Connate Fluids Research Articles

Vanadium mineralization at Los Chihuidos sediment-hosted Cu–V deposit, Neuquén Basin, Argentina: An approach to vanadium ore forming processes

Vanadium mineralization at Los Chihuidos deposit of the Neuquén Basin is linked to the development of a redox front system related to the inflow of hydrocarbons into the red sandstone of the Huincul Formation. Interaction of hydrocarbons with oxidized red beds and connate water generated redox reactions where hematite was dissolved due to iron reduction resulting in the discoloration of the red strata. At the contact between oxidized red sandstone and reduced white sandstone, precipitation of specific mineral phases resulted in the V ore with minor amounts of Cu. With the implementation of the redox interface, abundant V-montmorillonite and V-hematite precipitated at the more oxidizing conditions and Cu-V-corrensite-type at the more reducing conditions of the redox front. As the redox front advanced with fluids constantly migrating into the reservoir, more reducing conditions were stablished, promoting chloritization and minor illitization with V-Cu incorporation and continuous upgrading of the ore. Main ore mineralogy consists of clay minerals including V-bearing montmorillonite, Cu-V-corrensite-type, V-di-trioctahedral chlorite and Cu-tri-trioctahedral chlorite with minor V-illite–smectite mixed-layer minerals and associated secondary V-hematite. Chloritization over illitization was favored due to high amounts of Fe and Mg in detrital clasts and in connate fluids and by low K availability related to low amounts of detrital K-feldspar. The spatial transition of V and /or Cu bearing clay minerals observed through the mineralized redox front at Los Chihuidos deposit (kaolinite → smectite → illite/smectite → corrensite-type → di- trioctahedral -chlorite → tri- trioctahedral -chlorite) and the related variation of V–Cu concentrations in bulk rock are indicative of increasing pH and decreasing Eh of resident solutions from red to white sandstones during the hypogene mineralization process. Late influx of Cu-rich oxidized basinal brines precipitated main copper ore with Cu-sulfides in the white sandstone up to the contact with the redox front in contact with hydrocarbons. During uplift and exhumation, percolation of meteoric water promoted remobilization of V and Cu and the precipitation of oxidized V–Cu ore.

High-Resolution Time-Lapse Monitoring of Mud Invasion in Spatially Complex Rocks Using In-Situ X-Ray Radiography

Borehole measurements, such as electrical resistivity, neutron porosity, or nuclear magnetic resonance, are critical for the in-situ petrophysical assessment of subsurface rocks. However, the interpretation of borehole measurements is often subject to uncertainty arising from their sensitivity to the interplay between mud filtrate, connate fluids, and the rock’s pore structure. This uncertainty remains present even in homogeneous geological formations. Mudcake deposition on the borehole wall causes additional complexity, impacting both well construction and formation evaluation. It is, therefore, essential to account for the latter effects and perform appropriate corrections when interpreting borehole measurements. Recently, new experimental procedures were introduced to quantitatively describe the process of mud invasion under realistic rock and fluid conditions, focusing on gas-bearing rocks and without considering how original saturating fluids affected the process of invasion. Both mud-filtrate invasion and filter-cake deposition must be understood and incorporated into numerical and analytical models to reliably interpret borehole measurements and maximize value. This objective can only be fulfilled via experiments. We use X-ray microfocus radiography to examine in real time the processes of mud-filtrate invasion and internal and external mudcake deposition in thin rectangular rock samples. The high-resolution experimental procedure (10 to 30 μm) mimics the borehole and near-wellbore regions and facilitates the time-lapse visualization of in-situ fluid-transport processes in spatially complex rocks. Water- and oil-based muds were injected into rock samples initially saturated with a range of different connate fluids, including viscous liquids, while being continuously scanned with X-rays. Because the injected drilling muds were the same across all experiments, the observed discrepancies between experiments originate from differences in rock properties, heterogeneity and anisotropy, or initial fluid saturation conditions. Experimental results emphasize the effect of rock heterogeneity and initial connate fluid on the spatial distribution of fluids and mudcake formation ensuing from mud-filtrate invasion. Mud-filtrate invasion rates and final average mudcake thicknesses were similar across all cases for a given drilling mud, suggesting that mudcake properties, as opposed to rock properties, were the controlling factors. By contrast, the spatial distribution of fluids in each rock sample varied significantly between cases, highlighting the impact of rock heterogeneity/anisotropy on the process of invasion. Laboratory experiments also emphasize the impact of viscous and/or capillary forces on mud-filtrate flow behavior. The experimental method is efficient and reliable, allowing for a better understanding of the uncertainty of the effects of mud-filtrate invasion on borehole geophysical measurements acquired while or after drilling.

