Quantification of thermal anomalies in sediments around salt structures

Abstract

The temperature distribution through time in the subsurface has an important impact on the generation of hydrocarbons. It is therefore of interest to model the spatial variation of the temperature through time and 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 paths 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 of surrounding basal sediments. The focused heat re-enters the sediments near the apex of the salt structure, so that sediments around the salt apex arc warmer than the sediments at the same depth far from the salt structure, while sediments close to the salt, and in the secondary rim synclines, are cooler than sediments far from the salt at the same depth. A novel quantitative procedure enables modeling of the combined self-consistent evolution of salt and sediments. The impact of the salt on the temperature distribution through time can then be calculated. The relative effects of the height and width of the salt structures in producing significant anomalies arc demonstrated through different examples. The vertical extension of the oil window near the salt promotes earlier onset of maturation in sediments near the salt apex and delays conversion of trapped oil to gas in the deeper sediments when compared to the regional picture. Knowledge of the spatial temperature history can then be incorporated with that of the modeled structural evolution of migration pathways and trap development, thus providing an integrated picture of the dynamic evolution of salt and sediments. The examples presented illustrate the importance of modeling the thermal history in order to reduce the exploration risk near salt structures.