Evolution Of Formation Waters Research Articles

Geochemistry of formation waters and crude oils in the Shulu Sag, Bohai Bay Basin, NE-China, to assess quality and accumulation of hydrocarbons

Geochemical fingerprinting of formation waters and hydrocarbons can be utilized to track the migration, accumulation, and preservation of hydrocarbons. The present case study from the Shulu Sag, Bohai Bay Basin, NE-China analyzes the origin of the formation waters and the relationship between water chemistry and hydrocarbon preservation by combining the geochemical characteristics of formation waters and crude oils. There are three geochemical groups of formation waters with different origins, which represent different hydrodynamic environments and hydrocarbon preservation conditions. Group 1 waters are related to meteoric water with the lowest total dissolved solids (TDS), γ(Cl–Na)/γMg and the highest γNa/γCl, representing an open environment with poor hydrocarbon preservation conditions, accompanied by super heavy oils. Group 3 waters are connate water with the highest TDS, γ(Cl–Na)/γMg and the lowest γNa/γCl, representing a closed environment with the best hydrocarbon preservation conditions, accompanied by light oils. Group 2 waters are intermediate between Group 1 and Group 3 waters, representing a semi-open environment with intermediate hydrocarbon preservation conditions, accompanied by light, medium, and heavy oils. The clear corresponding relationship between oil quality and associated water type shows that water chemistry significantly affects the oil quality and accumulation in the Shulu Sag through secondary changes. Biodegradation is the dominant effect in the strata above 2100 m. Water washing is dominant in the depth range of 2100–3400 m, and there are no noticeable secondary changes in the strata below 3400 m. A model for the origin, evolution, and flow of formation waters in the Shulu Sag is established to analyze the hydrocarbon accumulation process. Meteoric waters flow into the buried hills from Ningjin Uplift as a result of gravity flow, resulting in modification of oil composition, destruction of the oil reservoirs, and the formation of Group 1 waters. During the hydrocarbon charging, the centrifugal flow driven by overpressure flows in two directions from the source rocks, one is to the western slope and mixes with meteoric waters to form Group 2 waters and heavy oil reservoirs, and the other is to the paleo-uplift to form Group 2 and Group 3 waters and light oil reservoirs. Water chemistry is sensitive to the hydrodynamic environment change and can be an important reference for oil and gas exploration. Buried hills with Group 3 waters should be the highest priority for further exploration, whose oil reservoirs are dominated by light oil with high commercial value. Although the oil quality is poor, buried hills with Group 2 waters in the slope can be secondary targets. In comparison, the buried hills with Group 1 waters should be avoided for oil and gas exploration. The geochemistry of formation waters helps understand the origin and evolution of formation waters, predict oil quality and distribution, and further aids oil and gas exploration.

Water-rock interactions and origin of formation water in the Bohai Bay Basin: A case study of the Cenozoic Formation in Bonan Sag

The faulted lacustrine Bohai Basin in eastern China contains abundant hydrocarbon resources. In these reservoirs, understanding the sandstone diagenesis and the resulting formation water provides a means to unravel the evolution processes in the basin. In most cases, the lack of isotopic and trace element analysis tests in this type of basin limits the research on the origin and evolution of formation water in this area. We have used multivariate statistical methods to classify the geochemical characteristics of the formation water for the Cenozoic Formation of Bonan Sag in the Bohai Bay Basin. Analysis of correlations among the evolution processes of different ions in different types of formation water provides an understanding of the primary factors influencing the ion content. We also evaluate the water-rock interactions of different types of formation water to evaluate their geologic significance, and we find three types. Type I formation water includes a mixture of river water, lake water, and atmospheric precipitation and exhibits weak water-rock interactions. Type II formation water contains primitive freshwater and brackish lake water that has undergone an evolution process similar to that of type I formation water, but that was followed by evaporation and concentration, the dissolution and precipitation of calcite and iron calcite, and feldspar dissolution. Type III formation water, which is a product of rock reconstruction, originates from saline lake sediment water. After undergoing evolution processes similar to those of types I and II, type III formation water is also affected by dissolution of evaporite, albite, dolomite, and iron dolomite. Thus, type III formation water is the product of water-rock interactions such as precipitation, SO42− reduction, and pyrite precipitation in which the water-rock reaction controls the development mechanism and characteristics of the reservoir space.

