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Geoffrey S. Ellis

Publications and source records attributed to Geoffrey S. Ellis.

At least 55 records · Page 3Linked to original sources
USGS2020

Assessment of undiscovered oil and gas resources in the Upper Cretaceous Austin Chalk and Tokio and Eutaw Formations, U.S. Gulf Coast, 2019

Using a geology-based assessment methodology, the U.S. Geological Survey estimated undiscovered, technically recoverable mean resources of 6.9 billion barrels of oil and 41.5 trillion cubic feet of natural gas in conventional and continuous accumulations in the Upper Cretaceous Austin Chalk and Tokio and Eutaw Formations onshore and in State waters of the U.S. Gulf Coast region.

Alabama, Arkansas, Louisiana, Mississippi, Oklahom↗
USGS2020

Sulfur isotope composition of individual compounds in immature organic-rich rocks and possible geochemical implications

We applied compound-specific sulfur isotope analysis (CSSIA) to organic matter (OM) extracted from ancient and immature organic-rich rocks from the Cretaceous Ghareb (Shefela Basin locality, Israel) and Miocene Monterey (Naples Beach locality, California, USA) Formations. Large variations in the δ 34 S values of different organosulfur compounds (OSCs), that reach up to 28‰ and 36‰, were observed in the Ghareb and Monterey samples, respectively. Additionally, some common OSCs in both locations showed consistent 34 S trends relative to each other. The consistent enrichment in 34 S of C 35 hopane thiophene relative to iC 20 thiophene in the studied sections probably resulted from differences in the timing of OM sulfurization. Reactive organic precursors quickly consume the most 34 S-depleted reduced S, while less reactive species incorporate the heavier residual S at a later time. Despite the differences in the depositional environments, ages, and the initial δ 34 S values of the reduced S (represented by the δ 34 S of pyrite) between the Ghareb and the Monterey Formations, the sulfurization order of common organic compounds seems to be similar. All of the δ 34 S values of OSCs are 34 S enriched relative to that of the coexisting pyrite with the exception of the C 25 highly branched isoprenoid (HBI) thiophene in several samples from the Monterey Formation. The existence of 34 S-depleted sulfurized HBI may point to OM sulfurization that occurred at or near the sediment-water interface during the deposition of the Monterey. Moreover, the δ 34 S of steroid sulfides shows an inverse trend with the pristane/phytane ratio, which may indicate that the sulfurization mechanism of these OSCs are affected by redox conditions. Further investigation of CSSI values in immature rocks from other basins may help constrain the OM sulfurization process, timescale, and depositional conditions and their possible use as paleoenvironmental proxies.

California↗
USGS2020

Position-specific distribution of hydrogen isotopes in natural propane: Effects of thermal cracking, equilibration and biodegradation

Intramolecular isotope distributions, including isotope clumping and position specific fractionation, can provide proxies for the formation temperature and formation and destruction pathways of molecules. In this study, we explore the position-specific hydrogen isotope distribution in propane. We analyzed propane samples from 10 different petroleum systems with high-resolution molecular mass spectrometry. Our results show that the hydrogen isotope fractionation between central and terminal positions of natural propanes ranges from −102‰ to +205‰, a much larger range than that expected for thermodynamic equilibrium at their source and reservoir temperatures (36–63‰). Based on these findings, we propose that the hydrogen isotope structure of catagenic propane is largely controlled by irreversible processes, expressing kinetic isotope effects (KIEs). Kinetic control on hydrogen isotope composition of the products of thermal cracking is supported by a hydrous pyrolysis experiment using the Woodford Shale as substrate, in which we observed isotopic disequilibrium in the early stage of pyrolysis. We make a more general prediction of KIE signatures associated with kerogen cracking by simulating this chemistry in a kinetic Monte Carlo model for different types of kerogens. In contrast, unconventional shale fluids or hot conventional reservoirs contain propane with an isotopic structure close to equilibrium, presumably reflecting internal and/or heterogeneous exchange during high temperature storage (ca. 100–150 °C). In relatively cold (<100 °C) conventional gas accumulations, propane can discharge from its source to a colder reservoir, rapidly enough to preserve disequilibrium signatures even if the source rock thermal maturity is high. These findings imply that long times at elevated temperatures are required to equilibrate the hydrogen isotopic structure of propane in natural gas host rocks and reservoirs. We further defined the kinetics of propane equilibration through hydrogen isotope exchange experiments under hydrous conditions; these experiments show that hydrogen in propane is exchangeable over laboratory timescales when exposed to clay minerals such as kaolinite. This implies rather rapid transfer of propane from sources to cold reservoirs in some of the conventional petroleum systems. Propane is also susceptible to microbial degradation in both oxic and anoxic environments. Biodegradation of propane in the Hadrian and Diana Hoover oil fields (Gulf of Mexico) results in strong increases in central-terminal hydrogen isotope fractionation. This reflects preferential attack on the central position, consistent with previous studies.

