Connate Waters Research Articles

Basin scale evolution of formation waters: a diagenetic and formation water study of the Triassic Chaunoy Formation, Paris Basin

Formation waters and their movements within basins are commonly attributed with responsibility for patterns of cementation and porosity-loss within reservoirs and aquifers. It is thus important to understand when and how waters move in the subsurface. We have studied the evolution and movement of formation water in the Triassic Chaunoy Formation of the Paris Basin, NW Europe to define the way in which the water has evolved and to interpret water movement patterns using mineral isotope and fluid inclusion data in conjunction with detailed formation water analyses. The Chaunoy Formation is cemented with different types of dolomite, calcite and quartz cement. We have studied the evolution of waters in terms of oxygen, carbon and strontium isotopes and salinity. Connate waters were meteoric in origin but strongly influenced by the proximity of a playa lake. Input of palaeo-meteoric water, which entered the Chaunoy via eastern halite bodies, resulted in highly saline formation water for the majority of the Chaunoy Formation from about 100 Ma until (and for some time after) maximum burial. Saline waters spread into the aquifer initially taking a west-north west trajectory and then switching to a west-south west pattern in the Eocene. Following the Alpine orogeny and localised uplift of the basin margin, the southern portion of Chaunoy received fresh, low salinity, meteoric water. It is likely that formation water was lost from the Chaunoy in a more central part of the basin, via sub-vertical cross formational flow to the Mid Jurassic Dogger Formation, to accommodate the influx of fresh water.

Multi-stage fluid circulation in a hydraulic fracture breccia of the Larderello geothermal field (Italy)

The deep well MV5A, drilled in the western part of the Larderello geothermal field, crossed a 20-cm-thick hydraulic fracture breccia unit at a depth of 1090 m below ground level (b.g.l.). This breccia occurs in a fine-grained Triassic metasandstone and consists of angular to subangular clasts of up to some centimeters in size. Pervasive alteration has affected the breccia clasts and wall rock around the breccia, with the formation of Mg–Fe chlorite. After such alteration, hydrothermal circulation caused the precipitation of two generations of calcite cement. Then, ankerite partially replaced these two calcite generations. Ankerite also precipitated in late veinlets with chlorite. Late hydrothermal activity led to the crystallization of albite, quartz and finally, anhydrite. The calcite contains vapor-rich inclusions and two populations of liquid-rich (L1 and L2) inclusions. L1 inclusions are characterized by homogenization temperatures between 304 and 361°C and salinities from 7.4 to 11.6 wt.% NaCl equivalent; L2 inclusions revealed homogenization temperatures in the range of 189–245°C and salinities from 2.6 to 6.3 wt.% NaCl equivalent. The fluids contained in L2 inclusions were probably trapped coevally with some vapor-rich inclusions under boiling conditions after the L1 inclusions formed. Some of the abundant vapor-rich inclusions in calcite may also represent early, low-temperature inclusions affected by decrepitation and/or stretching and/or leaking during L1 trapping. The liquid-rich (L) inclusions trapped at later stages in ankerite, albite and anhydrite display, respectively, homogenization temperature ranges of 189–198°C, 132–145°C, and 139–171°C, and salinities ranging from 1.6 to 1.7 wt.% NaCl equivalent, 1.4 to 2.1 wt.% NaCl equivalent and 3.7 to 6.2 wt.% NaCl equivalent. The inclusions studied record the evolution, over time, of the fluids flowing in the breccia level: L1 inclusions capture high-temperature fluid (about 300 to 350°C) of high salinity (around 10 wt.% NaCl equivalent) at above-hydrostatic pressures (up to about 150 bar). The L2 inclusions in calcite and liquid-rich inclusions in ankerite and albite represent subsequent hydrothermal fluid evolution toward lower temperatures (about 250 to 130°C), pressures (45 to a few bar) and salinities (6.3 to 1.4 wt.% NaCl equivalent). During this stage, boiling processes and infiltration of meteoric waters probably occurred. Finally, moderately saline fluids (around 5 wt.% NaCl equivalent) at a temperature (about 160°C) close to that of present-day in-hole measurements was trapped in the anhydrite inclusions. The liquids trapped in liquid-rich inclusions circulated at 41,000 years (maximum age of calcite) or later. This age represents an upper limit for the development of vapor-dominated condition, in this part of the geothermal system. The fluids circulating at the breccia level were probably meteoric and/or connate waters. These fluids may have interacted with the anhydrite and carbonate bearing formations present in the Larderello area. The occurrence of the hot and saline fluids, trapped in L1 inclusions at above-hydrostatic pressure, suggests that similar fluids but with higher pressure (≥167 bar) and temperature (≥360°C) may have been responsible for rock fracturing.

