and Lerche (1987) to calculate the temperature field around a cylindrical salt dome. The novel quantitative procedure of this paper uses a model procedure for evaluating evolving salt shapes through time (Lerche and Petersen, 1995). The salt shapes are thus known at each instant of time once thecombined evolution of salt and sediments is modeled.