Geochimica et Cosmochimica Acta↗
USGS2019

Kinetics of elemental sulfur reduction by petroleum hydrocarbons and the implications for hydrocarbon thermal chemical alteration

Although sulfur-containing compounds are known to play a significant role in the diagenic and catagenic processes that generate oil and gas, relatively little is known about the kinetics of reactions between elemental S and petroleum hydrocarbons. To investigate this subject, a series of closed-system pyrolysis experiments using paraffin, a low-sulfur oil, and a high-sulfur oil with and without elemental S were conducted, and first-order chemical kinetics were fit to the experimental results. The average value for the activation energy required to reduce elemental S to H 2 S and to thermochemically alter higher molecular weight hydrocarbons to methane was calculated to be 193 kJ mol -1 (46 kcal mol -1 ). The results of this study demonstrate that under typical geologic conditions the rate of reduction of elemental S to H 2 S by petroleum hydrocarbons is quite rapid. The maximum time for substantial amounts of elemental S to persist in contact with petroleum hydrocarbons is estimated to be no more than a few million years in cool reservoirs (e.g., <80 °C), and in hotter reservoirs (e.g., >120 °C) the half-life of elemental S may be as short as hundreds of years. Additionally, the presence of elemental S substantially lowers the onset temperature of hydrocarbon thermal chemical alteration (TCA). The activation energy for TCA of a low-sulfur oil to generate methane is estimated to be lowered by 92 kJ mol -1 (22 kcal mol -1 ) due to the presence of elemental sulfur. Consequently, the presence of elemental S in petroleum reservoirs is expected to lower the thermal stability of oil and decrease the maximum depth at which oil occurs within a basin (thermal deadline). The observed acceleration of hydrocarbon TCA is possibly due to organic sulfur compounds (e.g., thiols and sulfides) that form through the reaction of H 2 S or polysulfides with hydrocarbons and subsequently thermally degrade leading to the formation of sulfur radicals that in turn enhance TCA reactions.

Geochimica et Cosmochimica Acta↗
USGS2018

Position-specific 13C distributions within propane from experiments and natural gas samples

Site-specific carbon isotope measurements of organic compounds potentially recover information that is lost in a conventional, ‘bulk’ isotopic analysis. Such measurements are useful because isotopically fractionating processes may have distinct effects at different molecular sites, and thermodynamically equilibrated populations of molecules tend to concentrate heavy isotopes in one molecular site versus another. Most recent studies of site-specific 13 C in organics use specialized Nuclear Magnetic Resonance (NMR) techniques or complex chemical degradations prior to mass spectrometric measurements. Herein we present the first application of a new mass spectrometric technique that reconstructs the site-specific carbon isotope composition of propane based on measurements of the 13 C/ 12 C ratios of two or more fragment ions that sample different proportions of the terminal and central carbon sites. We apply this method to propane from laboratory experiments and natural gas samples to explore the relationships between site-specific carbon isotope composition, full-molecular δ 13 C, thermal maturity, and variation in organic matter precursors. Our goal is to advance the understanding of the sources and histories of short-chain alkanes within geologic systems. Our findings suggest that propane varies in its site-specific carbon isotope structure, which is correlated with increasing thermal maturity, first increasing in terminal position δ 13 C and then increasing in both center and terminal position δ 13 C. This pattern is observed in both experimental and natural samples, and is plausibly explained by a combination of site-specific, temperature-dependent isotope effects associated with conversion of different precursor molecules (kerogen, bitumen, and/or oil) to propane, differences in site-specific isotopic contents of those precursors, and possibly distillation of reactive components of those precursors with increasing maturity. We hypothesize that the largest changes in site-specific isotopic content of propane occur when bitumen and/or oil replace kerogen as the dominant precursors. If correct, this phenomenon could have significant utility for understanding gas generation in thermogenic petroleum systems.

