Published 1994 as part of Memoir 61
Copyright © 1994 The American Association of Petroleum Geologists.  All Rights Reserved.
 

Detailed geophysical, hydraulic, and geochemical data were compiled from the literature for sedimentary basins and for liquid-dominated geothermal fields of sediment-filled rift valleys. The objective was to use the geo-thermal data as a guide to the effects to be expected in sedimentary basins from the upflow along growth faults and other high-permeability zones.

These geothermal reservoirs usually lie 0.6-0.8 km below the surface. Representative conditions in such reservoirs are temperatures of 120-370°C, thermal gradients of 15-80°C/km, salinity of 3-27 g/l, pH of about 4.5-5.5, pressure from hydrostatic to lithostatic, average porosity of 10-20%, and permeability of 0.1-600 md, typically. Comparable zones in sedimentary basins with similar thermal, geochemical, hydrodynamic, and lithological conditions are, for example: (1) upflow zones along growth faults (e.g., Wilcox trend, northern Gulf of Mexico Basin, up to 60°C/km) or piercement structures (e.g., Danish Central Graben, North Sea Basin, up to 50°C/km), (2) upflow zones along deep, but permeable strata of sedimentary basins (e.g., Alberta basin, ~40°C/km), and (3) sediments near ancient rift zones (e.g., Gabon basin).

Average bulk permeability and porosity are typically reduced by hydrothermal upflow in both environments. For example, the bulk porosity at a depth of 0.6-2.5 km in unaltered sediments is up to 10% higher than in an adjacent, moderate-temperature (120-200°C) geothermal reservoir. Apparently, the upflow of hydrothermal fluids into sedimentary strata can cause significant reduction of porosity and permeability in a short time (<16,000 yr) even at moderate temperatures and geothermal gradients (15-60°C/km). Representative bulk porosities of hydrothermally altered sediments at temperatures exceeding 250°C are about 3-10%, comparable to the porosity of basins commonly found at depth below about 4-5 km.