Rare-Earth-Element and Isotopic Evidence for the Genesis of the Bajiazi Stratabound Sulfide Ore Bodies of Northeast China

This study examines the rare-earth-elemenl and isotopic geochemistry of sulfide ores and associated rocks in the Bajiazi sulfide deposits of northeastern China. The distribution and concentration of rare earth elements (REE) in sulfide ores and associated rocks from the Bajiazi deposits have been determined by inductively coupled plasma spectroscopy (ICP) methods. Birdwing-shaped rare-earth-element profiles are only observed in granitic rocks at Bajiazi. Diverse rare-earth-element profiles of the ores and dolostones are different from those of granite and are interpreted to reflect an early sedimentary-diagenetic mineralization stage during which rare earth elements were added to the ores and host rocks. The characteristic features of the abundances of REE in the Bajiazi samples, including granite and sedimentary host rocks, are demonstrated by a variety of Eu anomalies in the chondrite-normalized REE pattern. Eu is the most mobile element, exhibiting increasingly negative anomalies in granite rock samples relative to the depletion of the sulfide ores and associated sedimentary host rocks, which are considered to be caused by preferential scavenging of Eu from sea water to sediments. The δ34S values of sulfide minerals at Bajiazi generally range from −12.3 to 14.2%, suggesting reduction both of marine sulfate and biogenic sources. The δ34S values from different ore types, sulfide minerals, and mines indicate a sedimentary exhalative origin, although they were, to some extent, homogenized during late overprinting. The late-stage sulfides from Bajiazi are isotopieally remarkably homogeneous and are significantly, although slightly, enriched in δ34S. These results imply multiple sources of sulfur derived from biogenic reduction, and/or sulfate in oceanic and/or connate waters, or from marine evaporites, and/or from magmatic hydrothermal sources. Isotopic temperatures from intersulfide fractionations (pyrite, sphalerite, and galena) range from 75° to 542°C, indicating the Bajiazi sulfide minerals have experienced different mineralizing stages. Galena in the Bajiazi Proterozoic sediment-hosted Pb-Zn sulfide deposits has a very uniform Pb-isotope composition, with 206Pb/204Pb = 16.07 to 16.58, 207Pb/204Pb = 15.00 to 15.66, and 208Pb/204Pb = 36.13 to 36.92, suggesting that it all formed from a common mineralizing fluid. The variation of lead-isotopic values of ores is similar to that of their host sedimentary strata. The lead in the various ore types and host rocks within the Bajiazi district is virtually identical. Lead-isotope signatures, which generally plot very close to model crustal growth curves, characterized by a model age of ∼1350 Ma, indicate a normal, nonradiogenic origin and an Early Proterozoic upper-crustal source for the lead in the ores. That lead was unrelated to the radiogenic lead in the Mesozoic granite. The δ13C values range from +1 to −5% PDB, and the δ18O values from −6 to −15% PDB. Later crystallization generations are enriched in the light isotopes of carbon and oxygen compared to early generations. The characteristics of carbon and oxygen isotopes of the Bajiazi deposits indicate that: (1) primary ores were precipitated in the Proterozoic marine environment; (2) both ores and host carbonates from the Bajiazi district are remarkably homogeneous in their carbon- and oxygen-isotope compositions during later tectonic, conlact-metamorphic stages; (3) although the variations in isotopic composition of carbon and oxygen are relatively small, a significant evolution toward a lighter isotopic composition with advancing diagenetic, tectonic, and contact metamorphic processes is observed; and (4) this evolution is independent of the presence or absence of ore minerals.

Modelling Primary Hydrocarbon Expulsion in a Synthetic Sedimentary Section: Relative Contributions of Edge Conditions and other Geological Factors

The effects of numerical edge conditions and of the relative contributions of geological processes or factors to primary hydrocarbon migration are studied using the GEOPETII program developed by the Basin Modeling Group at USC. Lateral edge conditions can have important effects on the results of modelled sedimentary sections. The absolute mass of an oil or gas accumulation varies with the section length modelled, while more vertical grids (pseudo-wells) along a section result in less lateral continuity of accumulated oil and gas. A modelled section with a highly permeable lithology at the two lateral edges produces lower excess pressure between the three left-edge pseudo-wells and between the three right-edge pseudo-wells, and tends to have less oil and gas accumulated in the section center. If the modelled section has a low-permeability lithology at the two edges, the opposite effect is produced. A modelled section which has a thick basal layer, high permeability, and relatively few horizontal subdivisions produces less excess pressure in a shale source rock above the basal layer, and has smaller oil and gas accumulations in a reservoir unit above the source rock. By contrast, a modelled section with more subdivisions, and a smaller time-step for each layer calculation, produces higher excess pressures and tends to have more oil and gas accumulated in reservoirs above the source rock. Accordingly, the ratio of the thickness deposited in a time-step to the subdivision thickness used in the calculation should be kept small (less than 0.4 for the code used here). Twelve major processes or factors control oil and gas expulsion: (1) buoyancy; (2) the solubility of hydrocarbons in connate waters; (3) the saturation of irreducible water in a source rock; (4) capillary pressure; (5) critical pressure for fracturing a source rock; (6) the viscosity of hydrocarbons; (7) the relative permeabilities of oil and gas; (8) the lithology-dependent permeability ratio in the vertical to horizontal direction; (9) the total organic carbon in the source rock; (10) the kerogen type; (11) the ages of the source-rock strata; and (12) the palaeoheat flux of the area. The Shongliao Basin, China has been intensively drilled, and two volumes of downhole data are available. Those data were used to set limits on the ranges of parameter values and on geological factors. In this way, an evaluation is given of the relative importance of the twelve contributions influencing primary hydrocarbon expulsion. Both separate-phase gas-expulsion efficiency and the absolute mass amount of gas expelled from the source rock increase with increases in: buoyancy, hydrocarbon saturation in irreducible water in the source rock, capillary pressure, relative permeability, total organic carbon, source-rock age, and palaeoheat flux. However, gas expulsion efficiency and the amount of gas expelled decrease with increases in: hydrocarbon solubility in mobile waters in the source rock, critical fracture pressure, the viscosity of the hydrocarbons, and with changes in kerogen type to a more oil-prone variety. The object of the work presented here is to investigate simultaneously all the processes thought to control primary hydrocarbon expulsion. The Shongliao Basin in China was chosen as a case study because copious, basin-wide data are available.

