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Abstract: Thermal Anomalies in Sediments near Evolving Salt Structures: Importance for Modeling Timing of Oil Generation

K. Petersen, I. Lerche

The temperature distribution in the subsurface through time 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 two to three times higher than that of typical sediments. Salt structures often display large vertical reliefs 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 of surrounding basal sediments. The focused heat reenters the sediments near the apex of the salt structure so that sediments at the salt apex are warmer than the sediments far away from the salt structure, while sediments close to the salt base and in the secondary rim synclines are cooler than the sediments far away from the salt.

A novel quantitative procedure enables modeling of the combined evolution of salt and sediments. Once the evolving salt shape is modeled, the impact of the salt on the temperature distribution through time can be calculated. The relative effects of the height and width of the salt structures in producing significant anomalies are 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 compared to the modeled structural evolution of migration pathways and trap development and thus provide an integrated picture of the ynamic evolution of salt and sediments. The presented examples illustrate the importance of modeling the thermal history for reducing the exploration risk near salt structures.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994