Genesis of formation water in the northern sedimentary basin of South China Sea: Clues from hydrochemistry and stable isotopes (D, 18O, 37Cl and 81Br)

Formation water is an important fluid in sedimentary basins, hydrogeochemical characteristics of which has important instructions to oil and gas generation and preservation. The South China Sea (SCS) oilfield is a large oil field in China, but research on origin and evolution of formation waters in SCS oilfield has been poorly documented. In this study, major ions, as well as δ2H, δ18O, δ37Cl and δ81Br in formation waters of two basins (BBWB and ZJKB) of SCS were analyzed to fill the gap. The results showed that the chemical types of formation waters in both basins are ClNa. The relationship between Br vs major ions and 2H/18O compositions showed the origin of formation water are mainly ancient seawater. The formation waters are enriched in Ca and Sr, and depleted of Na and Mg compared with seawater mainly owing to albitization of plagioclase, whereas the dolomitization is minor. Comparing to the BBWB, the formation waters in ZJKB were more enriched in 2H and 81Br, and more depleted in 37Cl. The formation waters in ZJKB were stored in a relatively more closed environment and might have been affected more by evaporation, whereas those in BBWB might experience more influence from halite dissolution by ancient precipitation through open fracture structures.

Evolution of formation waters in the Permian Basin, United States: Late Permian evaporated seawater to Neogene meteoric water

Understanding subsurface waters is important for hydrocarbon exploration and development. Waters from the Permian Basin were collected and analyzed for stable isotopes, ionic concentrations, and strontium isotopes to determine their origin. Three main geochemical groups of waters are present. Group 1 has high δ18O and δD values with high total dissolved solids (TDS; 100–240 g/L), including high concentrations of Na+, Cl−, and Br−.Group 1 waters contain distinctly less sodium (Na+) than chlorine (Cl−) on a molar basis, similar to modern seawater. Group 2 and 3 waters have low δ18O and δD values. Group 2 waters have relatively low TDS (4–75 g/L). Group 3 waters have high TDS (170–225 g/L), Na+ and Cl− in approximately equal molar amounts, and low Br− concentrations. Group 1 waters are interpreted as forming from highly evaporated seawater during precipitation of uppermost Permian salts. Because of their high density, those waters displaced preexisting formation waters throughout the Permian Basin during the latest Permian. Waters in groups 2 and 3 came mainly from precipitation in the mountains of southeast New Mexico. Those mountains formed during Neogene tectonic uplifts. Group 3 waters acquired their Na+ and Cl− by dissolution of upper Permian salt. Many group 2 and 3 waters acquired their Ca2+ and SO42− by dissolution of Permian anhydrite or gypsum. Some waters contain a mixture of these groups. Understanding the origin of subsurface waters helps predict subsurface salinity, aquifer drive, sulfate reduction, chemical reactions during water injection, and chemistry of diagenetic waters.

Characterization of Origin and Evolution of Formation Water in Buried Hill of Jizhong Depression, China, Using Multivariate Statistical Analysis of Geochemical Data

Groundwater samples from buried hill of Jizhong Depression were evaluated using two statistical analyses: hierarchical cluster analysis (HCA) and principal component analysis (PCA). The samples were classified into four clusters, C1–C4, in HCA and the hydrochemical types of C1–C4 are HCO3-Na, Cl·HCO3-Na, Cl-Na, and Cl-Na·Ca. From C1 to C2, C3, and C4, the water-rock interaction becomes increasingly intensive, and rNa/rCl gets lower while total dissolved solids and r(Cl-Na)/rMg get higher. Three components of PCA explain 86.87% of the variance. Component1 (PC1), characterized by highly positive loadings in Na+ and Cl−, is related to evaporation concentration. Component2 (PC2) is defined by highly positive loading in HCO3- and is related to influence of atmospheric water. With high positive loadings in Ca2+ and high negative loadings in Na+ and SO42-, component3 (PC3) suggests plagioclase albitization. The combination of HCA and PCA within the hydrogeological contexts allowed the division of study area into five dynamic areas. From recharge area to discharge area, the influence of atmospheric water gets weaker and water-rock interactions such as evaporation concentration and plagioclase albitization become intensive. Therefore groundwater in buried hill showed paths of hydrochemical evolution, from C1, to C2, C3, and C4. Buried hill reservoir in Jizhong Depression is mainly distributed in hydrodynamic blocking and discharge area; therefore the two regions can be the favorable areas for petroleum migration.