Geochemical patterns of a sheared fluorite vein, Parzan, Spanish Central Pyrenees

The formation of Parzan fluorite-lead «extension-type» vein (Spanish Central Pyrenees) is related to the post-Hercynian hydrothermal activity widespread not only in the Pyrenees but in Western Europe as well. The lode was later strongly deformed during the Alpine orogeny with textural modifications and destruction of primary fluid inclusions. In order to determine if geochemical signatures of the vein were disturbed during Alpine deformation, post-ore fluid inclusions and quartz oxygen-isotopes have been addressed. Microthermometry and X-ray microanalysis of frozen inclusions confirm the Na-Ca-Cl character of post-ore fluid circulations and indicate that they were originally derived from a single source: Triassic connate waters. Oxygen isotope data on quartz formed during deformation along with trace element geochemistry suggest that the geochemical signatures of the vein (mainly REE and 87Sr/86Sr) were apparently not reset during deformation. The REE content of fluorites is very high (up to 0.12%); the chondrite normalized REE patterns display a continuous decrease from La to Lu. Sulphur isotope data for the Parzan vein is outside the typical “deep seated” range, suggesting dominantly crustal sources of sulphur for sulphides, while formation waters may be proposed for barite. Parzan fluorites have the most radiogenic Sr values of all the investigated fluorite deposits in Central Pyrenees. They scatter in the range of both the igneous rocks and the Cambrian-Ordovician and Triassic detrital sequences of the Parzan area. If REE contents and 87Sr/86Sr ratios reflect the primary stage of vein filling as suggested by the good correlation between both geochemical signatures, the linear array between 87Sr/86Sr and 1/ΣREE is suggestive of a mixing process. The δ34S, δ18O and 87Sr/86Sr parameters in barite, which postdates the fluorite, also provide strong evidence that barite deposition is a result of fluid mixing. Such an interpretation is in line with present-day ideas for the genesis of many of the F-Ba-Pb-Zn vein deposits in the Hercynian belt of Iberia.

The brine fields of the Benue Trough, Nigeria: a comparative study of geomorphic, tectonic and hydrochemical properties

The numerous isolated brine occurrences in the Benue Trough of Nigeria share some geomorphic, geological and tectonic features in common. Evaluation of these features as well as the hydrochemical properties of the brines, is important in speculating the origin of the brines. Field observations suggest that the brines are not confined to any bedrock type but are hosted in diverse lithologies: sandstones, shales, mudstones and limestones. Despite the varied bedrock types, the hydrochemical data are generally similar. The data, in conjunction with the hydroisotopes and the general tectonics of the Benue Trough, suggest the existence of connate waters pressed out during tectonic compaction and stored in the resulting fractures.

Origin of fibrous gypsum in the Newark rift basin, eastern North America

ABSTRACT Cores from the red beds constituting the Upper Triassic Passaic Formation, a syn-rift sequence in the Newark Basin, display subhorizontal fractures (veins) filled with satin spar gypsum. Such veins are apparent in the shallow subsurface, 100-300 m below present ground level, but are not observed in outcrops or in quarries because of groundwater dissolution. These fractures are roughly parallel to the present ground surface in the basin, suggesting topographic influence on their formation. Fractures observed in cores are seen to crosscut stratigraphic boundaries, indicating that the veins formed after stratal tilting and faulting and thereby post-date the early Late Jurassic. The gypsum crystals making up the vein fill grew perpendicular to the fracture walls, suggesting that opening and filling of the fractures was contemporaneous and that the fractures are extensional. That gypsum crystallizes from meteoric and connate waters and is derived by dissolution of associated evaporites is reflected in the D, 18O, 34S, and 87Sr/86Sr isotopic compositions of the sulfate. Fractures are interpreted to be a result of unloading and exhumation. Fractures were initiated after major depositional and tectonic events took place and post-date the Late Jurassic. Satin spar precipitated in those fractures, carried in by gypsum-saturated brines derived either by dissolution of the near-surface sulfate evaporites or rehydration of more deeply buried anhydrite. Such satin spar veins are not evaporites, but are composed of chemically remobilized sulfate

