Geology topics

Alon Amrani

Publications and source records attributed to Alon Amrani.

9 recordsLinked to original sources

USGS2022

Geochemical characterization of natural gases in the pre-salt section of the Santos Basin (Brazil) focused on hydrocarbons and volatile organic sulfur compounds

The objective of this work is to characterize the geochemistry of a suite of natural gas samples from five fields in order to improve the understanding of the lacustrine petroleum system of the pre-salt section from the Santos Basin (Brazil). Additionally, the distribution of volatile organic sulfur compounds (VOSC) in petroleum reservoirs was examined to investigate possible applications to petroleum systems assessments. The hydrocarbon gases were generated by thermogenic processes associated with the oil window. The 13 C-enriched values for C 1 (>−40‰) were interpreted as an organic source signature rather than an indication of thermal maturity, except for the oil occurrence (Field B), where a different fluid charge mainly composed of methane and CO 2 from a minor external kitchen area in the Santos Basin was identified. The molecular composition and the carbon and hydrogen isotopic data of the hydrocarbon gases, when combined with the VOSC molecular compositional data, allow the identification of four gas families associated with different kitchens and/or migrations pathways. The total VOSC concentrations range from 0.7 to 23.9 ppm by volume (ppmV). The organic sulfides are mainly composed of carbonyl sulfide (COS) and diethyl sulfide. The main thiol compound is ethanethiol. The cyclic VOSC are primarily composed of thiophene, with a negligible amount of branched thiophenes (<0.1 ppmV). H2S showed a strong positive Pearson's correlation with COS and methanethiol (MeSH) concentrations (r = 0.943 and 0.807, respectively). This suggests that COS and MeSH formation was linked to H 2 S generation and/or post-catagenetic interactions between hydrocarbons and H 2 S, mainly related to thermochemical sulfate reduction (TSR). In contrast, the distribution of higher molecular weight VOSC seems to be controlled by source rock facies, rather than H 2 S concentration. Principal component analysis of the VOSC compositional data identified some subgroups within the gas families mainly associated with TSR. The results presented in this work reveal that VOSC can be an important auxiliary tool in petroleum system studies.

Atlantic Ocean, Santos Basin↗

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↗

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↗

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 δ 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

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 δ34S 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 δ34S 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 δ34S 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 δ34S values that range from the δ34S values of BTs to those of the DBTs, but with general 34S depletion relative to one cage TDs. Hydrous pyrolysis experiments (360 °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 δ34S 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 δ34S values via addition of newly generated TDs that form predominantly during TSR. We therefore suggest that TDs will preserve their δ34S values even under high-temperature reservoir conditions and will reflect the original sulfates δ34S value. The combination of TDs, BTs, and DBTs δ34S 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 δ34S values (close to sulfate minerals) were affected by TSR or by a secondary charge of 34S-enriched H2S.