Metallogenesis of Ni–Co–Fe-arsenide type mineralization in the Purísima-Rumicruz deposit, Jujuy, Argentina

The Purísima-Rumicruz vein system is a Ni–Co–Fe-arsenide type or Five-Element deposit located in the Cordillera Oriental, northwestern Argentina. The host rocks are Ordovician black shales of the Acoite Formation. Three vein types, were identified (a) sulfides/sulfoarsenides + carbonates ± quartz; (b) barite; and (c) crystalline quartz veins. Sulfide-carbonate veins are narrow and exhibit multi-episode infill and are NW-SE and E-W oriented. Four successive stages of mineral precipitation were established. The first stage, sulfide-rich, comprises galena, pyrite, chalcopyrite, bornite, and Zn-rich tetrahedrite-tennantite. A second stage corresponds to native silver, arsenides, Ni-sulfarsenides and pitchblende, indicative of a rise in the As/S ratio in the hydrothermal fluids. The third stage evidences a decrease in the As/S ratio with sulfide precipitation of silver sulfides, chalcocite, chalcopyrite, tetrahedrite-tennantite, bornite and millerite. Finally, a supergene event with covellite, digenite, limonite and Cu-carbonates occurred. Microthermometric studies of Stage 1 quartz yield Th between 185 and 276 °C and 5.2–12 wt% eq. NaCl. Salinities. Stage 3 calcite yield Th 110-240 °C and salinities in two ranges, 1–11 and 16.8–24 %wt. NaCl eq. The δ34S values in chalcocite and galena range between +5.8 and + 7‰, and the δ18OSMOW values from quartz, are between +6.2 and + 9.0‰. δ18OSMOW and δ13CPDB values in calcite samples range between +10.9 and +14.7‰, and −3.05 to −4.07‰, respectively. Isotope data indicate a cortical source of the fluids, probably from connate waters. Early Cretaceous rifting produced a thermal anomaly and direct faults and fractures. Deep-seated brines mixed with connate fluids of the Acoite Formation led to the formation of the Purísima-Rumicruz veins.

Geological, geochemical and fluid inclusion investigations on the Duna Pb-Ba-(Ag) deposit, Central Alborz, North Central Iran

The Duna Pb-Ba-(Ag) mine is located ca. 155 km north of Tehran in the central Alborz structural zone, northern Iran. The ore mineralization occurs as stratabound, epigenetic, E-W and WSWENE trending veins and veinlets in fracture-controlled as well as massive and open-space filling textures within a Permian dolomitic limestone host rock. Field observations as well as mineralogical and petrographic studies show dolomitization, silicification and possibly haematisation in the host rock. In the mineralization zone, galena and barite are the main minerals, followed by pyrite, quartz, chalcopyrite, tetrahedrite, sphalerite, calcite and supergene minerals such as covellite, malachite, azurite, cerussite, anglesite, and Fe-oxides. The chemical analyses of the highgrade ore samples show an average grade of 18.66 wt. % for Pb, 19.99 wt. % for Ba, and 120 ppm for Ag together with substantial quantities of Zn (0.15 wt. %), As (690 ppm), Cu (0.86 wt. %), Sb (0.25 wt. %), and Sr (0.56 wt. %). The amount of silver in some samples from the tunnel and discordant layers is up to 7030 ppm. The positive Eu/Eu⃰ ratio and the weak negative Ce/Ce⃰ anomaly in the ore samples were most likely inherited from magmatic water. The presence of minerals such as pyrite and chalcopyrite together with the co-precipitation of sphalerite and chalcopyrite suggest a high-temperature for mineralizing fluids. The homogenization temperatures of fluid inclusions from barite in concordant layers span between 135 and 165 ºC with salinities between 18.54 and 23.65 wt. % NaCl equivalent, while the homogenization temperatures of fluid inclusions from barite of discordant layers span between 113 and 285 ºC with salinities between 7.34 and 23.65 wt. % NaCl equivalent. The structural, geological, geochemical, and mineralogical studies together with the paragenesis of the ore minerals and fluid inclusion data allow consideration of the Duna Pb-Ba-(Ag) mine as a two stage mineralization scenario; 1st stage /older/MVT-type (Early Cimmerian tectonic phase), and the second stage/younger/Irish-type (Laramide orogenic movements). The structural data, high temperature of the fluid inclusions, positive Eu/Eu⃰ ratio and high silver content, especially in the discordant layers, indicate the involvement of a magmatic water mixed with meteoric and connate fluids comparable to the Irishtype mineralization in the second stage, which formed along brecciated zones of the thrust faults. The second stage of mineralization was most likely influenced by the Akapol granitoid intrusive mass, which overprinted the 1st stage/older/MVT-type.

Pore-scale numerical simulation of supercritical CO2 migration in porous and fractured media saturated with water

A thorough understanding of the microscopic flow process in porous and fractured media is significant for oil and gas development, geothermal energy extraction and subsurface CO 2 storage etc. In CO 2 geological sequestration, the CO 2 is often injected at the supercritical state (scCO 2 ), which will displace the connate fluids in the pore spaces during the drainage process. However, when CO 2 injection stops, the connate brine or water flows back to displace the scCO 2 . Therefore, the configuration of migration paths in a specific reservoir plays a significant role in affecting the connectivity and storage efficiency of scCO 2 . In this paper, the two-phase (scCO 2 and water) boundary has been defined using the phase field method, and the COMSOL Multiphysics simulator is applied to study the migration of scCO 2 in porous/fractured media at the pore scale. The geological conditions of Shiqianfeng formation in the CO 2 capture and storage pilot site of the Ordos Basin in China is selected as the engineering background. Before using the actual microscopic geometry based on thin-section of Shiqianfeng sandstone, we get the general understanding on scCO 2 migration in fractured porous media that has the highly simplified configuration with circular particles, considering the impacts of wettability, geometry of formation mineral grains, interfacial tension, injection rates, and fracture geometry. Results show that the CO 2 preferential flow occurs at locations with high CO 2 flow rates and high CO 2 pore pressure. The preferential flow of scCO 2 occurs adjacent to the wall of grains while minimal or little flow takes place through the interior between the grains, considering the grains with irregular shapes. The interfacial tension of porous media plays a significant role in controlling the spatial distribution of the scCO 2 . A much lower interfacial tension results in a much thinner scCO 2 flow band with a much higher saturation. The geometry of fractures in porous media increases the complexity of the scCO 2 flow paths at the pore scale. Cited as : Liu, H., Zhu, Z., Patrick, W., Liu, J., Lei, H., Zhang, L. Pore-scale numerical simulation of supercritical CO 2 migration in porous and fractured media saturated with water. Advances in Geo-Energy Research, 2020, 4(4): 419-434, doi: 10.46690/ager.2020.04.07