Fluid inclusions in pre-ore minerals from the carbonate-hosted mercury deposits in the Espadán Ranges (eastern Spain)

The fluids associated with pre-ore minerals in the carbonate-hosted mercury veins from the Espadán Ranges (eastern Spain) have been studied using microthermometry, Cryo-SEM-EDS, Raman spectroscopy and crush-leach analyses, in order to determine their origin and the processes responsible for mineralization. Three primary fluid inclusion types have been found in quartz and dolomite: (1) type S, composed of liquid + vapour (mainly CO 2) + halite; (2) type S-V, composed of liquid + high-density CO 2 double bubble + halite; and (3) type V, composed of high-density CO 2N 2 mixtures. Type-S fluid inclusions are preferentially trapped in Betxí-type veins, characterized by the presence of hydrothermal dolomite, whereas type-V fluid inclusions are trapped almost exclusively in Eslida-type veins, which were without dolomite. From these data two different fluids can be inferred to occur during the period of formation of the veins: (1) a complex polysaline brine of NaClKClCaCl 2(MgCl 2, SO 4 2−)H 2O composition with a salinity of up to 33 eq wt% NaCl; and a (2) CO 2N 2-rich fluid. The brine halogen composition points to its secondary origin caused by the dissolution of underlying marine evaporites. It is thought that these deposits were formed during Jurassic or Cretaceous rifting periods, as a consequence of fluid mixing between a high-salinity secondary brine and connate waters. The generation of a CO 2N 2-rich phase caused the pressure increase of the fluids that favoured the hydraulic fracturing of the enclosing rock (dolostone). The origin of this volatile-rich phase probably occurred due to the oxidation and thermochemical breakdown of the organic matter contained in the host rock. This event is recorded by the coetaneous trapping of both aqueous and carbonic fluids in quartz crystals. The formation conditions for the hypogene sulphide paragenesis are obscure but the coincidence between the preferential trapping of type-V fluid inclusions, the absence of hydrothermal dolomite and the major abundance of mercury in Eslida-type veins is thought to be linked to a mercury fractionation event to the vapour phase during the release of the volatiles.

Stable isotope composition of F-Pb-Zn mineralization in the Valle de Tena (Spanish Central Pyrenees)

This study, dealing with the stable isotope (C, O, and S) geochemistry of F-Pb-Zn deposits, is part of an integrated metallogenetic study of the Valle de Tena (Central Pyrenees). The objective of this work is to place constraints on the origin of the mineralizing solutions and sources of sulphur and metals. Two types of mineralization have been identified: (1) stratabound ores linked to an emersion episode that occurred during the Namurian to Westfalian transition (Portalet mines), and (2) low-temperature hydrothermal veins related to post-Hercynian extensional tectonics (Yenefrito and Lanuza mines, and Tebarray occurrences). Stable isotopic study indicates that the source of mineralizing components was the local Upper Palaeozoic country rocks in the case of the stratabound Portalet deposit (biogenic sulphides and connate waters, modified by some meteoric input through a karst surface). In the case of vein deposits, stable isotopes point toward either modified connate water or marine water as the mineralizing fluid, with sulphur and metals derived from a mixture, in different proportions, of regional igneous and (meta)sedimentary country rocks.

Mineral waters and hydrogeothermal resources in Bulgaria

Thermal mineral waters and extractable geothermal energy are an important and still underestimated natural wealth of Bulgaria. Their diversity is due to the complex geological structure, intense neotectonic activity, and the resulting complex character of meteoric water circulation. Reproductive hydrogeothermal systems with low-mineralized thermal waters of meteoric origin and resrvoir temperatures ranging between 30 and 100 (maximum 120) °C occur in the southern and north-eastern part of the country. Their total reproductive potential is estimated at 14–15 m3s−1. In the northern part (Moesian platform), regional hydrogeothermal reservoir with connate (marine) and mixed (marine and meteoric) mineral waters and brines are identified, the temperatures varying from 40 to 140 (maximum 150) °C. Huge resources of geothermal energy with commercial importance are accumulated in them. The hydrogeothermal wealth of Bulgaria provides as important basis for the development of balneological and multi-seasonal tourism, bottling industries, geothermal heating of buildings and greenhouses, aquaculture and other related activities.