Geochimica et Cosmochimica Acta↗

USGS2008

The role of labile sulfur compounds in thermal chemical sulfate reduction

The reduction of sulfate to sulfide coupled with the oxidation of hydrocarbons to carbon dioxide, commonly referred to as thermochemical sulfate reduction (TSR), is an important abiotic alteration process that most commonly occurs in hot carbonate petroleum reservoirs. In the present study we focus on the role that organic labile sulfur compounds play in increasing the rate of TSR. A series of gold-tube hydrous pyrolysis experiments were conducted with n -octane and CaSO 4 in the presence of reduced sulfur (e.g. H 2 S, S°, organic S) at temperatures of 330 and 356 °C under a constant confining pressure. The in-situ pH was buffered to 3.5 (∼6.3 at room temperature) with talc and silica. For comparison, three types of oil with different total S and labile S contents were reacted under similar conditions. The results show that the initial presence of organic or inorganic sulfur compounds increases the rate of TSR. However, organic sulfur compounds, such as 1-pentanethiol or diethyldisulfide, were significantly more effective in increasing the rate of TSR than H 2 S or elemental sulfur (on a mole S basis). The increase in rate is achieved at relatively low concentrations of 1-pentanethiol, less than 1 wt% of the total n -octane, which is comparable to the concentration of organic S that is common in many oils (∼0.3 wt%). We examined several potential reaction mechanisms to explain the observed reactivity of organic LSC. First, the release of H 2 S from the thermal degradation of thiols was discounted as an important mechanism due to the significantly greater reactivity of thiol compared to an equivalent amount of H 2 S. Second, we considered the generation of olefines in association with the elimination of H 2 S during thermal degradation of thiols because olefines are much more reactive than n -alkanes during TSR. In our experiments, olefines increased the rate of TSR, but were less effective than 1-pentanethiol and other organic LSC. Third, the thermal decomposition of organic LSC creates free-radicals that in turn might initiate a radical chain-reaction that creates more reactive species. Experiments involving radical initiators, such as diethyldisulfide and benzyldisulfide, did not show an increase in reactivity compared to 1-pentanethiol. Therefore, we conclude that none of these can sufficiently explain our observations of the initial stages of TSR; they may, however, be important in the later stages. In order to gain greater insight into the potential mechanism for the observed reactivity of these organic sulfur compounds during TSR, we applied density functional theory-based molecular modeling techniques to our system. The results of these calculations indicate that 1-pentanethiol or its thermal degradation products may directly react with sulfate and reduce the activation energy required to rupture the first S–O bond through the formation of a sulfate ester. This study demonstrates the importance of labile sulfur compounds in reducing the onset timing and temperature of TSR. It is therefore essential that labile sulfur concentrations are taken into consideration when trying to make accurate predictions of TSR kinetics and the potential for H 2 S accumulation in petroleum reservoirs.

Geochimica et Cosmochimica Acta↗

USGS2008

Experimental investigation on thermochemical sulfate reduction by H2S initiation

Hydrogen sulfide (H 2 S) is known to catalyze thermochemical sulfate reduction (TSR) by hydrocarbons (HC), but the reaction mechanism remains unclear. To understand the mechanism of this catalytic reaction, a series of isothermal gold-tube hydrous pyrolysis experiments were conducted at 330 °C for 24 h under a constant confining pressure of 24.1 MPa. The reactants used were saturated HC (sulfur-free) and CaSO 4 in the presence of variable H 2 S partial pressures at three different pH conditions. The experimental results showed that the in - situ pH of the aqueous solution (herein, in - situ pH refers to the calculated pH of aqueous solution under the experimental conditions) can significantly affect the rate of the TSR reaction. A substantial increase in the TSR reaction rate was recorded with a decrease in the in - situ pH value of the aqueous solution involved. A positive correlation between the rate of TSR and the initial partial pressure of H 2 S occurred under acidic conditions (at pH ∼3–3.5). However, sulfate reduction at pH ∼5.0 was undetectable even at high initial H 2 S concentrations. To investigate whether the reaction of H 2 S (aq) and HSO 4 - "> HSO4- occurs at pH ∼3, an additional series of isothermal hydrous pyrolysis experiments was conducted with CaSO 4 and variable H 2 S partial pressures in the absence of HC at the same experimental temperature and pressure conditions. CaSO 4 reduction was not measurable in the absence of paraffin even with high H 2 S pressure and acidic conditions. These experimental observations indicate that the formation of organosulfur intermediates from H 2 S reacting with hydrocarbons may play a significant role in sulfate reduction under our experimental conditions rather than the formation of elemental sulfur from H 2 S reacting with sulfate as has been suggested previously (Toland W. G. (1960) Oxidation of organic compounds with aqueous sulphate. J. Am. Chem. Soc. 82 , 1911–1916). Quantification of labile organosulfur compounds (LSC), such as thiols and sulfides, was performed on the products of the reaction of H 2 S and HC from a series of gold-tube non-isothermal hydrous pyrolysis experiments conducted at about pH 3 from 300 to 370 °C and a 0.1-°C/h heating rate. Incorporation of sulfur into HC resulted in an appreciable amount of thiol and sulfide formation. The rate of LSC formation positively correlated with the initial H 2 S pressure. Thus, we propose that the LSC produced from H 2 S reaction with HC are most likely the reactive intermediates for H 2 S initiation of sulfate reduction. We further propose a three-step reaction scheme of sulfate reduction by HC under reservoir conditions, and discuss the geological implications of our experimental findings with regard to the effect of formation water and oil chemistry, in particular LSC content.

