Regina M. Capuano

The temperature dependence of hydrogen isotope fractionation between clay minerals and water: Evidence from a geopressured system

Abstract δD values of water samples from two Gulf Coast geopressured fields vary linearly with temperature, while the δD values from the coexisting clays remain constant and are independent of temperature. Fluid How rates on the order of millimeters per year, characteristic of Gulf Coast geopressured systems, provide adequate residence time for hydrogen isotope equilibrium to be achieved between the extant porewaters and clay minerals. Since equilibrium is achieved, these δD values are used to calculate the temperature dependence of the hydrogen isotope fractionation factor between illite-smectite and water, αclay-waterH, between 0 and 150°C: 1000 In α clay-water H = −45.3 × 10 3 T + 94.7 . This equation predicts values for αclay-waterH at 96°C which are equal to those predicted by a similar equation derived by Yeh (1980). Above and below 96°C, this new equation predicts values which diverge from those predicted by the equation of Yeh (1980) with a difference of +9 at 150°C and −25 at 0°C. The possibility is raised that the very slow upward component of flow characteristic of Gulf Coast geopressured sediments, combined with the slow downward subsidence of the clay-rich sediments, can result in a system which is rock-dominated with respect to hydrogen.

Ca/Mg of brines in Miocene/Oligocene clastic sediments of the Texas Gulf Coast: Buffering by calcite/disordered dolomite equilibria

The compositions of brine collected from geopressured and nongeopressured sediments of the northeast Texas Gulf Coast were used to predict the Ca/Mg activity ratios of water in equilibrium with calcite and dolomite to 150°C. Ca/Mg activity ratios of these water samples show an excellent linear correlation with temperature giving the equation, log(aCa++/aMg++) = −0.22 + 7.21T(°C)/1000 (r = 0.94, n = 51). This line parallels and is very near to the equilibrium surface for calcite/disordered dolomite, suggesting that equilibrium between calcite and poorly ordered dolomite may control the Ca/Mg activity ratios in the water samples. In these sediments, Fe-rich dolomite and calcite cements are the last cementation phases (Loucks et al., 1984), indicating they are the most likely minerals affecting the present water chemistry. In addition, in these sediments calcite cements precipitated in oxygen isotopic equilibrium with present-day water and calcite and dolomite cements coprecipitated in oxygen isotopic equilibrium (Milliken et al., 1981). Other possible processes such as mixing, cation exchange, and water-rock interaction involving other carbonate and silicate minerals may act as sinks or sources for both Ca and Mg, but they appear not to control the Ca/Mg activity ratios at the study site. The Ca/Mg activity ratios of brines from other groundwater systems (clastics and dolomite-poor carbonates) of the U.S. Gulf Coast agree well with the Ca/Mg temperature trend determined in this study, suggesting equilibrium with calcite and similar composition disordered dolomite in all these sediments. Water from dolomite-rich carbonate sediments did not fit the Ca/Mg activity ratio curve presented in this study and is more likely in equilibrium with calcite and more ordered dolomite, a possible effect of dedolomitization. This linear Ca/Mg trend was used to estimate the ordering parameter s = 0.4 and with that the ΔG°f = −516.05 kcal/mol, ΔH°f = −554.20 kcal/mol, and a solubility product constant of 10−16.9 at 25°C and 1 bar were calculated for the disordered dolomite found in these sediments. This dolomite is likely typical of that found in other clastic and dolomite-poor carbonate sediments.

Fluid-mineral equilibria in a hydrothermal system, Roosevelt hot springs, Utah

The availability of fluids and drill cuttings from the active hydrothermal system at Roosevelt Hot Springs allows a quantitative comparison between the observed and predicted alteration mineralogy, calculated from fluid-mineral equilibria relationships. Comparison of all wells and springs in the thermal area indicates a common reservoir source, and geothermometer calculations predict its temperature to be higher (288°C ± 10°) than the maximum measured temperature of 268°C. The composition of the deep reservoir fluid was estimated from surface well samples, allowing for steam loss, gas release, mineral precipitation and ground-water mixing in the well bore. This deep fluid is sodium chloride in character, with approximately 9700 ppm dissolved solids, a pH of 6.0, and gas partial pressures of O2 ranging from 10−32 to 10−35 atm, CO2 of 11 atm, H2S of 0.020 atm and CH4 of 0.001 atm. Comparison of the alteration mineralogy from producing and nonproducing wells allowed delineation of an alteration pattern characteristic of the reservoir rock. Theoretical alteration mineral assemblages in equilibrium with the deep reservoir fluid, between 150° and 300°C, in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-H4SiO4-H2O-H2S-CO2-HCl, were calculated. Minerals theoretically in equilibrium with the calculated reservoir fluid at >240°C include sericite, K-feldspar, quartz, chalcedony, hematite, magnetite and pyrite. This assemblage corresponds with observed higher-temperature (>210°C) alteration assemblage in the deeper parts of the producing wells. The presence of montmorillonite and mixed-layer clays with the above assemblage observed at temperatures <210°C corresponds with minerals predicted to be in equilibrium with the fluid below 240°C. Alteration minerals present in the reservoir rock that do not exhibit equilibrium with respect to the reservoir fluid include epidote, anhydrite, calcite and chlorite. These may be products of an earlier hydrothermal event, or processes such as boiling and mixing, or a result of errors in the equilibrium calculations as a result of inadequate thermochemical data.