Geochronology of calcite‐filled joints, southeast Canada: Insight into Late Cretaceous deformation of eastern North America

AbstractResolving the timing of brittle tectonism is a challenge. We examined intra‐cratonic Middle Ordovician limestone bedrock that overlies Mesoproterozoic crystalline basement, which are both cut by ENE‐WSW trending fault zones that have historic M4‐5 earthquakes along their trace. Calcite‐filled mode I joints are ubiquitous within the strata, and are parallel to the modern stress field. Calcite δ18O and δ13C values are analogous to the bulk composition of Middle Ordovician limestones, and suggest vein formation from a source dominated by connate fluids of the host rock. U‐Pb geochronology of six calcite veins sealing ENE‐WSW oriented joints produced similar Cretaceous dates and were regressed together yielding a date of 96.8 ± 1.1 Ma (MSWD: 2.9). A Cenomanian age is consistent with the most recent reorientation of North America's compressive stress field, which is attributed to a change in spreading direction along the northern Mid‐Atlantic Ridge and is linked to a global plate reorganization event at ca. 105–100 Ma. These results suggest that the most recent redistribution of lateral stress was not a passive event, and generated brittle deformation and fluid flow at least 500 km inland of North America's Atlantic passive margin.

Changes in fluid regime in syn-orogenic sediments during the growth of the south Pyrenean fold and thrust belt

Abstract The eastern sector of the south Pyrenean fold and thrust belt developed during the Alpine compression and affected Upper Cretaceous to lower Oligocene foreland basin deposits. In this study, we determine the changes in fluid regime and fluid composition during the growth of this fold and thrust belt, integrating petrographic and geochemical data obtained from fracture-filling cements. Hydrothermal fluids at temperatures up to 154 °C, migrated from the Axial zone to the foreland basin and mixed with connate fluids in equilibrium with Eocene sea-water during lower and middle Eocene (underfilled foreland basin). As the thrust front progressively emerged, low-temperature meteoric waters migrated downwards the foreland basin and mixed at depth with the hydrothermal fluids from middle Eocene to lower Oligocene (overfilled non-marine foreland basin). The comparison of the fluid flow models from the Southern Pyrenees with other orogens worldwide, seems to indicate that the presence or absence of thick evaporitic units highly control fluid composition during the development of fold and thrust belts. Whereas in thrusts not detached along thick evaporite units, mixed fluids are progressively more depleted in δ18O and have a lower temperature and lower Fe and Sr contents as the thrust front emerges, in thrust detachments through thick evaporite units, the mixed fluids are enriched in δ18O.

The source and evolution of paleofluids responsible for secondary minerals in low-permeability Ordovician limestones of the Michigan Basin

In this study we report on the source and evolution of fluids associated with secondary vein and replacement minerals in low-permeability carbonate units in the Michigan Basin. This petrogenetic information was collected using thermometric data from fluid inclusions combined with C-, O- and Sr-isotope data, and focuses on mm- or cm-wide vein and vug minerals from Ordovician limestones of the Trenton and Black River groups and Cambrian sandstones in SW Ontario, Canada.Primary fluid inclusions in dolomite represent fluid stage I and have the highest trapping temperatures (Ttrap) in the sedimentary succession, between 88 and 128 °C. Primary inclusions in calcite (stage II), and celestine and anhydrite (stage III), represent the final stages of secondary mineral formation and have Ttrap values of 54–78 °C. All three stages of vein minerals formed from brines with salinities of 31–37 wt% [CaCl2+NaCl]eq that were saturated in halite and methane gas at the time of mineral growth. Three subsequent stages of secondary fluid inclusions were observed in the samples (stages IV–VI), however, no secondary vein minerals formed during these stages and they are interpreted as re-mobilization and/or minor fluid ingress along grain boundaries and micro-fractures. Notably, stage IV secondary inclusions are gas-undersaturated with salinities of 32–34 wt% [CaCl2+NaCl]eq and minimum Ttrap values of 57–106 °C, and are interpreted to have formed during a Late Devonian–Mississippian regional heating event.Secondary minerals from the Cambrian units and the Shadow Lake Formation have δ13C values of −6.1 to −2.5‰ (VPDB), δ18O values of +14.6 to +24.2‰ (VSMOW) and 87Sr/86Sr of 0.70975–0.71043. Relative shifts of approximately +2 to +3‰ in δ13C, >+4‰ in δ18O and −0.002 in 87Sr/86Sr are observed upward across the boundary between the Cambrian–Shadow Lake units and the overlying Gull River Formation. Samples from the Gull River and Coboconk formations and the Trenton Group (Kirkfield, Sherman Fall and Cobourg formations) have δ13C values of −1.0 to +1.9‰, δ18O values of +18.9 to +28.1‰ and 87Sr/86Sr values of 0.70790–0.70990.The combined microthermometric and isotopic data for secondary minerals in the Cambrian–Shadow Lake units suggest they formed from hydrothermal brine with a geochemical signature obtained by interaction with the underlying Precambrian shield, or shield-derived minerals in the Cambrian sandstones. Previous U-Pb dating of vein calcite and Rb-Sr dating of secondary K-feldspar from the region suggests that brine ingress occurred during the Late Ordovician–Silurian. The O- and Sr-isotope variability in vein samples from the Gull River and Coboconk formations is interpreted as localized mixing of 18O-enriched, connate fluids with hydrothermal brine. In comparison, isotopic data for the Trenton Group indicate secondary mineral formation from connate fluids, sourced from 18O-enriched, evolved seawater and/or modified hydrothermal brine that experienced fluid–rock interaction during transit through the underlying stratigraphy.