Geochimica et Cosmochimica Acta↗

USGS2008

Theoretical study on the reactivity of sulfate species with hydrocarbons

The abiotic, thermochemically controlled reduction of sulfate to hydrogen sulfide coupled with the oxidation of hydrocarbons, is termed thermochemical sulfate reduction (TSR), and is an important alteration process that affects petroleum accumulations in nature. Although TSR is commonly observed in high-temperature carbonate reservoirs, it has proven difficult to simulate in the laboratory under conditions resembling nature. The present study was designed to evaluate the relative reactivities of various sulfate species in order to provide greater insight into the mechanism of TSR and potentially to fill the gap between laboratory experimental data and geological observations. Accordingly, quantum mechanics density functional theory (DFT) was used to determine the activation energy required to reach a potential transition state for various aqueous systems involving simple hydrocarbons and different sulfate species. The entire reaction process that results in the reduction of sulfate to sulfide is far too complex to be modeled entirely; therefore, we examined what is believed to be the rate limiting step, namely, the reduction of sulfate S(VI) to sulfite S(IV). The results of the study show that water-solvated sulfate anions SO 4 2- are very stable due to their symmetrical molecular structure and spherical electronic distributions. Consequently, in the absence of catalysis, the reactivity of SO 4 2- is expected to be extremely low. However, both the protonation of sulfate to form bisulfate anions ( HSO 4 - ) and the formation of metal-sulfate contact ion-pairs could effectively destabilize the sulfate molecular structure, thereby making it more reactive. Previous reports of experimental simulations of TSR generally have involved the use of acidic solutions that contain elevated concentrations of " HSO 4 - relative to SO 4 2- . However, in formation waters typically encountered in petroleum reservoirs, the concentration of HSO 4 - is likely to be significantly lower than the levels used in the laboratory, with most of the dissolved sulfate occurring as SO 4 2- , aqueous calcium sulfate ([CaSO 4 ] (aq) ), and aqueous magnesium sulfate ([MgSO 4 ] (aq) ). Our calculations indicate that TSR reactions that occur in natural environments are most likely to involve bisulfate ions ( HSO 4 - ) and/or magnesium sulfate contact ion-pairs ([MgSO 4 ] CIP ) rather than ‘free’ sulfate ions ( SO 4 2- ) or solvated sulfate ion-pairs, and that water chemistry likely plays a significant role in controlling the rate of TSR.

Geochimica et Cosmochimica Acta↗

USGS2006

Experiments on δ 34 S mixing between organic and inorganic sulfur species during thermal maturation

Reduced sulfur species were studied to constrain isotopic exchange-mixing with synthetic polysulfide cross-linked macromolecules (PCLM), model sulfur containing molecules and natural sulfur-rich kerogen, asphalt and oil of the Dead Sea area. PCLM represents protokerogens that are rich in sulfur and thermally unstable. Mixing rates of PCLM with HS-(aq) (added as (NH 4 ) 2 S (aq) ) at low to moderate temperatures (50–200 °C) are rapid. Elemental sulfur and H 2 S (gas) fully mix isotopes with PCLM during pyrolysis conditions at 200 °C. During these reactions significant structural changes of the PCLM occur to form polysulfide dimers, thiolanes and thiophenes. As pyrolysis temperatures or reaction times increase, the PCLM thermal products are transformed to more aromatic sulfur compounds. Isotopic mixing rates increase with increasing pyrolysis temperature and time. Polysulfide bonds (S–S) in the PCLM are responsible for most of these structural and isotopic changes because of their low stability. Conversely, sulfur isotope mixing does not occur between dibenzothiophene (aromatic S) or hexadecanthiol (C–SH) and HS-(aq) at 200 °C after 48 h. This shows that rates of sulfur isotope mixing are strongly dependent on the functionality of the sulfur in the organic matter. The order of isotopic mixing rates for organic matter is kerogen > asphalt > oil, which is inverse to their sulfur thermal stability. Asphalt and oil with more refractory sulfur show significantly lower isotopes mixing rates than the kerogen with more labile sulfur. Based on the findings of the present study we suggest that sulfur isotopes mixing can occur from early diagenesis into catagenesis and result in isotopic homogenization of the inorganic and organic reduced sulfur pools.

Geochimica et Cosmochimica Acta↗