The potential for free and mixed convection in sedimentary basins

Free thermal convection and mixed convection are considered as potential me chanisms for mass and heat transport in sedimentary basins. Mixed conventio n occurs when horizontal flows (forced convection) are superimposed on ther mally driven flows. In cross section, mixed convection is characterized by convection cells that migrate laterally in the direction of forced convecti ve flow. Two-dimensional finite-element simulations of variable-density gro undwater flow and heat transport in a horizontal porous layer were performe d to determine critical mean Rayleigh numbers for the onset of free convect ion, using both isothermal and semi-conductive boundaries. Additional simul ations imposed a varying lateral fluid flux on the free-convection pattern. Results from these experiments indicate that forced convection becomes dom inant, completely eliminating buoyancy-driven circulation, when the total f orced-convection fluid flux exceeds the total flux possible due to free con vection. Calculations of the thermal rock alteration index (RAI=q.del T) de lineate the patterns of potential diagenesis produced by fluid movement thr ough temperature gradients. Free convection produces a distinct pattern of alternating positive and negative RAIs, whereas mixed convection produces a simpler layering of positive and negative values and in general less diage netic alteration.