Origin and evolution of formation water from the Ordovician carbonate reservoir in the Tazhong area, Tarim Basin, NW China

Chemistry, H, O and Sr isotopes of formation waters were determined from the Ordovician carbonate reservoirs in the Tazhong area, Tarim basin. The aim is to elucidate their origin and migration and their alteration by thermochemical sulfate reduction (TSR) and water-rock interactions. The waters were originated from evaporated seawater, and are enriched in Ca and Sr, and depleted in Mg and SO4 compared with the seawater evaporation trajectory (SET), thus are considered to result from dolomitization, anhydrite dissolution and in situ TSR. δ18O values show positive shift due to water-rock interactions, and δD values show negative shift with increasing TSR extents or total thiaadamantanes concentrations of the associated oils, likely resulting from hydrogen isotope exchange with TSR-H2S. However, the δD-δ18O relationship was mainly controlled by the mixing with paleo-meteoric water with δD of −50‰ and δ18O of −7.5‰. Paleo-meteoric water may have leached the Silurian siliciclastic rocks and thus has 87Sr/86Sr up to 0.715. Upper Ordovician (O3l) formation water shows more negative δD and δ18O values, higher 87Sr /86Sr ratios and lower total dissolved solids (TDS) than the O1–2y water, which may have resulted from mixing of paleo-meteoric water. Lower Ordovician (O1–2y) formation water shows 87Sr /86Sr ratio of 0.7095–0.7105, TDS from 160 to 240g/l, and Sr concentrations from 300 to 1000mg/L, which may have resulted from mixing with hydrothermal fluid up-migrated from Ediacaran and Lower Cambrian siliciclastic rocks. Significant mixings with paleo-meteoric water and hydrothermal fluid occurred at paleo-highland at the east of No.1 Fault-Slope Zone and active faulting zone near No.10 Structural Belt, respectively. Hydrothermal fluid and oils probably shared similar migration pathways in No.10 Structural Belt. Formation water geochemistry may thus provide clues to constraint on petroleum migration.

Origin and evolution of formation water in North China Plain based on hydrochemistry and stable isotopes (2H, 18O, 37Cl and 81Br)

Origin and evolution of formation water are of great hydrogeochemical interest, the research on which can provide indicator for genesis and production of oilfield. Although many studies were carried out to investigate the origin and evolution of formation water in North China Plain (NCP), the research is still insufficient because of its complicated history. Considering these, based on hydrogeochemistry and stable isotopes (2H, 18O, 37Cl and 81Br), the origin and evolution of formation waters in North China Plain were further discussed. Results indicate that formation waters both in Jizhong Depression and Huanghua Depression in NCP are derived from meteoric water (river water and/or lake water) and the primary evolution processes are evaporation and mixing with seawater. The difference between two tectonic units is that the evolution of formation waters in Jizhong Depression is similar to that of salt lake in arid area, which underwent more evaporation, while formation waters in Huanghua Depression show more trace affected by seawater. This geochemical difference reflects the tectonic difference between two depressions. It can also be concluded that 81Br behaves more conservatively than 37Cl in the evolution of groundwater under certain circumstances.