Geochimica et Cosmochimica Acta↗
USGS2018

Equilibrium and non-equilibrium controls on the abundances of clumped isotopologues of methane during thermogenic formation in laboratory experiments: Implications for the chemistry of pyrolysis and the origins of natural gases

Multiply isotopically substituted molecules (‘clumped’ isotopologues) can be used as geothermometers because their proportions at isotopic equilibrium relative to a random distribution of isotopes amongst all isotopologues are functions of temperature. This has allowed measurements of clumped-isotope abundances to be used to constrain formation temperatures of several natural materials. However, kinetic processes during generation, modification, or transport of natural materials can also affect their clumped-isotope compositions. Herein, we show that methane generated experimentally by closed-system hydrous pyrolysis of shale or nonhydrous pyrolysis of coal yields clumped-isotope compositions consistent with an equilibrium distribution of isotopologues under some experimental conditions (temperature–time conditions corresponding to ‘low,’ ‘mature,’ and ‘over-mature’ stages of catagenesis), but can have non-equilibrium (i.e., kinetically controlled) distributions under other experimental conditions (‘high’ to ‘over-mature’ stages), particularly for pyrolysis of coal. Non-equilibrium compositions, when present, lead the measured proportions of clumped species to be lower than expected for equilibrium at the experimental temperature, and in some cases to be lower than a random distribution of isotopes (i.e., negative Δ 18 values). We propose that the consistency with equilibrium for methane formed by relatively low temperature pyrolysis reflects local reversibility of isotope exchange reactions involving a reactant or transition state species during demethylation of one or more components of kerogen. Non-equilibrium clumped-isotope compositions occur under conditions where ‘secondary’ cracking of retained oil in shale or wet gas hydrocarbons (C 2-5 , especially ethane) in coal is prominent. We suggest these non-equilibrium isotopic compositions are the result of the expression of kinetic isotope effects during the irreversible generation of methane from an alkyl precursor. Other interpretations are also explored. These findings provide new insights into the chemistry of thermogenic methane generation, and may provide an explanation of the elevated apparent temperatures recorded by the methane clumped-isotope thermometer in some natural gases. However, it remains unknown if the laboratory experiments capture the processes that occur at the longer time and lower temperatures of natural gas formation.

Geochimica et Cosmochimica Acta↗
USGS2018

Experimental investigation of the role of rock fabric in gas generation and expulsion during thermal maturation: Anhydrous closed-system pyrolysis of a bitumen-rich Eagle Ford Shale

Gold-tube pyrolysis experiments were conducted on miniature core plugs and powdered rock from a bitumen-rich sample of Eagle Ford Shale to investigate the role of rock fabric in gas generation and expulsion during thermal maturation. The samples were isothermally heated at 130, 300, 310, 333, 367, 400, and 425 °C for 72 h under a confining pressure of 68.0 MPa, corresponding to six levels of induced thermal maturity: pre-oil generation (130 °C/72 h), incipient oil/bitumen generation (300 and 310 °C/72 h), early oil generation (333 °C/72 h), peak oil generation (367 °C/72 h), early oil cracking (400 °C/72 h), and late oil cracking (425 °C/72 h). Experimental results show that gas retention coupled with compositional fractionation occurs in the core plug experiments and varies as a function of thermal maturity. During the incipient oil/bitumen generation stage, yields of methane through pentane (C 1 –C 5 ) from core plugs are significantly lower than those from rock powder, and gases from core plugs are enriched in methane. However, the differences in C 1 –C 5 gas yield and composition decrease throughout the oil generation stage, and by the oil cracking stage no obvious compositional difference in C 1 –C 5 gases exists. The decrease in the effect of rock fabric on gas yield and composition with increasing maturity is the result of an increase in gas expulsion efficiency. Pyrolysis of rock powder yields 4–16 times more CO 2 compared to miniature core plugs, with δ 13 C CO2 values ranging from −2.9‰ to −0.6‰, likely due to carbonate decomposition accelerated by reactions with organic acids. Furthermore, lower yields of gaseous alkenes and H 2 from core plug experiments sugge