The Northwest Geysers high-temperature reservoir: evidence for active magmatic degassing and implications for the origin of the Geysers geothermal field

Noble gas isotope abundances in steam from the Coldwater Creek field of the Northwest Geysers, California, show mixing between a nearly pure mid-ocean ridge (MOR) type magmatic gas with high 3He/ 4He and low radiogenic 40∗Ar ( R/ Ra > 8.3 and 40∗Ar/ 4He < 0.07), and a magmatic gas diluted with crustal gas ( R/ Ra < 6.6 and 40∗Ar/ 4He > 0.25). The 3He-enriched component is positively correlated with 4 He/ 40∗ Ar ratios, total helium to non-condensable gas ratios, and the ratios of total helium to atmospheric noble gases, and is accompanied by mantle-like 3He/CO 2 and 4 He/ 36 Ar ratios. The steam samples most enriched in this high 3Hecomponent are produced from a high-temperature reservoir (HTR) and are also the most enriched in total gas and HCl. These results support the hypothesis of active magma degassing beneath the Northwest Geysers, suggest that a significant fraction of the non-condensable gases produced with steam from the HTR is magmatic, and add new constraints to genetic models of the system and its evolution. The intensity of the magmatic signal is inconsistent with deep boiling of connate or metamorphic waters and suggests active magma degassing. A correlation between magmatic helium and non-condensable gases implies that the HTR high-gas component is also magmatic and that the formation of the HTR was possibly related to magmatic intrusion. Magmatic input is unlikely to decline on the time scale of production, but injection of water into the HTR would have multiple benefits: (1) pressure and steam flow would increase; (2) gas concentrations would decrease by dilution with low-gas steam from vaporized liquid; and (3) HCl in steam would be removed once a liquid phase is established. High 3He/ 4He ratios in steam produced from the reservoirs in the southern parts of The Geysers suggest the presence of a similar HTR underlying these portions of the field. Monitoring of 3He content may provide early warning of potential pressure drawdown and entry of corrosive, high-gas HTR steam.

Controls on halogen concentrations in sedimentary formation waters

AbstractChlorine is the most abundant halogen in sedimentary formation waters with concentrations from &lt;100 to &gt;250000 mg/l. Bromine is the second most abundant halogen at &lt;1 mg/l to &gt;6000 mg/l with iodine from &lt;0.1 mg/l to &gt;100 mg/l and fluorine from &lt;0.1 mg/l to 30 mg/l. Chlorine and bromine show a strong systematic covariation suggesting that they are subject to the same controlling mechanisms. Fluorine only shows relatively high concentrations at elevated chlorine and bromine concentrations showing that fluorine, chlorine and bromine are possibly controlled by the same processes. Iodine does not correlate with any of the other halogens indicating that unique processes control iodine.Key geological parameters that influence chlorine and bromine (and possibly fluorine) concentrations are the presence of salt in a basin, the age of the reservoir unit and the kerogen-type within the main hydrocarbon source rock in a basin. The presence of salt in a basin shows that sea water was evaporated to halite saturation producing connate waters with high concentrations of chlorine and bromine. The presence of salt also leads to high salinity waters through water-salt interaction during burial and diagenesis. Tertiary reservoirs typically have much lower chlorine and bromine concentrations than Mesozoic or Palaeozoic reservoirs. The age of the reservoir unit may simply reflect the different amounts of time available for formation water to interact with salt. The dominance of type II marine kerogen in a basin leads to higher bromine concentrations. This may reflect the dominance of a marine influence in a basin which is more likely to lead to salt deposition than terrestrial depositional environments. Iodine concentrations are independent of all these parameters. Other geological parameters such as depth of burial, temperature, basin forming mechanism and reservoir lithology have no influence upon halogen concentrations.Key processes that affect halogen concentrations are sea water evaporation and dilution, water—salt interaction and input from organic sources. Chlorine and bromine data lie close to the experimentally-derived sea water evaporation trend showing that sea water evaporation may be an important general control on halogens. Sea water dilution is probably responsible for most low salinity formation water chlorine and bromine concentrations for the same reason. Sea water dilution can occur either by meteoric invasion during burial, or following uplift and erosion, or by diagenetic dehydration reactions. Water can interact with salt in a variety of ways: halite dissolution by congruent processes, halite recrystallization by incongruent processes, sylvite dissolution or recrystallization and halite fluid inclusion rupture. Halite dissolution will lead to high chlorine and relatively low bromine waters because halite contains little bromine. In contrast, halite recrystallization will lead to bromine-enhanced waters because NaBr dissolves preferentially to NaCl. The occurrence of dissolution or recrystallization will depend on the water rock ratio, greater volumes of water will lead to more dissolution and waters with higher Cl/Br ratios. Sylvite is usually rich in bromine so dissolution will lead to bromine-enhanced waters. Primary aqueous inclusions in halite contain high bromine concentrations so that rupture, during deformation or recrystallization, will lead to bromine-enhanced formation water. A combination of these processes are responsible for the very limited range of Cl/Br ratios although congruent halite dissolution must have a limited role due to the absence of waters with high Cl/Br ratios.Iodine is strongly concentrated in organic materials in the marine environment; oils and organic rich-source rocks have high I/Cl and I/Br ratios relative to sea water or evaporated sea water. All formation waters are enriched in iodine relative to sea water implying that there has been input from organic matter or interaction with oil. However, hydrocarbon source rock type in a basin has no discernible effect upon iodine concentrations.