Origin of Formation Water from the Unayzah and Khuff Petroleum Reservoirs, Saudi Arabia

Chemical and isotopic fingerprinting techniques provide support for tracing the origin and evolution of formation waters taken during downhole and drill stem tests from exploration wells in Saudi Arabian oil and gas fields. Low molar Cl/Br ratios (85-93) and a δ18O reverse loop suggests an evaporative seawater origin for hypersaline water from the Upper Permian-Lower Triassic Khuff Formation. In contrast, formation water from the Upper Carboniferous-Lower Permian Unayzah Group shows a heterogeneous trend between low salinity/low δ18O freshwater and hypersaline/intermediate δ18O brine, the latter being a product of evaporated seawater. Unayzah water salinity increases linearly with depth at the rate of 77 to 98mg/L per meter, allowing quantitative prediction of water resistivity (Rw) at specific reservoir depths. The increase in Ca and depletion of Mg and HCO3 with Unayzah depth suggests the impact of temperature-triggered ion exchange processes that alter the primary host rock and formation water composition.

Origin, chemical and isotopic evolution of formation water in geopressured zones in the Pannonian Basin, Hungary

In the central part of the Pannonian Basin we investigate formation waters from two deep sub-basins with sedimentary sequences up to 7000 m thick, and from fields above basement highs where the total thickness of the sediments is only 2000 m. In each study region the depth interval involves the Lower (lower part of the Upper Miocene) and Upper (upper part of the Upper Miocene and Pliocene) Pannonian sediments separated with a widespread aquitard, named the Algyő Formation, at the top of the Lower Pannonian. However, in the fields above basement highs, it occasionally contains sand lenses, sedimentological discontinuities, faults and fractures resulting in a more conductive character. We investigate the sources and evolution of the formation waters in two geological settings: in sub-basins and in fields above an elevated basement. We distinguish NaHCO 3-type water in the sub-basins and NaCl-dominated water in the fields above the elevated basement. The isotopic composition of most NaHCO 3-type samples indicates a pure paleometeoric origin, while a few samples are a mixture of paleometeoric and non-meteoric waters. The NaCl-type samples all have a non-meteoric contribution. We have developed a model for the origin and evolution of the formation water that operates for both water types and both geological conditions. The Cl − dominated water is a mixture of ascending pre-Pannonian evaporated water and freshening Lake Pannon water trapped during the Late Miocene and Pliocene sedimentation in the area above the elevated basement, where in some places the Algyő Formation is conductive. The source and evolution of the NaHCO 3 non-meteoric water in the sub-basins are similar to those of the NaCl water, with the main difference being the aquitard character of the Algyő Formation in the sub-basins. The aquitard consists of siltstone and clay-marl, which has resulted in overpressure in the layers under the Algyő Formation. The ascending pre-Pannonian water, which mixes with in-situ pore water squeezed through the compacted clay by overpressure, is exposed to microfiltration. The Cl − remains behind, and the squeezed water is depleted of the heavier isotopes. These processes are shown by recent chemical and isotopic data ( δ 18O and δD), as well as by archival chemical and hydrological data.

Origin and evolution of formation water at the Jujo–Tecominoacán oil reservoir, Gulf of Mexico. Part 1: Chemical evolution and water–rock interaction

