[Wilhelm] The Geothermal Field of the Chesapeake Bay Impact Structure
German Title: The Geothermal Field of the Chesapeake Bay Impact Structure
Current Status: completed with report
Main Applicant:Prof. Dr. Helmut Wilhelm
Petrophysical investigations on 361 samples from the boreholes Cape Charles (STP2), Kiptopeke, Newport News Park II, North, Fentress, and Jenkins Bridge in the Chesapeake Bay impact region (Virginia, USA) have been carried out during this project. In addition the results of corresponding investigations on 497 core samples from the ICDP-USGS borehole Eyreville in previous ICDP-projects were used to investigate thermo-physical properties for a lithological and geothermal interpretation of the measured data and to derive petrophysical constraints for a numerical thermohydraulic model of the Chesapeake Bay impact region. As a corresponding geothermal data set, equilibrated high-resolution temperature logs were recorded in these boreholes which reveal prominent stratigraphic, petrophysical and hydraulic variations. To enhance the resolution of the thermal measurements the method of reduced temperatures has been applied to thermal log sections of interest. The thermal regime of the Chesapeake Bay impact area is strongly variable due to compositional and structural diversity of the sedimentary cover and compositional heterogeneities within the crater fill, varying convective contributions to the local heat transport because of well-known water extraction from and less-known or unknown infiltration in aquifers, regional variations in ground surface temperatures because of the surface land-water distribution and unknown influences from lateral and deep crustal boundary conditions. A convective contribution to the heat transport can be excluded for the breccia crater fill with a possible exception near the outer crater rim and in the Chickahomini confining unit, i.e. in the breccia cap. The regional variability of the thermal conditions is clearly expressed in the different curvature trends and the different mean temperature gradients of the measured temperature profiles in the boreholes Fentress and Jenkins Bridge. The temperature profiles recorded in the post-impact and the Cretaceous aquifers show clear signs of vertical groundwater migration. The vertical volume fluxes (Darcy velocities) determined for the aquifers using the method of Mansure and Reiter (1979) are in the range of 0.1 – 0.2 m a^-1. The extension of our thermal investigations from the ICDP-USGS borehole Eyreville into the whole Chesapeake Bay region has confirmed the results obtained from this borehole but it has considerably enhanced the view of the physical processes influencing the local temperature profiles measured in boreholes of that region.