Groundwater Flow With Heat and Solute Transport in Sedimentary Basins.

Numerical experiments were undertaken to understand (1) heat and fluid transport in an uplifted foreland basin where fluid flow is driven by topographic gradient, and (2) effects of expulsion of geopressured fluids on the thermodynamic system in salt basins. Thermal evolution of an uplifted foreland basin is strongly affected by thermal buffering of basement rocks. Thermal buffering by basement rocks does not permit constant heat flow along the sediment-basement contact, in contrast to many previous numerical studies. Ignoring basement effects can result in serious errors in temperature prediction, such as a false transient thermal pulse in the discharge area. Carbonaceous sediments are common in many sedimentary basins in the Mid-continent of North America that developed during the Late Paleozoic. Thermal insulation by a low conductivity carbonaceous layer can result in high basin heat storage, lower fluid viscosities, high efficiency of heat transport and heat retention during the discharge of hot fluids. This is a possible mechanism for elevated temperatures associated with the formation of Mississippi Valley-type (MVT) ore deposits even under the continental mean of basal heat flow about 60 mW/m$\sp2.$. Expulsion of geopressured fluids along a fault zone can inject a large quantity of low salinity fluids into the overlying hydropressured sediments within a few hundred years, inducing fluid pressure and temperature anomalies in the overlying hydropressured sediments. The short term expelled hot and low salinity fluids are gravitationally unstable and can cause long term ($\u3e$10,000 years) thermohaline convection in a salt basin. Salinity analysis from SP logs and numerical simulation in South Eugene Island, Block 330 field suggest that the present fault zone in the Block 330 area may provide a permeable conduit to the deeper reservoirs (depth $\u3e$ 2000 m) immediately overlying the frictional failure zone. The shallow reservoirs (depth $\u3c$ 1600 m) are believed to be currently fault-sealed. Sediment compressibility can maintain excess fluid pressures and drive fluid flow even thousands of years after fluid expulsion stops. Compressibility of sediments may play an important role in the low decline rate of the production history in the study area