Origin and role of fluids involved in the seismic cycle of extensional faults in carbonate rocks

We examine the potentially-seismic right-lateral transtensional–extensional Tre Monti Fault (central Apennines, Italy) with structural and geochemical methods and develop a conceptual evolutionary model of extensional faulting with fluid involvement in shallow (≤3 km depth) faults in carbonate rocks. In the analysed fault zone, multiscale fault rock structures include injection veins, fluidized ultracataclasite layers, and crackle breccias, suggesting that the fault slipped seismically. We reconstructed the relative chronology of these structures through cross-cutting relationship and cathodoluminescence analyses. We then used C- and O-isotope data from different generations of fault-related mineralizations to show a shift from connate (marine-derived) to meteoric fluid circulation during exhumation from 3 to ≤1 km depths and concurrent fluid cooling from ∼68 to <30 °C. Between ∼3 km and ∼1 km depths, impermeable barriers within the sedimentary sequence created a semi-closed hydrological system, where prevalently connate fluids circulated within the fault zone at temperatures between 60° and 75 °C. During fault zone exhumation, at depths ≤1 km and temperatures <30 °C, the hydrological circulation became open and meteoric-derived fluids progressively infiltrated and circulated within the fault zone. The role of these fluids during syn-exhumation seismic cycles of the Tre Monti Fault has been substantially passive along the whole fault zone, the fluids being passively redistributed at hydrostatic pressure following co-seismic dilatancy. Only the principal fault has been characterized, locally and transiently, by fluid overpressures. The presence of low-permeability clayey layers in the sedimentary sequence contributed to control the type of fluids infiltrating into the fault zone and possibly their transient overpressures. These results can foster the comprehension of seismic faulting at shallow depths in carbonate rocks of other fold-thrust belts involved in post-collisional seismogenic extensional tectonics.

CO2-Energized-Acid Treatment Reduces Freshwater Use, Boosts Well Performance

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 172620, “Successful Implementation of CO2-Energized-Acid Fracturing Treatment in Deep, Tight, and Sour Carbonate Gas Reservoir in Saudi Arabia That Reduced Freshwater Consumption and Enhanced Well Performance,” by Ataur R. Malik, SPE, Alaa A. Dashash, Saad M. Driweesh, SPE, and Yousef M. Noaman, SPE, Saudi Aramco, and Eduardo Soriano, SPE, and Alfredo Lopez, Halliburton, prepared for the 2015 SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, 8–11 March. The paper has not been peer reviewed. Carbon dioxide (CO2) with 30% foam quality (FQ) has been introduced for the first time during acid fracturing treatments in a tight, sour, high-pressure/high-temperature carbonate gas reservoir in Saudi Arabia to reduce consumption of fresh water, minimize reservoir damage, reduce the flowback period, and eliminate the need for nitrogen lifting with coiled tubing. The addition of liquid CO2 to hydrochloric acid (HCl) in quantities sufficient to produce emulsion allows live acid to retard and penetrate much deeper than HCl alone. Introduction Gases were introduced to the oil and gas industry primarily as an aid to recover pumped stimulation fluids. This application still accounts for the majority of use of nitrogen and CO2. Special applications, such as foaming stimulation fluids, have reduced consumption of the liquid phase significantly. Stimulation activities have been increasing dramatically in Saudi Arabia. During each stage of fracturing stimulation in Saudi Arabia, up to 3,000 bbl of groundwater is used. Up to 70% of that groundwater consumption can be reduced through the CO2-foam fracturing treatment. The application of CO2 also reduces the flowback period and eliminates the need to perform coiled-tubing nitrogen lifting, which is a unique requirement for tight and depleted reservoirs. The addition of liquid CO2 to HCl in quantities sufficient to produce viscous foam (emulsion) is one of the more significant improvements in recent years. The addition of CO2 to acid accomplishes the following: It increases the viscosity of the commingled fluid. It controls leakoff of acid. Most CO2 foamed-acid treatments are performed at matrix rates. It can easily be pumped at fracturing rates. The fracture length is determined by acid reaction rate, injection rate, fracture width, and rate of fluid loss from the fracture to the formation. It dramatically increases the efficiency of HCl. Laboratory testing with cores from the canyon reef formation indicates an improvement in penetration of live acid of almost ninefold over a conventional acid system. It cleanses the formation. Many formations are only partially soluble in HCl. The energy and viscosity of the foamed acid aid in removal of these undissolved fines from the formation. The CO2 also strips hydrocarbon from the rock, exposing it to dissolution by the acid. It improves formation permeability by the removal of stimulation and connate fluids. It dissolves more rock. An 80%-quality foam (only 20% of the volume is acid) has been proved to remove as much rock as when the entire solution is acid. It reduces or eliminates the need for swabbing. The load to recover is significantly lower than a conventional-treatment volume. The balance of the treatment (the CO2) will vaporize and aid in the recovery of undissolved fines, spent acid, and flush volumes. Because CO2 at the surface is above the critical pressure (1,071 psi) but below the critical temperature (87.8°F), CO2 is pumped into the wellbore as a liquid. It remains a liquid until heated by the formation downhole. Conversion of CO2 to a gas downhole causes no disruption of the two-phase fluid because supercritical CO2 has a high density and no abrupt expansion of CO2 occurs.