The origin and evolution of formation water from Upper Jurassic to Upper Cretaceous mudstone–packstone–dolomite host rocks at the Jujo–Tecominoacán oil reservoir, located onshore in SE-Mexico at a depth from 5200 to 6200 m.b.s.l., have been investigated, using detailed water geochemistry from 12 producer wells and six closed wells, and related host rock mineralogy. Saline waters of Cl–Na type with total dissolved solids from 10 to 23 g/L are chemically distinct from hypersaline Cl–Ca–Na and Cl–Na–Ca type waters with TDS between 181 and 385 g/L. Bromine/Cl and Br/Na ratios suggest the subaerial evaporation of seawater beyond halite precipitation to explain the extreme hypersaline components, while less saline samples were formed by mixing of high salinity end members with surface-derived, low salinity water components. The dissolution of evaporites from adjacent salt domes has little impact on present formation water composition. Geochemical simulations with Harvie-Mφller-Weare and PHRQPITZ thermodynamic data sets suggest secondary fluid enrichment in Ca, HCO3 and Sr by water–rock interaction. The volumetric mass balance between Ca enrichment and Mg depletion confirms dolomitization as the major alteration process. Potassium/Cl ratios below evaporation trajectory are attributed to minor precipitation of K feldspar and illitization without evidence for albitization at the Jujo–Tecominoacán reservoir. The abundance of secondary dolomite, illite and pyrite in drilling cores from reservoir host rock reconfirms the observed water–rock exchange processes. Sulfate concentrations are controlled by anhydrite solubility as indicated by positive SI-values, although anhydrite deposition is limited throughout the lithological reservoir column. The chemical variety of produced water at the Jujo–Tecominoacán oil field is related to a sequence of primary and secondary processes, including infiltration of evaporated seawater and original meteoric fluids, the subsequent mixing of different water types and the formation of secondary minerals by water–rock interaction. A best fit between measured and calculated reservoir temperatures was obtained with the Mg–Li geothermometer for high salinity formation water (TDS > 180 g/L), whereas Na–K, Na–Ka–Ca and quartz geothermometers are partially applicable for less salinite water (TDS < 23 g/L).

Origin and evolution of formation water at the Jujo–Tecominoacán oil reservoir, Gulf of Mexico. Part 2: Isotopic and field-production evidence for fluid connectivity

Abstract The chemical and isotopic characterization of formation water from 18 oil production wells, extracted from 5200 to 6100 m b.s.l. at the Jujo–Tecominoacan carbonate reservoir in SE-Mexico, and interpretations of historical production records, were undertaken to determine the origin and hydraulic behavior of deep groundwater systems. The infiltration of surface water during Late Pleistocene to Early Holocene time is suggested by 14C-concentrations from 2.15 to 31.86 pmC, and by 87Sr/86Sr-ratios for high-salinity formation water (0.70923–0.70927) that are close to the composition of Holocene to modern seawater. Prior to infiltration, the super-evaporation of seawater reached maximum TDS concentrations of 385 g/L, with lowest δ18O values characterizing the most hypersaline samples. Minor deviations of formation water and dolomite host rocks from modern and Jurassic 87Sr/86Sr-seawater composition, respectively, suggest ongoing water–rock interaction, and partial isotopic equilibration between both phases. The abundance of 14C in all sampled formation water, 87Sr/86Sr-ratios for high-salinity water close to Holocene – present seawater composition, a water salinity distribution that is independent of historic water-cut, and a total water extraction volume of 2.037 MMm3 (1/83–4/07) excludes a connate, oil-leg origin for the produced water of the Jurassic–Cretaceous mudstone-dolomite sequence. Temporal fluctuations of water chemistry in production intervals, the accelerated migration of water fronts from the reservoir flanks, and isotopic mixing trends between sampled wells confirms the existence of free aquifer water below oil horizons. Vertical and lateral hydraulic mobility has probably been accelerated by petroleum extraction. The combination of interpreting historical fluctuations of salinity and water percentage in production wells with chemical-isotopic analysis of formation water resulted in a successful method to distinguish four groundwater bodies, stratified vertically within the Jujo–Tecominoacan reservoir. Two with low TDS from 10 to 23 g/L are preserved in the upper reservoir section, mainly in Lower Cretaceous and Kimmeridgian strata. For the deeper part of the reservoir, 87Sr/86Sr trends indicate an affiliation of most samples to two independent mixing trends between “intermediate saline” (TDS ∼200 g/L) and hypersaline (>350 g/L) groundwater end-members.

