Comparative Hydrodynamic and Thermal Characteristics of Sedimentary Basins and Geothermal Systems in Sediment-Filled Rift Valleys
Wolfgang Polster, H. L. Barnes, 1994. "Comparative Hydrodynamic and Thermal Characteristics of Sedimentary Basins and Geothermal Systems in Sediment-Filled Rift Valleys", Basin Compartments and Seals, Peter J. Ortoleva
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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 geothermal 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/1, 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. Such hydrothermal processes may easily form basinal seals that are effective traps for hydrocarbon fluids.
Sedimentary basins have temperatures and gradients similar to those in geothermal systems but at greater depth. For example, a temperature range from 120-190°C and a gradient of 45°C/km are typical at 2.2-3.8 km depth in sedimentary basins and 0.6-2.2 km depth in geothermal reservoirs. This means that, given similarities in geochemical conditions, similar water/rock interactions can occur. For instance, diagenetic alterations as a result of the influx of hot brines into clastic sediments are often similar in both cases.
Geothermal systems differ markedly from basins in having higher surface-heat flow, much higher near-surface gradients, and much lower gradients in the presence of cold water recharge. Additionally, organic solutes are absent in this type of geothermal reservoir while organic and biological reactions play an important role in sedimentary basins.
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Basin Compartments and Seals
Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.