A Practical Method of Predicting Calcium Carbonate Scale Formation in Well Completions

Summary Scale formation in downhole tubular-flow passages can cause partial to complete plugging that will affect production or injection rates adversely. In an intelligent-well completion in which the interval-control-valve (ICV) opening must be changed to control flow rate, the completion will become ineffective if plugging of clearances prevents valve actuation. To mitigate these problems, a method to predict the potential rate of scale formation under realistic conditions has been developed. This empirical method allows prediction of tool performance under scale-forming conditions for downhole applications, and uses chemical data and flow fields generated by computational-fluid-dynamics (CFD) models for downhole tools. Chemical data are obtained from laboratory tests on coupons by use of brines matching the chemistry of connate fluids. Tests were conducted in a high-pressure, corrosion-resistant vessel over a range of high pressures (100 to 10,000 psi) and high temperatures (75 to 150 °C) to simulate downhole well conditions. Two test sets were conducted, each with fluid at rest and with an impeller generating low velocity in the reaction vessel, ranging from 4 hours to 4 days, with scaling rates determined from coupon-weight gain. Concentrations in the range of 50 to 125% of the typical connate-fluid concentration were used. The weight-gain data obtained from the coupon tests and from a tube-plugging test were used to develop an empirical model for scale-growth rate at a given point on a solid surface with pressure, pressure gradient, temperature, fluid velocity, and brine concentration as independent variables. Artificial-intelligence methodology was used to develop this model, which can be used to predict the scale-growth rate for any arbitrary geometry. By use of the internal geometry of any tool to be modeled, a CFD model is prepared and the pressure, fluid velocity, and pressure-gradient data are generated for the entire internal solid surface of the tool for a given flow rate through the tool. These data are fed into the empirical model to calculate the scale-growth rate, which is integrated to obtain scale thickness at each point of the internal solid boundary. To verify accuracy, scale formation in a 4.5-in. ICV was predicted at high-pressure, high-temperature conditions at a low flow rate. Laboratory tests on the valve matched the model predictions well enough, which enabled Petrobras to design a better completion and fluid-handling system for a presalt well.

Organic compounds in produced waters from shale gas wells.

A detailed analysis is reported of the organic composition of produced water samples from typical shale gas wells in the Marcellus (PA), Eagle Ford (TX), and Barnett (NM) formations. The quality of shale gas produced (and frac flowback) waters is a current environmental concern and disposal problem for producers. Re-use of produced water for hydraulic fracturing is being encouraged; however, knowledge of the organic impurities is important in determining the method of treatment. The metal content was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Mineral elements are expected depending on the reservoir geology and salts used in hydraulic fracturing; however, significant levels of other transition metals and heavier main group elements are observed. The presence of scaling elements (Ca and Ba) is related to the pH of the water rather than total dissolved solids (TDS). Using gas chromatography mass spectrometry (GC/MS) analysis of the chloroform extracts of the produced water samples, a plethora of organic compounds were identified. In each water sample, the majority of organics are saturated (aliphatic), and only a small fraction comes under aromatic, resin, and asphaltene categories. Unlike coalbed methane produced water it appears that shale oil/gas produced water does not contain significant quantities of polyaromatic hydrocarbons reducing the potential health hazard. Marcellus and Barnett produced waters contain predominantly C6-C16 hydrocarbons, while the Eagle Ford produced water shows the highest concentration in the C17-C30 range. The structures of the saturated hydrocarbons identified generally follows the trend of linear > branched > cyclic. Heterocyclic compounds are identified with the largest fraction being fatty alcohols, esters, and ethers. However, the presence of various fatty acid phthalate esters in the Barnett and Marcellus produced waters can be related to their use in drilling fluids and breaker additives rather than their presence in connate fluids. Halogen containing compounds are found in each of the water samples, and although the fluorocarbon compounds identified are used as tracers, the presence of chlorocarbons and organobromides formed as a consequence of using chlorine containing oxidants (to remove bacteria from source water), suggests that industry should concentrate on non-chemical treatments of frac and produced waters.