P-T conditions in the South Wales Coalfield: evidence from coexisting hydrocarbon and aqueous fluid inclusions

Siderite nodules in the Carboniferous Coal Measures of South Wales contain cavities which are often infilled with quartz, carbonates, sulphides, and hydrocarbons. The quartz contains a mixture of hydrocarbon and aqueous fluid inclusions. The aqueous fluid inclusions consist of a dilute brine (3 wt % NaCl equivalent) and have homogenization temperatures in the range 97–212 ºC (mean 143 ºC). The hydrocarbon fluid inclusions are dominated by methane with a small component of higher order hydrocarbons; their homogenization temperatures are in the range 35–78 °C (mean 54 °C). It is assumed that the two fluids were trapped simultaneously during growth of the quartz and thus a P–T estimate of entrapment can be obtained by graphical intersection of the hydrocarbon isochores and the aqueous fluid bubble point (homogenization) temperatures assuming hydrocarbon saturation. This method gives temperatures between 130 and 160 ºC, and pressures between 40 and 55 MPa. The timing of mineralization is uncertain, but it is suggested that it took place during burial and low grade metamorphism of the subsiding sedimentary basin (i.e. in the Upper Carboniferous). The hydrothermal fluids were probably derived from evolved meteoric or connate waters expelled during subsidence and sediment compaction.

Mineralogy and Origin of Well-Bore Scales in an Active Steamflood: Tar Zone, Fault Block IIA, Wilmington Field, California: ABSTRACT

Precipitation of well-bore scale is a significant problem in the active steam drive project in the Tar Zone of Fault Block IIA, Wilmington Field. This present study is part of the US Department of Energy Class III program to improve oil recovery in slope and basin reservoirs. The aim of the study is to design the least expensive treatment to minimize scale precipitation. As the first step in this program, it is necessary to precisely identify the mineralogy and origin of the scales. Three different families of well-bore scale occur. (1) Carbonate scale, dolomite, high and low Mg-calcite; Mg content increases with decreasing well bore temperature. Calcium originates from connate waters: Mg originates from waterflood seawater Mn and some Mg originate from framework grains. Routine acidizing with 15% HCI readily removes this scale. (2) Sulfate scale, barite and anhydrite, is not common. Sulfates can be interlayered with carbonates and were precipitated at relatively low temperatures. Sulfates have small attachment areas and active well-bore brushing removes them with or without acid treatment. (3) Hydrous Magnesium Silicate scale occurs sparingly. The actual mineral form is difficult to identify because of poorly developed crystal structure. This mineral represents precipitation at the lowest temperaturemore » of all scales in the field. It is non-soluble in acid and must be mechanically removed.« less

Fluid Evolution Interpreted from Diagenetic Assemblages and Salinity Data in Permo-triassic Sandstone, Northern Perth Basin, Western Australia

ABSTRACT Shallow-marine Permo-Triassic sandstone of the northern Perth Basin, Western Australia, shows a progressive range down-dip in pore-water salinities, and in the mineralogy of diagenetic assemblages. These features have been interpreted as resulting from a history involving three hydrocarbon injections, and the introduction of meteoric waters. The sandstone units, some containing oil and gas, dip and thicken southward toward the basin depocenter. Salinity in both irreducible water of the hydrocarbon column and pore water of the water leg increases southward. Isohalines in the hydrocarbon column are broadly parallel to those in the water leg, but water salinity in the oil column is higher than in the water leg. In the north, typical diagenetic mineral assemblages include kaolinite, siderite, pyrite, Ti-oxides, and quartz, but farther south they include chlorite, pyrite, albite, Ti-oxides, and quartz. Detrital feldspar and mica in the north are partly dissolved and altered to kaolinite, whereas to the south, they show chlorite alteration. The authigenic phosphate xenotime increases from north to south. The contrast in diagenetic assemblages from north to south, together with the increase southward in water salinity, point to southward flow of introduced water, and siderite zonation indicates that the introduced water ranged from meteoric to saline. The water probably originated on basement near the northern margin of the basin, which, although now exposed, may have been repeatedly submerged and exposed since the Permo-Triassic. It flowed down dip, generally diluting saline connate waters of the marine sequence as it penetrated the basin. Flow began during early diagenesis. There were at least two episodes of oil emplacement. The presence of the first oil halted most mineral diagenesis in the reservoir, and solid bitumen envelopes developed around radioactive monazite grains. As the oil leaked out, mineral diagenesis recommenced, and traces of residual oil were trapped in the growing quartz. Emplacement of the second (present) oil again halted mineral diagenesis, and a little of the new oil adhered to envelope surfaces or penetrated envelope cracks. A late gas inflow displaced the oil-water contacts. The fresh-water influx continued below the hydrocarbon column, lowering salinity in the water leg and promoting quartz cementation. The influx may still be going on.

