Abstract 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.

Abstract The mineralogies of hydrothermal sediments in cores and other samples from the Atlantis II Deep of the Red Sea were examined by optical and scanning electron microscopy, and by X-ray diffraction and electron microprobe methods. The bottom 20 percent of the studied section consists of 1 to 5 wt percent sulfides, dominantly as pyrrhotite, cubic cubanite, chal-copyrite, and pyrite, plus other hydrothermal minerals including vermiculite, anhydrite, hematite, chamosite, and. ilvaite: Above this zone is a sulfide-enriched unit about 4 m thick, of 8 to 66 wt percent sulfides. Chalcopyrite, pyrite, and sphalerite with 3.5 to 4.5 mole percent FeS accompany iron-rich smectite, anhydrite, manganosiderite, amorphous silica, and amorphous ferric oxyhydroxides. Mineralogic breaks between these units indicate variations in tectonic activity which affect both detrital accumulation and locations of hydrothermal vents. Veins crosscutting the sediments consist mainly of anhydrite, silicates, pyrrhotite, pyrite, and chalcopyrite, with minor valleriite, cubic cubanite, and sphalerite. Early vein sphalerite contains 14.0 to 23.3 and averages 17.3 mole percent FeS, but later vein sphalerite averages 3.6 mole percent FeS. The observed vein assemblage—-pyrrhotite, cubic cubanite, high iron sphalerite, and anhydrite—indicates disequilibrium between H 2 S and > in the depositing fluid. Apparently, mixing between two circulating hydrothermal fluids, one shallow with SO 4 > H 2 S at <250°C, and a deeper, hotter, fluid with H 2 S > SO 4 , produced disequilibrium mineral assemblages before discharging onto the sea floor. Precipitation at 200° to 250°C is implied by the assemblage cubic cubanite + chalcopyrite + monoclinic pyrrhotite. However, temperatures beneath the sea floor >334° ± 17°C are indicated by 100 µm grains of cubic cubanite + chalcopyrite + pyrite that were apparently carried upward by the hydrothermal fluid. Vertical transport of these grains to the sea floor required rapid flow and coojing to preserve the high-temperature cubic form of cubanite. The rmodynamic evaluation shows the hydrothermal fluid at 250°C to haver a H2s = 0.001, log a S2 , = -11.8 to -13.7, log a O2 = -36.5 to -38.5, and pH = 4.56 ± 0.5. At 200°C: a H2S - 0.001, log a S2 = -12.9 to -13.0, log a O2 , = —41.7 to -41.9, and pH = 4.64 ± 0.5. Comparisons show that the Atlantis II sediments resemble volcanogenic massive sulfides in most characteristics, including associated volcanism, tectonic setting, pyrrhotite-chalcopyrite-sphalerite zoning, metal grade, sedimentary textures, and most fluid properties. The few differences are related to a flanking evaporite-shale sequence and to the immature state of the Atlantis II deposit.