Origin of an anhydrite sheath encircling a salt diapir and implications for the seismic imaging of steep-sided salt structures, Egersund Basin, Northern North Sea

The ingress of meteoric fluids into the crests of salt structures typically results in the formation of anhydrite-dominated caprock. The migration of connate fluids up the margins of salt structures has also been documented, although the products related to this are rarely sampled in the deep subsurface. We use 3D seismic reflection and borehole data to document the geometry and lithology of a salt diapir. Seismic data suggest that a weld is developed along the diapir stem, although borehole data indicate that the stem consists of an inner, c . 1500 m thick, halite-dominated zone, and an outer, c . 250 m thick, anhydrite-dominated ‘sheath’. The anhydrite may have formed early in the basin history, as a depositional anhydrite or crestal caprock. An alternative interpretation is that the anhydrite represents ‘lateral caprock’, which formed late in the basin history in response to the migration of NaCl-poor fluid up the margins of the diapir and dissolution of halite. The possibility that lateral caprock may form adjacent to salt structures has implications for understanding the patterns and vigour of groundwater flow in sedimentary basins. Furthermore, our study shows the margins of steep-sided salt structures may be misidentified by several hundred metres if time-migrated seismic reflection data are used.

Variation of mineralizing fluids and fractionation of REE during the emplacement of the vein-type fluorite deposit at Bozijan, Markazi Province, Iran

Fluorite vein mineralization occurs mainly in slates and phyllites of Lower Jurassic Shemshak Formation and scarcely in Triassic limestones in Bozijan, which is situated at the west of Mahallat city in the Markazi Province of Iran. The ore consists mainly of fluorite, quartz, calcite, and iron oxides. The subordinate components are galena, pyrite, manganese oxides, and malachite. Local wall-rock alterations include argilization and silicification. The nature of the mineralization and ore–host rock relationships indicate an epigenetic mode of formation for fluorite mineralization. Fluid inclusions in early fluorites and quartz consist of aqueous and aqueous-carbonic inclusions, thereby showing that during deposition of bulk fluorites, two immiscible fluids are involved: aqueous and carbonic. The aqueous fluid is a mixture of two low- and high-salinity (< 15 and > 26.24 wt.% NaCl equivalent) H 2O–NaCl–(CaCl 2–KCl–MgCl 2) brines, as is evidenced by T h -salinity plots. The aqueous inclusions are homogenized at temperatures between 152.5 and 312 °C. The aqueous-carbonic inclusions exhibit salinity and homogenization temperature in ranges between 22.04 and 24.26 wt.% NaCl equivalent and between 253 and 390.5 °C, respectively. Fluid inclusions in late fluorites show the fluorites were precipitated from a colder (102.4–175 °C) and less saline (15.96–24.45 wt.% NaCl equivalent) fluid than the early fluorites. Rare earth element (REE) analysis in fluorites revealed extremely low values in ranges between 2.87 and 34.39 ppm for early fluorites and between 1.91 and 6.4 ppm for late fluorites, thus indicating that fluorites had been derived from a sedimentary environment. However, Tb/Ca and Y/Ho ratios invariably suggest a hydrothermal origin for Bozijan fluorites. The Ce/Yb ratios and chondrite-normalized patterns revealed that the fluorites (early as well as late ones) are enriched in LREE (light rare earth element) relative to HREE (heavy rare earth element). This indicates that REE leaching from source rocks and fluid migration occurred under high-temperature and low-pH conditions. The enigmatic LREE-enriched late fluorites suggest that deposition of fluorites in Bozijan did not occur during a long-lived episode of mineralization. Europium represents positive as well as negative anomalies that have been probably caused by fluorite precipitation from mixing two fluids possessing opposite Eu anomalies. The Ce/Ce* ratios portray persistent negative Ce anomalies, thus indicating reducing conditions in the hydrothermal fluids. In the (La/Yb)n–Eu/Eu* diagram, data points do not overlap with the fields represented by fluorite-bearing Au–Ag deposits from elsewhere, thus indicating a very low potential for precious-metal mineralization in the studied area. This interpretation is well in line with the low contents of precious metals in the Bozijan deposit, thus suggesting that REE geochemistry is a reliable tool for exploring precious metals in fluorite deposits. The Bozijan deposit is classified here as an “unconformity-related fluorite deposit.” According to a conceptual model, mineralization occurred when the ascending hypersaline brines mixed with the less saline connate fluids in sediments of Triassic and predominantly Jurassic ages. The less saline connate brines were assumed to be the fluoride-bearing solutions, as suggested by the high F contents of the rocks containing them and REE patterns.