The behavior of lithium and its isotopes in oilfield brines: evidence from the Heletz-Kokhav field, Israel

Twenty-four brine samples from the Heletz-Kokhav oilfield, Israel, have been analyzed for chemical composition and Li isotope ratios. The chemical composition of the brines, together with geological evidence, suggests derivation from (Messinian) seawater by evaporation that proceeded well into the gypsum stability field but failed to reach the stage of halite crystallization. The present salinity of the samples (18–47 g Cl/L) was achieved by dilution of the original evaporitic brine by local fresh waters. Like brines from other sedimentary basins, the Li/Cl ratios in the Heletz-Kokhav samples show a prominent Li enrichment (five-fold to eight-fold) relative to modern seawater. The isotopic ratios of Li, expressed in the δ 6Li notation, vary from −26.3 to −17.9‰, all values being significantly higher than that of modern seawater (−32‰) irrespective of their corresponding Li concentration (1.0–2.3 mg/L). The isotopic composition of Li and the Li/Cl ratio in the oilfield brines were acquired in two stages: (a) The original evaporated seawater gained isotopically light Li during the diagenetic interaction between the interstitial Messinian brine and the basin sediments. A parent brine with an elevated Li/Cl ratio was formed. The brine was later diluted in the oilfields. (b) The δ 6Li values of the final brines were determined during epigenetic interaction with the Heletz-Kokhav aquifer rocks. At the same time, the Li/Cl ratio inherited from stage (a) remained largely unchanged. This work represents the first use of lithium isotopic composition to elucidate the origin and evolution of formation waters in sedimentary basins.

ABSTRACT: Effects of Basin-Scale Fluid Flow and Diagenesis on the Evolution of Formation Waters in the West-Central Part of the Alberta Basin, Canada

The origin and evolution of formation waters were investigated in the west-central part of the Alberta Basin in Canada, with the aim to determine how these waters were affected by fluid flow and diagenesis. The major ion chemistry of formation waters from various aquifers throughout the entire stratigraphic succession was used to identify: a) how deep meteoric water has penetrated into the subsurface, and to what extent it has flushed connate formation waters, b) areas of cross-formational flow and mixing, where intervening aquitards are weak or missing, and c) processes that influence and alter formation water chemistry. Elucidation of these issues helps to better understand the interaction of different flow systems in the Alberta Basin and to characterize the evolution of formation waters with respect to the basin history. Using mainly salinity and Na/Cl distributions, the formation waters can be divided into four main groups: 1) Tertiary-Upper Cretaceous - mainly meteoric water, 2) Lower Cretaceous various degrees of mixing between seawater and meteoric water, 3) JurassicMississippian - evaporated seawater that was altered by halite dissolution and dolomitization, and 4) Devonian-Cambrian - evaporated seawater that underwent a high degree of water-rock interaction (dolomitization, halite dissolution), and albitization of feldspar. Mixing of these waters is inhibited by the combination of competent intervening shale aquitards and weak flow-driving mechanisms in the deep parts of the stratigraphic succession.

Abstract :Basin-Scale Evolution of Formation Waters revealed from Diagenetic Minerals: Does It Help Us Predict Reservoir Quality?

Abstract :Basin-Scale Evolution of Formation Waters revealed from Diagenetic Minerals: Does It Help Us Predict Reservoir Quality?

Glacial recharge and paleohydrologic flow systems in the Illinois basin: Evidence from chemistry of Ordovician carbonate (Galena) formation waters

The Illinois basin provides an opportune setting for elucidating the roles of remnant evaporite brines and meteoric waters in the evolution of formation waters in an intracratonic sedimentary basin. Formation waters from carbonate reservoirs in the Upper Ordovician Galena Group have been analyzed geochemically to study the origin of their salinity, their chemical and isotopic evolution, and their relationship to paleohydrologic flow systems. Chloride/bromide ratios and CI/Br-Na/Br relations indicate that initial brine salinity resulted from subaerial evaporation of seawater rather than from halite dissolution. Subsequent subsurface dilution of the brines by meteoric waters is disclosed by δD-δ 18 O covariance; however, the remnant evaporite brine has not been completely expelled from these Ordovician strata. Galena formation waters have 87 Sr/ 86 Sr ratios that range from 0.708 17 (a value nearly equal to that of coeval seawater) to 0.710 43. This is the greatest range of Sr isotopic ratios found in waters from any stratigraphic unit in the basin. Two fluid mixing events are revealed in plots of 87 Sr/ 86 Sr vs. 1/Sr: introduction of 87 Sr-enriched fluids from a siliciclastic source, probably overlying Maquoketa shale, and a later event that only affected reservoir waters in the western shelf of the basin. General covariance between Sr and H-O isotopes suggests that the later event is related to meteoric water recharge. The point of intersection of the δD-δ 18 O trend with the meteoric water line implies that this mixing event involved Pleistocene glacial meltwater that recharged Galena reservoirs near outcrops along the western margin of the basin. Ordovician Galena formation waters are geochemically distinct from those in both Silurian-Devonian and Mississippian-Pennsylvanian strata, a distinction that has evidently been maintained by the overlying Maquoketa regional aquitard.