Organic Geochemistry↗
USGS2017

East African weathering dynamics controlled by vegetation-climate feedbacks

Tropical weathering has important linkages to global biogeochemistry and landscape evolution in the East African rift. We disentangle the influences of climate and terrestrial vegetation on chemical weathering intensity and erosion at Lake Malawi using a long sediment record. Fossil pollen, microcharcoal, particle size, and mineralogy data affirm that the detrital clays accumulating in deep water within the lake are controlled by feedbacks between climate and hinterland forest composition. Particle-size patterns are also best explained by vegetation, through feedbacks with lake levels, wildfires, and erosion. We develop a new source-to-sink framework that links lacustrine sedimentation to hinterland vegetation in tropical rifts. Our analysis suggests that climate-vegetation interactions and their coupling to weathering/erosion could threaten future food security and has implications for accurately predicting petroleum play elements in continental rift basins.

Geology↗
USGS2017

Experimental investigation on thermochemical sulfate reduction in the presence of 1-pentanethiol at 200 and 250 °C: Implications for in situ TSR processes occurring in some MVT deposits

Organic sulfur compounds are ubiquitous in natural oil and gas fields and moderate-low temperature sulfide ore deposits. Previous studies have shown that organic sulfur compounds are important in enhancing the rates of thermochemical sulfate reduction (TSR) reactions, but the details of these reaction mechanisms remain unclear. In order to assess the extent of sulfate reduction in the presence of labile sulfur species at temperature conditions near to those where TSR occurs in nature, we conducted a series of experiments using the fused silica capillary capsule (FCSS) method. The tested systems containing labile sulfur species are MgSO 4 + 1-pentanethiol (C 5 H 11 SH) + 1-octene (C 8 H 16 ), MgSO 4 + 1-octene (C 8 H 16 ), MgSO 4 + 1-pentanethiol (C 5 H 11 SH), 1-pentanethiol (C 5 H 11 SH)+H 2 O, and MgSO 4 + 1-pentanethiol (C 5 H 11 SH) + ZnBr 2 systems. Our results show that: (1) intermediate oxidized carbon species (ethanol and acetic acid) are formed during TSR simulation experiments when 1-pentanethiol is present; (2) in the presence of ZnBr 2 , 1-pentanethiol can be oxidized by sulfate to CO 2 at 200 °C, which is within the temperature range observed in natural TSR; and (3) the precipitation of sulfide minerals may significantly promote the rate of TSR, indicating that the rates of in situ TSR reactions in ore deposits could be much faster than previously thought. This may be important for understanding the possibility of in situ TSR as a mechanism for the precipitation of metal sulfides in some ore deposits. These findings provide important experimental evidence for understanding the role of organic sulfur compounds in TSR reactions and the pathway of TSR reactions initiated by organic sulfur compounds under natural conditions.

Ore Geology Reviews↗
USGS2017

Effects of thermal maturation and thermochemical sulfate reduction on compound-specific sulfur isotopic compositions of organosulfur compounds in Phosphoria oils from the Bighorn Basin, USA

