Abstract:

The Ordos Basin is not only an important uranium mineralization province but also a major producer of oil, gas and coal in China. The genetic relationship between uranium mineralization and hydrocarbons has been recognized by a number of previous studies, but it has not been well understood in terms of hydrodynamics of basin fluid flow. In a previous study we have demonstrated that the preferential localization of uranium mineralization in the upper part of the Jurassic strata may have been related to the interface of an upward flowing, reducing fluid and a downward flowing, oxidizing fluid, and that this interface may have been controlled by the interplay between the fluid overpressure which was related to disequilibrium sediment compaction and drove the upward flow and the topographic relief which drove the downward flow. In the present study, we carried out numerical modeling for the contribution of oil and gas generation to the development of fluid overpressure, in addition to sediment compaction and heating. Our results indicate that when hydrocarbon generation was taken into account, fluid overpressure during the Cretaceous was more than doubled in comparison with the simulation when hydrocarbon generation was not considered. Furthermore, the fluid overpressure dissipation after the ceasing of sedimentation slowed down relative to the nonhydrocarbon generation case. These results suggest that hydrocarbon generation may have played an important role in uranium mineralization, not only in providing reducing agents required for the mineralization, but also in contributing to the driving force to maintain the upward flow against the pushing of topographydriven, downward flow, thus helping stabilize the interface between the two fluid system and the localization of uranium mineralization.

Key words: Ordos Basin, uranium deposits, hydrodynamics, hydrocarbon generation, fluid overpressure, fluid flow, numerical modeling