Geochemistry and origin of formation brines from the Paris Basin, France: 2. Saline solutions associated with oil fields

Oil fields from the Paris Basin are located in three geological levels: Dogger (Middle Jurassic), Rhaetian and Keuper (Upper Triassic). The origin of water and dissolved salts in saline solutions associated with oil is investigated, using the hypothesis that Cl − and Br − are conservative in solution. In a diagram of Cl − Br − ratios vs. Cl −, the points of the Dogger and the Keuper aquifers lie in the zone of primary brines whereas those of the Rhaetian are located in the zone of secondary brines. The Na + Cl − ratio is very high in all of the saline solutions, suggesting either a relatively poorly evolved brine (at the beginning of halite deposition) or some halite dissolution. All of the saline solutions, especially those from the Dogger aquifer, are clearly diluted. Dilution by meteoric waters would not alter the Cl − Br − ratio. High concentrations of lithium, rubidium and boron are attributed to the presence of an extremely evolved brine of marine origin in mixtures of secondary brines formed by dissolution of halite and small amounts of sylvite and Ca-sulphate. Mass-balance calculations of the excess of Cl − with respect to Na + + K + allows the fraction x of water derived from the primary brine to be calculated. Assumptions are that all Na + and K + is derived from the secondary brine and that the Cl − content of the primary brine is constrained by bischofite saturation at ∼ 11,000 mmol kg −1. The typical proportion of water from the primary brine waters is ∼ 0.7% in the Dogger formation and ∼ 2.4% in the Keuper. Values of the concentration factor CF, expressed as the ratio of the molal concentration of a conservative ion in the evolved brine to its concentration in seawater have been calculated for lithium. Solutions from the Dogger and Keuper aquifers have CF Li + -values of 1.8 · 10 3 and of 7.7 · 10 3, respectively. In the Rhaetian aquifer, the linear relationship between heavy-isotope contents of the brine ( 2H and 18O) and the Cl − content indicates a mixture of two water sources. The first one is meteoric water as defined by the intersection of the formation water line with the global meteoric water line ( δ 2 H = −65‰ , δ 18 O = −9.3‰ ). The other end-member is enriched in heavy isotopes and in Cl −, and is attributed to seawater that has dissolved evaporites (mainly halite). However, the presence of small amounts of an extremely evolved brine is required to account for the bromide, lithium, rubidium and boron contents of the formation brine from the Rhaetian aquifer. As this contribution is very limited in amount, it cannot affect the heavy-isotope content of the formation water. Mass-balance equations are solved for ternary mixtures of meteoric water, seawater and brine. The same constraints as above (no Na + and K + supplied by the primary brine and Cl Brine ≈ 11,000 mmol kg −1) are applied. It is also assumed that the seawater component has the same ionic contents as modern seawater and that the salt content of the meteoric component is negligible. The fraction of primary brines is x ≈ 1.4% according to the Cl − balance. This estimate agrees with that obtained from the lithium balance assuming that the primary brine component was the same as that of the Keuper formation water. Mass-balance calculations also indicate that the SO 4 2− content of the Rhaetian formation water is exclusively due to the secondary brine component. In the three saline solutions, the very high Ca 2+ contents and high Ca 2+ Mg 2+ ratios, may be attributed to reaction of the very evolved brine (free of SO 2− 4), with gypsum or anhydrite. The result is an exchange of Mg 2+ for Ca 2+, which causes a release of Ca 2+ to the solution and a precipitation of secondary Mg-sulphate. This process can also account for the high strontium contents of the analysed samples. Values of the ratio 87 Sr 86 Sr are very high in the Keuper formation water due to close contact of the solutions with the granitic basement or with granite-derived detrital deposits. Values in the Rhaetian and Dogger aquifers equal those of Upper Triassic marine deposits. The stable isotope ( 34S and 18O) content of the dissolved aqueous SO 2− 4 indicates a Triassic origin for the bulk of sulphate involved in the evolution of formation waters from the Rhaetian and from the Dogger. All geochemical indicators thus show a vertical migration of Triassic brines. However, aqueous SO 2− 4 from the Keuper has a Permian isotopic signature suggesting a reworking in solution of salts from this period.