Compound-specific sulfur isotope analysis was applied to a suite of 18 crude oils generated from the Permian Phosphoria Formation in the Bighorn Basin, western USA. These oils were generated at various levels of thermal maturity and some experienced thermochemical sulfate reduction (TSR). This is the first study to examine the effects of thermal maturation on stable sulfur isotopic compositions of individual organosulfur compounds (OSCs) in crude oil. A general trend of 34 S enrichment in all of the studied compounds with increasing thermal maturity was observed, with the δ 34 S values of alkyl-benzothiophenes (BTs) tending to be enriched in 34 S relative to those of the alkyl-dibenzothiophenes (DBTs) in lower-maturity oils. As thermal maturity increases, δ 34 S values of both BTs and DBTs become progressively heavier, but the difference in the average δ 34 S value of the BTs and DBTs (Δ 34 S BT-DBT) decreases. Differences in the isotopic response to thermal stress exhibited by these two compound classes are considered to be the result of relative differences in their thermal stabilities. TSR-altered Bighorn Basin oils have OSCs that are generally enriched in 34 S relative to non-TSR-altered oils, with the BTs being enriched in 34 S relative to the DBTs, similar to the findings of previous studies. However, several oils that were previously interpreted to have been exposed to minor TSR have Δ 34 S BT-DBT values that do not support this interpretation. The δ 34 S values of the BTs and DBTs in some of these oils suggest that they did not experience TSR, but were derived from a more thermally mature source. The heaviest δ 34 S values observed in the OSCs are enriched in 34 S by up to 10‰ relative to that of Permian anhydrite in the Bighorn Basin, suggesting that there may be an alternate or additional source of sulfate in some parts of the basin. These results indicate that the sulfur isotopic composition of OSCs in oil provides a sensitive indicator for the extent of TSR, which cannot be determined from other bulk geochemical parameters. Moreover, when combined with additional geochemical and geologic evidence, the sulfur isotopic composition of OSCs in oils can help to identify the source of sulfate for TSR alteration in petroleum reservoirs.

Montana, Wyoming↗
USGS2017

Depositional environment and organic matter accumulation of Upper Ordovician–Lower Silurian marine shale in the Upper Yangtze Platform, South China

The main controlling factors of organic matter accumulation in the Upper Ordovician Wufeng–Lower Silurian Longmaxi Formations are complex and remain highly controversial. This study investigates the vertical variation of total organic carbon (TOC) content as well as major and trace element concentrations of four Ordovician–Silurian transition sections from the Upper Yangtze Platform of South China to reconstruct the paleoenvironment of these deposits and to improve our understanding of those factors that have influenced organic matter accumulation in these deposits. The residual TOC content of the Wufeng Formation averages 3.2% and ranges from 0.12 to 6.0%. The overlying lower Longmaxi Formation displays higher TOC content (avg. 4.4%), followed upsection by consistent and lower values that average 1.6% in the upper Longmaxi Formation. The concentration and covariation of redox-sensitive trace elements (Mo, U and V) suggest that organic-rich intervals of the Wufeng Formation accumulated under predominantly anoxic conditions. Organic-rich horizons of the lower Longmaxi Formation were deposited under strongly anoxic to euxinic conditions, whereas organic-poor intervals of the upper Longmaxi Formation accumulated under suboxic conditions. Positive correlations between redox proxies and TOC contents suggest that organic matter accumulation was predominantly controlled by preservation. Barium excess (Ba xs ) values indicate high paleoproductivity throughout the entire depositional sequence, with an increase in the lower Longmaxi Formation. Increased productivity may have been induced by enhanced P recycling, as evidenced by elevated C org /P tot ratios. Mo–U covariation and Mo/TOC values reveal that the Wufeng Formation was deposited under extremely restricted conditions, whereas the Longmaxi Formation accumulated under moderately restricted conditions. During the Late Ordovician, the extremely restricted nature of ocean circulation on the Upper Yangtze Platform in tandem with enhanced stratification of the water column promoted anoxic conditions favorable for the preservation of organic matter. During Early Silurian time, organic matter accumulation was principally controlled by changes in sea level, which affected terrigenous flux, redox conditions, and the degree of nutrition recycling.

