Heat transfer in the subsurface can vary with depth due to variations in the thermal conductivity of the geologic medium as well as variations in groundwater flow velocity. However, traditional thermal response tests (TRTs) do not allow for evaluation of the depth variability of heat transfer. We investigate the potential for using discrete-depth heat dissipation tests in open, water-filled boreholes to evaluate variations in heat exchange rate with depth. Heat dissipation tests were initiated at target depths in a test well using an electrical resistance heater. Heat dissipation was monitored by measuring borehole water temperature through time using a fiber-optic distributed temperature sensing system. Temperature data were used to compare the thermal response at different depths in the borehole. To account for both the thermal conductivity of the geologic medium and the groundwater flow velocity, we used a numerical groundwater flow model (MODFLOW) and solute transport model (MT3DMS) to simulate heat dissipation tests. Simulation results indicate the measured response to a heat dissipation test in an open borehole is strongly dependent on the measurement location within the borehole; thus, data are ambiguous when the measurement location is uncontrolled. However, modeling results also indicate that the thermal response of a heat dissipation test as measured at the center of the borehole is sensitive to variations in thermal conductivity and groundwater flow velocity, suggesting that heat dissipation tests are a potentially useful method for characterizing depth variability in thermal properties if a centralized temperature measurement method is used to monitor the tests.

Abstract Gravity-driven groundwater discharge from intracratonic basins has been identified as the source of hydrothermal brines responsible for the genesis of Mississippi Valley-Type (MVT) ore deposits. Fluid flow processes that controlled the location of ore mineralization on the local and district scale have not previously been evaluated quantitatively for the Upper Mississippi Valley zinc-lead district. The objective of this study was to evaluate conceptual models and identify processes that regulated heat and fluid flow in the district. The study used numerical modeling of groundwater flow in fractured carbonate host strata and coupled groundwater flow and heat transport in a district scale model. Two-dimensional modeling of fracture networks was performed based on mapping of fracture patterns in carbonate host rocks of the Sinnipee Group. Numerical modeling of groundwater flow through stochastic realizations of discrete fracture networks was used to characterize the relationship between scale and heterogeneity of equivalent permeability of carbonate strata. Groundwater and heat flow were modeled on the district scale using a three-dimensional finite difference technique implemented with the HST3D code. The results of groundwater flow modeling identified processes that influenced the localization of MVT ore mineralization at three scales. At the smallest scale investigated, stochastic variability of permeability within fracture networks in carbonate host strata resulted in flow focusing and mineralization in small, more randomly distributed, ore bodies. Variability in the vertical permeability of the Maquoketa Shale confining unit, at a somewhat larger scale, caused increased groundwater velocities and higher temperatures in the underlying host strata. Larger individual ore bodies in the district may have been localized by the relationship between discrete structural features and enhanced permeability in the overlying Maquoketa Shale. On the district scale, hydraulic communication between major aquifers across the sub-Tippecanoe unconformity resulted in a large area along the axis of the district of anomalously high temperatures and convergent groundwater flow and was responsible for the general location of the district.

Abstract In this chapter we examine pressures in the St. Peter Sandstone and associated formations of the deep Michigan basin. A comparison of computed brine heads to surface elevations reveals a large area of overpressures withinthe St. Peter Sandstone and the Glenwood Formation to the west and north of Saginaw Bay. Contrary to the patterns expected for a steady-state, topographically driven flow system, heads in these formations are highest in theregional discharge area. Vertical gradients between the Glenwood and the St. Peter and between the St. Peter and the Prairie du Chien Group would generate downward flow in the regional discharge area, which is also inconsistent with a steady-state flow system. Low-permeability zones must exist within the Glenwood and St. Peter to inhibit equilibration with normal pressures in the underlying and overlying units. Overpressures appear to be dissipating by both upward and downward leakage through high-permeability zones located in anticlinal structures and possibly related to basement faults. Within the larger area of anomalous pressures, repeat formation test data from selected wells reveal even greater overpressures locally within the St. Peter. These vertical variations in pressures are associated with vertical variations in permeability, suggesting a stacked system of compartments separated by low-permeability zones of diagenetic origin.

Abstract Low-permeability seals associated with abnormal pressures are most commonly identified by examining vertical pressure profiles and noting the depths at which a major change in the pressure gradient occurs. Alternatively, zones of very low permeability that may act as fluid seals may be identified on the basis of core analyses and well tests. Often, however, there are an insufficient number of direct pressure or permeability measurements to adequately identify the depth and lateral extent of these seals. A method has been developed for estimating porosity and permeability through the use of wireline logs. Multivariate statistical techniques are used to segment the logs and group the segments into electrofacies types. Application of this technique to 18 wells within the St. Peter Sandstone of the Michigan basin shows that the electrofacies characterization reflects both the hydraulic and diagenetic characteristics of the formation. Six electrofacies types have been identified, one of which has characteristics similar to those found within seals in other basins.

Abstract Diagenetic banding commonly occurs in association with zones of abnormal fluid pressures that have been identified as pressure compartments. Although diagenetically banded intervals may contain layers of moderately high porosity, the bands act collectively as a low-permeability unit and are therefore important as potential low-permeability seals for pressure compartments. This study focuses on a diagenetically banded interval in the Middle Ordovician St. Peter Sandstone of the Michigan basin. This interval is located within a large area of anomalous pressures identified by Bahr et al. (this volume) in the deep Michigan basin and is composed of a seal-forming lithology. The banded interval is characterized by millimeter- and centimeter-scale diagenetic banding, with alternating quartz-cemented bands, pressure solution-dominated bands, and porous bands. Point counting techniques and an image analysis system were used to quantify porosity, textural properties, and quartz cement. A theoretical model was used in conjunction with these data to estimate the amount of silica dissolved by intergranular pressure solution. Porosity variations in the St. Peter Sandstone are controlled by the combined effects of quartz cementation and intergranular pressure solution. A silica budget calculated for the banded interval indicates that more silica was dissolved by intergranular pressure solution than is present as quartz cement, suggesting that pressure solution alone could have produced enough silica to account for the banded quartz cement. On a local scale, the banded interval served as an exporter of silica. However, a larger-scale silica budget analysis computed for another well in the same region of the basin indicates that the St. Peter may actually be balanced on a regional scale. Results of this study were used to investigate the controls on diagenetic band formation. No significant correlation exists between porosity and grain size or porosity and sorting in the banded interval, suggesting that depositional textural parameters are not important in controlling the distribution of porosity and cement within the banded interval itself. However, original.