A stable and 40Ar/39Ar isotope study of a major thrust in the Helvetic nappes (Swiss Alps): Evidence for fluid flow and constraints on nappe kinematics

Stable isotope and 40 Ar/ 39 Ar measurements were made on samples associated with a major tectonic discontinuity in the Helvetic Alps, the basal thrust of the Diablerets nappe (external zone of the Alpine Belt) in order to determine both the importance of fluids in this thrust zone and the timing of thrusting. A systematic decrease in the δ 18 O values (up to 6‰) of calcite, quartz, and white mica exists within a 10- to 70-m-wide zone over a distance of 37 km along the thrust, and they become more pronounced toward the root of the nappe. A similar decrease in the δ 13 C values of calcite is observed only in the deepest sections (up to 3‰). The δD SMOW (SMOW = standard mean ocean water) values of white mica are −54‰ ± 8‰ ( n = 22) and are independent of the distance from the thrust. These variations are interpreted to reflect syntectonic solution reprecipitation during fluid passage along the thrust. The calculated δ 18 O and δD values (versus SMOW) for the fluid in equilibrium with the analyzed minerals is 12‰ to 16‰ and −30‰ to +5‰, respectively, for assumed temperatures of 250 to 450 °C. The isotopic and structural data are consistent with fluids derived from the deep-seated roots of the Helvetic nappes where large volumes of Mesozoic sediments were metamorphosed to the amphibolite facies. It is suggested that connate and metamorphic waters, overpressured by rapid tectonic burial in a subductive system escaped by upward infiltration along moderately dipping pathways until they reached the main shear zone at the base of the moving pile, where they were channeled toward the surface. This model also explains the mechanism by which large amounts of fluid were removed from the Mesozoic sediments during Alpine metamorphism. White mica 40 Ar/ 39 Ar ages vary from 27 Ma far from the Diablerets thrust to 15 Ma along the thrust. An older component is observed in micas far from the thrust, interpreted as a detrital signature, and indicates that regional metamorphic temperatures were less than about 350 °C. The plateau and near plateau ages nearest the thrust are consistent with either neocrystallization of white mica or argon loss by recrystallization during thrusting, which may have been enhanced in the zones of highest fluid flow. The 15 Ma 40 Ar/ 39 Ar age plateau measured on white mica sampled exactly on the thrust surface dates the end of both fluid flow and tectonic transport.

Fluid inclusion and isotopic systematics of an evolving magmatic-hydrothermal system

The Geysers, California, is the site of a long-lived hydrothermal system that initially developed 1.5–2 m.y. ago in response to the intrusion of a hypabyssal granitic pluton. Although wells drilled into The Geysers produce only dry steam, fluid inclusion, isotopic, and mineralogic data demonstrate that the present vapor-dominated regime evolved from an earlier and more extensive, liquid-dominated hydrothermal system. Circulation of these early fluids produced veins characterized by tourmaline ± biotite ± actinolite ± clinopyroxene within the pluton and adjacent biotite-rich hornfels, actinolite ± ferroaxinite ± epidote and epidote ± chlorite within the intermediate parts of the thermal system and calcite in the outer parts. Potassium feldspar and quartz are present in all assemblages. Pressure-corrected homogenization temperatures and apparent salinities of fluid inclusions trapped in vein minerals range from 440°C and 44 wt% NaCl equivalent within the hornfels (<600 m from the pluton) to 325°C and 5 wt% NaCl equivalent at distances of approximately 1500 m from the intrusion. We suggest that the shallow, moderate salinity fluids are connate waters modified by water-rock interactions while the high-salinity fluids are interpreted as magmatic brines. Halite-dissolution temperatures of inclusions in the hornfels and pluton indicate that the magmatic fluids were trapped at lithostatic pressures (300–900 bars). In contrast, homogenization temperatures of the connate fluids suggest trapping under hydrostatic pressures of less than several hundred bars. Whole-rock δ 18O values of samples from The Geysers display systematic variations with respect to depth, location within the field, and grade of alteration. At depths below +610 m relative to mean sea level, the δ 18O values are strongly zoned around a northwest-southeast trending low located near the center of the steam reservoir. As the pluton beneath The Geysers is approached, the δ 18O values decrease from approximately + 14 per mil near the surface to +4 to +7 per mil within the hornfels. The δ 18O values then increase to +8 to + 10 per mil at the intrusive contact, and thereafter, vary little within the pluton. Calculated rock-water fractionation factors suggest that temperature was the dominant control on the vertical decrease in the isotopic values of the reservoir rocks. In contrast, the increase in the δ 18O values within the hornfels suggest equilibration with an isotopically heavy fluid and/or decreasing water:rock ratios. The formation of vapor-dominated conditions is reflected in the abrupt appearance of low salinity (0.0–0.4 wt% NaCl equivalent) fluid inclusions with homogenization temperatures near 265°C. These inclusion fluids are thought to represent steam condensate that formed as the early liquid-dominated system boiled off. Bladed calcite, which is corroded in places, and late-stage clays provide mineralogic evidence of boiling and the formation of an acidic condensate in the upper part of the reservoir and in the overlying caprock. At greater depths, however, clear mineralogic evidence of boiling is lacking. The present vapor-dominated regime at The Geysers consists of two hydraulically connected steam reservoirs. Within the main steam reservoir, pressures are vaporstatic and temperatures are nearly constant at 240°C. Beneath this reservoir in the northwestern third of the field, temperatures as high as 342°C have been encountered. Wells drilled into this deeper reservoir discharge corrosive, Cl-bearing steam.