Upper Yangtze Platform↗
USGS2016

Study of thermochemical sulfate reduction mechanism using compound specific sulfur isotope analysis

The sulfur isotopic fractionation associated with the formation of organic sulfur compounds (OSCs) during thermochemical sulfate reduction (TSR) was studied using gold-tube pyrolysis experiments to simulate TSR. The reactants used included n -hexadecane ( n -C 16 ) as a model organic compound with sulfate, sulfite, or elemental sulfur as the sulfur source. At the end of each experiment, the S-isotopic composition and concentration of remaining sulfate, H 2 S, benzothiophene, dibenzothiophene, and 2-phenylthiophene (PT) were measured. The observed S-isotopic fractionations between sulfate and BT, DBT, and H 2 S in experimental simulations of TSR correlate well with a multi-stage model of the overall TSR process. Large kinetic isotope fractionations occur during the first, uncatalyzed stage of TSR, 12.4&permil; for H 2 S and as much as 22.2&permil; for BT. The fractionations decrease as the H 2 S concentration increases and the reaction enters the second, catalyzed stage. Once all of the oxidizable hydrocarbons have been consumed, sulfate reduction ceases and equilibrium partitioning then dictates the fractionation between H 2 S and sulfate (&sim;17&permil;). Experiments involving sparingly soluble CaSO 4 show that during the second catalytic phase of TSR the rate of sulfate reduction exceeds that of sulfate dissolution. In this case, there is no apparent isotopic fractionation between source sulfate and generated H 2 S, as all of the available sulfate is effectively reduced at all reaction times. When CaSO 4 is replaced with fully soluble Na 2 SO 4 , sulfate dissolution is no longer rate limiting and significant S-isotopic fractionation is observed. This supports the notion that CaSO 4 dissolution can lead to the apparent lack of fractionation between H 2 S and sulfate produced by TSR in nature. The S-isotopic composition of individual OSCs record information related to geochemical reactions that cannot be discerned from the &delta; 34 S values obtained from bulk phases such as H 2 S, oil, and sulfate minerals, and provide important mechanistic details about the overall TSR process.

Geochimica et Cosmochimica Acta↗
USGS2015

Qualitative and quantitative analysis of Dibenzofuran, Alkyldibenzofurans, and Benzo[b]naphthofurans in crude oils and source rock extracts

Dibenzofuran (DBF), its alkylated homologues, and benzo[b]naphthofurans (BNFs) are common oxygen-heterocyclic aromatic compounds in crude oils and source rock extracts. A series of positional isomers of alkyldibenzofuran and benzo[b]naphthofuran were identified in mass chromatograms by comparison with internal standards and standard retention indices. The response factors of dibenzofuran in relation to internal standards were obtained by gas chromatography-mass spectrometry analyses of a set of mixed solutions with different concentration ratios. Perdeuterated dibenzofuran and dibenzothiophene are optimal internal standards for quantitative analyses of furan compounds in crude oils and source rock extracts. The average concentration of the total DBFs in oils derived from siliciclastic lacustrine rock extracts from the Beibuwan Basin, South China Sea, was 518 &mu;g/g, which is about 5 times that observed in the oils from carbonate source rocks in the Tarim Basin, Northwest China. The BNFs occur ubiquitously in source rock extracts and related oils of various origins. The results of this work suggest that the relative abundance of benzo[b]naphthofuran isomers, that is, the benzo[b]naphtho[2,1-d]furan/{benzo[b]naphtho[2,1-d]furan + benzo[b]naphtho[1,2-d]furan} ratio, may be a potential molecular geochemical parameter to indicate oil migration pathways and distances.

Energy & Fuels↗
USGS2015

Compound-specific sulfur isotope analysis of thiadiamondoids of oils from the Smackover Formation, USA