The relationship between the McLaughlin gold–mercury deposit and active hydrothermal systems in the Geysers–Clear Lake area, northern Coast Ranges, California

The Geysers–Clear Lake area has a long history of research on its active hydrothermal systems. It is a unique area containing a number of hydrothermal systems which include: the Geysers steam field, one of the largest vapor-dominated geothermal systems yet recognized; the McLaughlin gold deposit, an extremely well preserved hot-spring style gold deposit; and the Sulphur Bank mercury deposit, one of the first locations where geothermal systems were recognized as modern analogues to epithermal deposits. There is also a variety of active hot- and mineral-springs, including Wilbur Springs, or the Sulphur Creek district, which has been considered one of the type localities for connate fluids. The McLaughlin gold–mercury deposit is a fossil hot-spring system dominated by meteoric waters that exchanged with sedimentary rocks of the Great Valley sequence. Mineralization was syntectonic, occurring contemporaneously with fault movement. The fluids circulated in syntectonic dilation zones that resulted in, and maintained, high permeability of the fluid conduits permitting large volumes of fluid flow. The fluids precipitated metals in response to physical and chemical changes associated with boiling. The hydrothermal fluids that formed the McLaughlin deposit have the highest reservoir temperature, salinity and are isotopically the most enriched, of the Coast Range hydrothermal systems. The McLaughlin deposit is considered an end-member “fluid-dominated” hydrothermal system. The Geysers steam field, in its earliest phase was likely similar to the McLaughlin deposit being fluid-dominated and forming, at least on a small scale, a vein system enriched in silver and anomalous in gold, base metals, antimony and mercury. The hydrothermal system evolved into a vapor-dominated system as a result of decreased permeability of the reservoir, decreased recharge and/or increased heat flow. The modern day reservoir is encapsulated in impermeable rocks and is a “vapor-dominated” end-member hydrothermal system. Active hot- and mineral-springs in the Coast Ranges of northern California are intermediate between the fluid- and vapor-dominated end-member systems. The chemical and isotopic compositions of these fluids are the result of thermal processes and are not explained by simple mixing models between connate fluids and meteoric groundwater. Their isotopic and chemical composition is best explained by meteoric-dominated systems with repeated non-equilibrium subsurface vapor loss (evaporation) in a near closed system, with the relative deuterium and 18O enrichment proportional to the reservoir temperature.

Diagenesis associated with cooling during burial: An example from Lower Cretaceous reservoir sandstones (Sirt basin, Libya)

Fluid inclusions in quartz and dolomite cements were used to determine temperature, timing, origin, and function of fluids in the diagenetic evolution of the Lower Cretaceous Upper Sarir Sandstone (Sirt basin, Libya). Transmitted light, backscattered electron, and scanning electron microscope (SEM) cathodoluminescence analyses were carried out to determine the paragenesis. In quartz overgrowths, fluid inclusions along the grain-overgrowth boundary yield homogenization temperatures from 130 to 141 degreesC and salinities between seawater and freshwater. Aqueous fluid inclusions in later quartz overgrowths have homogenization temperatures from 114 to 129 degreesC and salinities between 18 and 21 wt. % NaCl equivalent. Fluid inclusions in dolomite have variable homogenization temperatures (120-141 degreesC) and high salinities (20-22 wt. % NaCl equivalent). This trend in homogenization temperatures of fluid inclusions is interpreted as the result of quartz cement precipitation in the presence of hot, low-salinity, connate fluids connected with an increased heat flow related to regional rifting. Later quartz overgrowths precipitated during regional cooling concurrent with the end of the rifting and after invasion of saline fluids, possibly brought in from adjacent areas or refluxed downward during or after times of evaporite deposition. This diagenetic history linked to a decreasing heat flow could be common in other aborted-rift basins, which are important targets for oil exploration.

Fluid inclusions record thermal and fluid evolution in reservoir sandstones, Khatatba Formation, Western Desert, Egypt: A case for fluid injection

A fluid inclusion and petrographic study, focused on quartz overgrowths, was performed in reservoir sandstones from the Jurassic Khatatba Formation (Salam oil field, Egypt's Western Desert). The combination of detailed fluid inclusion petrography and scanning electron microscope (SEM) cathodoluminescence imaging has allowed us to relate individual fluid inclusion assemblages, that is, the most finely discriminated groups of petrographically associated fluid inclusions, to specific growth zones of authigenic quartz, establishing the relative timing of entrapment of the inclusions. After entrapment, fluid inclusions in authigenic quartz have been preserved without reequilibration, as indicated by the narrow ranges of homogenization temperatures (<4-5 degreesC) in most fluid inclusion assemblages. Three main growth zones are distinguished under SEM cathodoluminescence in the quartz overgrowths and are termed Q1, Q2, and Q3 from the oldest to the youngest. Zone Q1 is further subdivided into three subzones and contains abundant primary aqueous inclusions. Their homogenization temperatures range from 162 to 130 degreesC, with the earliest assemblages having the highest temperatures and with some large temperature fluctuations indicated between successive assemblages. Most Q1 inclusions have salinities in the freshwater to seawater range, with a trend toward increasing salinity through time. Zone Q2 contains primary aqueous inclusions with homogenization temperatures (overall range 148-125 degreesC) also recording large temperature fluctuations and cooling events. The Q2 fluid inclusions have high salinities (~5-20 wt. % NaCl equivalent), with salinity increasing through time. Zone Q3 contains both aqueous and oil inclusions of primary origin. The Q3 aqueous inclusions have homogenization temperatures (overall range 134-112 degreesC) recording overall cooling and high salinities (21-24 wt. % NaCl equivalent). In early Q3 subzones, oil inclusions appear to be of medium gravity, undersaturated with respect to gas. In later Q3 subzones, oil inclusions are of gas-saturated lighter oil. Our results indicate that fluid flow, involving drastic changes in temperature and salinity, was responsible for the precipitation of some of the quartz cement. The earliest quartz (Q1) precipitated from freshwater and seawater at temperatures significantly higher than those expected from the burial history and thermal maturity of these rocks. This quartz is interpreted to have precipitated during cooling of injected fluids that originated as hot connate fluids from deeper parts of the basin. The Q2 precipitation is interpreted to have resulted from episodes of injection of hot saline brines from below. Late quartz cement (Q3) precipitated during oil charge, from progressively cooler and more saline brines interpreted to have refluxed from the surface; it preserves a record of increasing oil maturity and gas saturation through time.