Near-surface and deep groundwaters

Groundwater occurs in all climatic zones and differs in hydrodynamic, isotopic and chemical terms according to its activity within the water cycle and its interaction with rocks. Among the many chemical components in groundwaters, tritium concentrations allow differentiation between groundwater of high and low mobility and thus permit the distinction between shallow and deep groundwaters. In Germany shallow groundwaters range in depth between 10 and 100 m and account for more than 85% of groundwater recharge. Deep groundwaters, in contrast, participate in the water cycle only at intervals of hundreds or thousands of years and reach depths of several hundred metres. Both types of groundwater overlie connate waters that have been isolated from the water cycle since the accumulation of the sediments in which they occur. The exploitation of deep groundwaters mostly causes long-term non-steady-state conditions, large-scale changes of flow patterns of the groundwaters and hydraulic short circuits to shallow aquifers. Thus, persistent contaminants may penetrate extraction wells in deep groundwaters even if the wells are believed to be well protected against pollution. In deep groundwaters it is difficult to apply the conventional use of groundwater protection zones to prevent this kind of intrusion of contaminants.

Near-surface and deep groundwaters

Groundwater occurs in all climatic zones and differs in hydrodynamic, isotopic and chemical terms according to its activity within the water cycle and its interaction with rocks. Among the many chemical components in groundwaters, tritium concentrations allow differentiation between groundwater of high and low mobility and thus permit the distinction between shallow and deep groundwaters. In Germany shallow groundwaters range in depth between 10 and 100 m and account for more than 85% of groundwater recharge. Deep groundwaters, in contrast, participate in the water cycle only at intervals of hundreds or thousands of years and reach depths of several hundred metres. Both types of groundwater overlie connate waters that have been isolated from the water cycle since the accumulation of the sediments in which they occur. The exploitation of deep groundwaters mostly causes long-term non-steady-state conditions, large-scale changes of flow patterns of the groundwaters and hydraulic short circuits to shallow aquifers. Thus, persistent contaminants may penetrate extraction wells in deep groundwaters even if the wells are believed to be well protected against pollution. In deep groundwaters it is difficult to apply the conventional use of groundwater protection zones to prevent this kind of intrusion of contaminants.

Pore-Water Chemistry and Diagenesis of the Modern Fraser River Delta

ABSTRACT Mixing of meteoric and marine water and diagenetic reactions within sediments of the modern (< 10,000 yr old) Fraser River Delta have been studied using sediment cores and pore-water compositions from 15 cored holes. Pore waters in the Fraser River delta sediments appear to be connate waters, and the concentration of major ions (Cl-, Mg2+, Na+ reflects the degree of mixing of meteoric water and marine water that occurred at the sediment-water interface as the sediments were deposited. Deviations from simple mixing of meteoric and marine water are observed for Ca2+, Mn2+, Fe2+, SO42-, and HCO3-, suggesting diagenetic modification of the pore water since sediment deposition. Although Na+ concentrations appear to be controlled by simple mixing rather than by diagenetic reactions, examination of reactions between clay minerals and water suggests that pore waters are in cation exchan e equilibrium with smectite. Some chemical modification of the pore waters has occurred subsequent to sediment deposition, but there is no chemical evidence either for extensive meteoric-water flushing of the sediments or for tidal influxes of marine water into the sediments. Framboidal pyrite and calcite are the only diagenetic minerals observed in the sediments. Calcite concretions formed at, or near, the sediment-water interface in distributary channels. Calcite in carbonate concretions is depleted in 13C, relative to PDB, because of oxidation near the surface of 13C-depleted methane formed at greater depths by methanogenesis. Oxidation of methane caused elevated HCO-3 concentrations that reacted with Ca2+ supplied by tidal influxes of marine water to promote calcite precipitation in this localized environment.