Thiadiamondoids (TDs) are diamond-like compounds with a sulfide bond located within the cage structure. These compounds were suggested as a molecular proxy for the occurrence and extent of thermochemical sulfate reduction (TSR). Compound-specific sulfur-isotope analysis of TDs may create a multi-parameter system, based on molecular and &delta;34S values that may be sensitive over a wider range of TSR and thermal maturation stages. In this study, we analyzed a suite of 12 Upper Jurassic oil and condensate samples generated from source rocks in the Smackover Formation to perform a systematic study of the sulfur isotope distribution in thiadiamondoids (one and two cages). For comparison we measured the &delta;34S composition of benzothiophenes (BTs) and dibenzothiophenes (DBTs). We also conducted pyrolysis experiments with petroleum and model compounds to have an insight into the formation mechanisms of TDs. The &delta;34S of the TDs varied significantly (ca 30&permil;) between the different oils depending on the degree of TSR alteration. The results showed that within the same oil, the one-cage TDs were relatively uniform, with 34S enriched values similar to those of the coexisting BTs. The two-cage TDs had more variable &delta;34S values that range from the &delta;34S values of BTs to those of the DBTs, but with general 34S depletion relative to one cage TDs. Hydrous pyrolysis experiments (360 &deg;C, 40 h) with either CaSO4 or elemental S (equivalent S molar concentrations) and adamantane as a model compound demonstrate the formation of one cage TDs in relatively low yields (<0.2%). Higher concentrations of TDs were observed in the elemental sulfur experiments, most likely because of the higher rates of reaction with adamantane under these experimental conditions. These results show that the formation of TDs is not exclusive to TSR reactions, and that they can also form by reaction with reduced S species apart from sulfate reduction, though at low yields. Oxygenated compounds, most notably 2-thiaadamantanone and phenol, were also formed during these pyrolysis experiments. This may represent the first stage in the formation of sulfurized compounds and the oxidation of organic matter under TSR conditions. Pyrolysis experiments with elemental S and a TD-enriched oil showed that the &delta;34S values of the TDs did not change, whereas the BTs did change significantly. It is therefore concluded that TDs do not exchange S atoms with coexisting inorganic reduced sulfur species. They can only change their &delta;34S values via addition of newly generated TDs that form predominantly during TSR. We therefore suggest that TDs will preserve their &delta;34S values even under high-temperature reservoir conditions and will reflect the original sulfates &delta;34S value. The combination of TDs, BTs, and DBTs &delta;34S values and concentrations allowed for a more reliable detection of the occurrence and extent of TSR than either proxy alone. It showed that except for two oils, all of the oils that were measured in this study were affected by TSR or TSR-sourced H2S, to some degree. It is still not known if some of the oils with the lower concentrations of TDs and enriched &delta;34S values (close to sulfate minerals) were affected by TSR or by a secondary charge of 34S-enriched H2S.

Geochimica et Cosmochimica Acta↗
USGS2015

Lake formation, characteristics and evolution in retroarc deposystems: A synthesis of data from the modern Andean orogen and its associated basins

Lake deposystems are commonly associated with retroarc mountain belts in the geological record. These deposystems are poorly characterized in modern retroarcs, placing limits on our ability to interpret environmental signals from ancient deposits. To address this problem, we have synthesized our existing knowledge about the distribution, morphometrics, and sedimentary geochemical characteristics of tectonically formed lakes in the central Andean retroarc. Large, active mountain belts such as the Andes frequently create an excess of sediment, to the point that modeling and observational data both suggest their adjacent retroarc basins will be rapidly overfilled by sediments. Lake formation, requiring topographic closure, demands special conditions such as topographic isolation and arid climatic conditions to reduce sediment generation, and bedrock lithologies that yield little siliciclastic sediment. Lacustrine deposition in the modern Andean retroarc has different characteristics in the six major morphotectonic zones discussed. (1) High-elevation hinterland basins of the arid Puna-Altiplano Plateau frequently contain underfilled and balanced-filled lakes that are potentially long-lived and display relatively rapid sedimentation rates. (2) Lakes are rare in piggyback basins, although a transition zone exists where basins that originally formed as piggybacks are transferred to the hinterland through forward propagation of the thrust belt. Here, lakes are moderately abundant and long-lived and display somewhat lower sedimentation rates than in the hinterland. (3) Wedge-top and (4) foredeep deposystems of the Andean retroarc are generally overfilled, and lakes are small and ephemeral. (5) Semihumid Andean back-bulge basins contain abundant small lakes, which are moderately long-lived because of underfilling by sediment and low sedimentation rates. (6) Broken foreland lakes are common, typically underfilled, large, and long-lived playa or shallow systems.

Book chapter↗