Free convection has been defined as a flow driven by density variations. The increasing temperatures with depth in a sedimentary basin may cause a thermal expansion, thus leading to a decrease in density of subsurface fluids with depth and production of a vertical density difference. Under certain conditions, the vertical density difference may induce free convection of the fluids. The fluids carry a lot of dissolved solids and hydrocarbon in free convection. Because the dissolved solids and hydrocarbon have a temperature-dependent solubility, they will dissolve or precipitate in the hotter or cooler region of a convection cell, which has an important effect on diage-nesis and hydrocarbon migration and accumulation.