The gas-bearing Devonian Presqu'ile Dolomite of the Cordova embayment region of British Columbia, Canada: Dolomitization and the stratigraphic template

White dolospar of the Presqu'ile Dolomite forms the reservoir for many gas fields of northeastern British Columbia within the Keg River, Sulphur Point, and Slave Point formations along the shelf margin of the Cordova embayment. The embayment fill succession of the organic-rich Klua Shale, the Otter Park shale, and marlstones equivalent to the Sulphur Point and Slave Point formations are aquicludes that blocked the upward and lateral flow of dolomitizing brines. Presqu'ile dolomitization along the Slave Point shelf edge is interrupted in places where the Klua Shale extends shelfward underneath the Slave Point Formation. The Watt Mountain shale was also a regional aquiclude that prevented upward circulation of dolomitizing fluids in areas behind the Slave Point shelf edge. Flow of dolomitizing NaCl-CaCl2-MgCl2-H2O brine solutions began in the shallow subsurface and precipitated dolomite cements (type 1) at temperatures less than 50 degreesC, possibly even during Slave Point deposition, as northwestward-directed shallow subsurface brine reflux from the Elk Point basin began. This was followed by upward-directed subsurface convective flow of NaCl-CaCl2-MgCl2-H2O and NaCl-MgCl2-H2O brines and precipitation of dolomite cements at temperatures of about 130 degreesC during the Late Devonian-Carboniferous. Lower salinity brines that precipitated calcite cements and some dolomite (type 2) may have interacted with connate fluids of marine origin or, less likely, with meteoric fluids. Modeling of organic maturation indicates that the Muskwa shales above the Slave Point Formation passed through the oil generation window in the Permian and the onset of dry gas generation in the middle Mesozoic. Postdolomitization calcite cements precipitated during these times contain abundant liquid hydrocarbon-filled and methane-filled inclusions. High temperatures of close to 200 degreesC experienced by Slave Point strata in the Mesozoic caused stretching, but no leakage or refilling, of fluid inclusions in all dolomite cements.

Pore fluid evolution and quartz diagenesis in the Khatatba Formation, Western Desert, Egypt

In the Khatatba sandstones (Middle Jurassic, Salam oil field, Egypt), detailed fluid inclusion analyses in the quartz overgrowths reveal a complex history. The earliest cement zones precipitated from hot, connate freshwater and seawater (Th=133–162°C; salinity=0–3.5wt.% NaCl eq.). A second phase of quartz precipitated from brines of variable salinities (Th=125–148°C; salinity=5–20–wt.% NaCl eq.), inconsistent with connate fluids. Both phases of quartz cementation resulted from multiple events of injection of hot fluids related to fracturing, quartz precipitation being caused by cooling after injection. The final phase of quartz precipitated from consistently high salinity brine (Th=114–133°C; salinity=21–24wt.% NaCl eq.) associated with oil (Th=79–127°C) during a period of overall cooling.

δ34 S of Lower Carboniferous anhydrite, Cumbria and its implications for barite mineralization in the northern Pennines

Recent hydrocarbon exploration in the Solway Basin has revealed the existence of Lower Carboniferous paralic sediments which contain numerous discrete anhydrite beds. Anhydrite S isotope ratios from the Easton-1 Well are compared with new isotopic data from barite mineralization that occurs along the faulted southern margin of the Northumberland-Solway Basin and published data from the Northern Pennine orefield. Similarities betw een the δ 34 S of anhydrite and barites from major mines at Haydon Bridge and the Alston district of the Northern Pennine orefield supports a Lower Carboniferous marine sulphate source for mineralization at these sites. Identification of extensive evaporite deposits hi the Solway Basin resolves several longstanding problems associated with mineralization hi the Northern Pennines including the source of barite sulphate and the origin of hypersaline mineralizing fluids. In an earlier investigation of the S isotope geochemistry of Carboniferous-hosted barite mineralization in and around the Alston district of theNothern Pennine orefield (NPO, Fig. 1a), Solomon et al. 1971 drew attention to the similarity between the δ 34 S of Lower Carboniferous marine sulphate and the sulphur isotopic composition of barite deposits. A model for barite mineralization was proposed in which deep, circulating, BaCl 2 brines mixed with shallow (&lt; 1 km depth), sulphate-rich, connate fluids composed of Lower Carboniferous sea water. Although the model proposed by Solomon et al. 1971 is consistent with aspects of barite mineralization, more recent studies of the evolution of pore fluid geochemistry in sedimentary basins ( Land 1987 ; Hanor 1994a , b ) argue against a marine connate origin for the sulphate component