The effect of organic matter and the H2O2 organic-matter-removal method on the δD of smectite-rich samples

Abstract The H 2 O 2 organic-matter-removal method (Jackson, 1985) used routinely to remove organic matter from sediments in preparation for δD analyses, was found to alter the δD of the smectite standard, H26, up to −8‰ and of Gulf Coast shale samples up to −13‰. Before the H 2 O 2 treatment, analytical error for the δD analyses was low ranging from ±1‰ to ±3‰. After treatment the error increased ranging from ±4 to ±13‰ for 46% of the treated samples. This decreased reproducibility suggests the deuterium is no longer evenly distributed within the samples, and that the shift in δD may be the result of retention or removal of a hydrogen bearing phase. Total organic carbon (TOC) analyses of the shale samples before and after treatment with H 2 O 2 show that up to 0.80 wt.% TOC remains in the samples even after the samples appear bleached, which according to Jackson (1985) indicates complete removal of the organic matter. An additional 24 h of treatment (two more additions of H 2 O 2 ) or more, depending on the initial TOC content, was necessary beyond bleaching of the samples to achieve a final TOC of 0.10 wt.% or less. The δD values of the untreated Gulf Coast shale samples with 1 wt.% TOC or less, lie within a narrow range, −38 to −32‰, and within this range lack any correlation with TOC, suggesting the presence of the organic matter is not effecting the δD values. A single sample with TOC greater than 1 wt.%, PB2–4762 m with 10 wt.% TOC, has a δD value of −56‰ outside of the range of the low TOC samples. Organic matter isolated from this high TOC sample was added incrementally to four of the other shale samples. δD analyses of these shale-organic matter mixtures show a decrease of 0.23‰ with each mole % increase in hydrogen from organic matter. By using this slope to calculate the δD of PB2–4762 m without the organic matter gives a value of −40‰. This new value is within 2‰ of the range of values for the low TOC shales, −38 to −32‰, and is within the analytical error of the δD analyses.

Hydrochemical constraints on fluid-mineral equilibria during compaction diagenesis of kerogen-rich geopressured sediments

Abstract Hydrochemical conditions characteristic of kerogen-rich geopressured sediments contribute to the development of a predictable alteration mineralogy. The following conditions result in the geopressured fluid becoming enriched, and in some cases, saturated with CO 2 and CH 4 : 1. (1) the upward and generally restricted flow of fluids in geopressured sediments, in contrast to the generally more rapid flow of fluids in nongeopressured sediments; 2. (2) the coincidence of the depths of geopressuring with the geothermal temperatures necessary for CO 2 and CH 4 release; and 3. (3) the opposing rates of sediment subsidence and CO 2 and CH 4 transfer into the upward-flowing fluid. Calculation of the phase relations in the systemCaO-FeO-MgO-Na 2 O-Al 2 O 3 -CO 2 -H 2 S-H 2 SO 4 -SiO 2 -H 2 O-HCl, as constrained by the above hydrochemicalcriteria, allows prediction of three patterns of mineral deposition during diagenesis of kerogen-rich geopressured sediments. Quartz deposition is favored in the upper portions and margins of the geopressured section and at the contact between geopressures and normal pressures because of increased fluid flux atshallower depths within the geopressured section. Carbonate deposition could occur above the zone of CO 2 release from kerogen degradation,as a result of the upward flux of CO 2 - and CH 4 -enriched (andpossibly saturated) fluids, and the subsequent decrease in fluid temperature, pressure, and CO 2 solubility.Kaolinite-carbonate could deposit within and above the zone of CO 2 release from kerogen as a result ofsilicate dissolution by CO 2 -rich acid pore fluids, followed by the potential for illite-carbonate and albitecarbonate deposition upon CO 2 depletion. In contrast, laumontite and anhydrite should not depositduring diagenesis of kerogen-rich geopressured sediments but could deposit during diagenesis of nongeopressured or kerogen-poor geopressured sediments. These mineralogic relationships compare favorablywith observed relationships in the kerogen-rich geopressured sediments of the Frio Formation from theTexas Gulf Coast.