Origin and evolution of formation waters, Alberta Basin, Western Canada Sedimentary Basin. II. Isotope systematics and water mixing

Isotopic measurements (Sr, O, D) on formation waters from the Alberta Basin have been made, covering a stratigraphic range from Devonian to Upper Cretaceous. These measurements, combined with chemical compositional trends, give evidence for two distinct water regimes. One hydrological regime is composed of waters hosted in Devonian-Lower Cretaceous reservoirs, the other waters from Upper Cretaceous and younger sedimentary rocks. The two regimes are separated by a regional transgressive shale in the Colorado Group, the Second White Speckled Shale Formation. The waters within the Devonian-Lower Cretaceous regime exhibit a large range in 87Sr/ 86Sr values (0.7076–0.7129), but have similar Sr concentrations, regardless of host lithology. Bulk rock and late-stage diagenetic cements are less radiogenic than present brines. Importantly, brines from Devonian carbonates possess the most radiogenic Sr isotopic signatures of the waters examined. Devonian shales and/or Cambrian shales may be sources of high 87Sr/ 86Sr ratios in the carbonate-hosted waters. Waters from the Upper Cretaceous clastic units, which have ratios as low as 0.7058, and diagenetic cements from Upper Cretaceous clastic units appear to have precipitated from fluids similar in Sr isotopic value to modern brines. High Sr concentrations in the Cretaceous clastic waters and sedimentary rocks and correspondingly low 87Sr/ 86Sr ratios suggest that volcanism in Montana during the Cretaceous may have provided a source of sediments to the study area. Cross-formational upward water migration, superimposed on lateral fluid flow, is required to explain the geochemistry and isotopic systematics in the brines from Devonian-Lower Cretaceous reservoirs. Strontium isotope ratios and Sr contents suggest a two component mixing relation for these waters. This system of waters also exhibits δD values characteristic of meteoric values in the Neogene, reflecting post-Laramide flushing of Tertiary waters throughout the basin, with subsequent hydrochemical isolation from more modern waters. In contrast, waters in Upper Cretaceous reservoirs have O and D isotopic compositions similar to those of present day rainfall; these, in conjunction with very dilute Sr concentrations and low Sr ratios, suggest hydrological isolation from the stratigraphically lower system.

Origin and evolution of formation waters from oil fields on the Norwegian shelf

Stable isotopes of H and O and chemical analyses of formation waters from Upper Triassic/Jurassic clastic reservoirs and Cretaceous carbonate reservoirs offshore Norway are employed to interpret the origin and evolution of these waters. In both types of reservoir the formation waters become progressively enriched in total dissolved solids and the heavy isotope of O with increasing depth, whereas deuterium varies erratically between −20 and −40ℵ (SMOW). Bromine analyses and stable isotope data indicate that the formation waters are mixtures of a brine end member, formed by evaporation of sea water beyond the point of halite saturation, and meteoric water. The chemical and isotopic composition of the formation waters and general geological considerations suggest that the brines originated as connate waters in Permian evaporites and were later expelled into overlying strata. There is a disequilibrium stable isotope distribution between the bulk chalk formations and the formation waters, and only minor diagenetic modification of the formation waters appears to have taken place in these reservoirs. The formation waters from the clastic reservoirs, on the other hand, are modified by diagenetic reactions.