Enhanced activity of oligotrophic endogenous bacteria in clay‐rich sediments by nutrient injection

A controlled field experiment was performed in which the microbiology and geochemistry of clay‐rich fluvial‐deltaic sediments were characterized both before and after nutrient injection into a shallow well. Acetate addition (with nitrogen and phosphate) initially increased the heterotrophic bacteria population in the groundwater within 21 days after nutrient addition. Consumption of oxygen and injected nutrient resulted in an expected stimulation of copiotrophic bacterial growth (31–48 days), then a noticeable “trough phase”; reflecting minimal or no bacterial growth followed by a secondary peak reflecting another bacterial population growth surge (62–85 days). This secondary surge was apparently supported by the nutrients generated by the decomposing biomass (initial population peak), and by oxygen replenishment supplied by continual ground water flow. During the intervening trough phase, bacterial counts by most‐probable‐number analysis of soil samples indicated that the denitrifying population increase...

Multielement analysis of geologic materials by inductively coupled plasma-atomic emission spectroscopy

Atomic emission spectroscopy using an inductively coupled plasma (ICP) source permits the rapid acquisition of multielement geochemical data from a wide variety of geologic materials. Rocks or other solid samples are taken into solution with a four acid digestion procedure and introduced directly into the plasma; fluid samples are acidified or analyzed directly. The entire process is computer-controlled, fully-automated, and requires less than five minutes per sample for quantitative determination of 37 elements. The procedures and instrumentation employed at the ESL for multielement ICP analysis of geologic materials are described and these are intended as a guide for evaluating analytic results reported from this laboratory. The quality of geochemical data can be characterized by precision, limits of quantitative determination, and accuracy. Precision values are a measure of the repeatability of analyses. In general, major element and analyses have precision of better than 5% and trace elements of better than 10% of the amount present. (MHR)

Initial investigation of soil mercury geochemistry as an aid to drill site selection in geothermal systems

A mercury-in-soil survey was conducted at the Roosevelt Hot Springs Known Geothermal Resource Area (KGRA), Utah, to evaluate mercury soil geochemistry as a method of selecting exploration well sites in a hot-water geothermal system. Samples of -80 mesh soil were collected at 30.5 m intervals along traverses crossing known structures, surficial geothermal alteration, and exploration well sites, and were analyzed using a Gold Film Mercury Detector. Strong mercury anomalies occur at locations along known structures in close proximity to subsurface thermal activity; examples include areas over hot spring deposits and near a shallow producing well. In contrast, background mercury concentrations are present in nearby locations with little or no indication of subsurface thermal activity, such as areas around deep marginal producing wells and dry wells, and areas lacking hot spring deposits. These results indicate that mercury geochemical surveys can be useful for identifying and mapping structures controlling fluid flow in geothermal systems and for delineating areas overlying near-surface thermal activity. Soil mercury geochemistry thus provides information which may aid in the cost-effective selection of exploratory well sites.

Petroleum generation in the southeast Texas basin: Implications for hydrocarbon occurrence at the South Liberty salt dome

Geochemical characteristics of hydrocarbons from the South Liberty field, Liberty County, Texas, were integrated with local stratigraphy, pressure, temperature, seismic data, and formation water chemistry to determine the source, maturity, and migration pathways of hydrocarbons associated with the salt dome in the southeast Texas Gulf Coast. Fourteen crude oil samples from the soft-geopressured Cook Mountain and Yegua (both Eocene) and Frio (Oligocene) reservoirs (989–2886 m [3245–9469 ft]) were analyzed by whole-oil high-resolution gas chromatography (GC) and GC–mass spectrometry. Pressure and temperature data from 36 wells were used to model the thermal maturation history in the vicinity of the dome. South Liberty oils were found to belong to a single genetic family, sourced from rocks with a similar level of maturity. Biomarkers indicate they were formed in a marginal marine environment with notable terrestrial input from a likely lower Tertiary source rock (probably the downdip lower Claiborne and/or Wilcox Group). Oils were generated within the peak oil window (vitrinite reflectance, ~0.6–0.9%) at expulsion temperatures of 125 to 130C. Thermal modeling indicates the lower Claiborne and/or upper Wilcox beds attained optimum maturity in and around the dome between 8 to 20 Ma and 18 to 31 Ma, respectively. Oil migration toward the structure probably occurred through faults breaching the deep-seated lower Claiborne/upper Wilcox source. Two severely biodegraded samples were found, and these were associated with cooler formation water (50 to 55C) with a greater meteoric water component (48–60%). Comparison with Brazoria County oils (100 km [62 mi] to the southwest) indicates that the South Liberty oils belong to the same genetic family, with minor differences resulting from a greater input from terrestrial kerogen.

Potential for a Low-Temperature Geothermal Resource Near Mackay, Idaho

Four water samples were collected from springs in the Mackay, Idaho area to investigate the potential for a direct-heat geothermal resource. The maximum measured temperature was 22 C for a spring south of Mackay. Calculation of the mineral equilibrium relationships in the calcium-bicarbonate water samples indicates that these samples equilibrated with the carbonate reservoir rocks. The temperatures of equilibration suggest that the subsurface temperatures of these water samples are probably no higher than measured surface temperatures.