Origin of the unusually high dibenzothiophene oils in Tazhong-4 Oilfield of Tarim Basin and its implication in deep petroleum exploration

Unusually high dibenzothiophene (DBT) concentrations are present in the oils from the Tazhong-4 Oilfield in the Tazhong Uplift, Tarim Basin. Positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used in combination with conventional geochemical approaches to unravel the enrichment mechanisms. Significant amounts of S₁ species with relatively low DBE values (0–8), i.e., sulfur ethers, mercaptans, thiophenes and benzothiophenes, were detected in three Lower Ordovician oils with high thermal maturity, which were suggested to be the products of thermochemical sulfate reduction (TSR) in the reservoir. The occurrence of TSR was also supported by the coexistence of thiadiamondoids and abundant H₂S in the gases associated with the oils. Obviously low concentrations of the DBE = 9 S₁ species (mainly equivalent to C₀–C₃₅ DBTs) compared to its homologues were observed for the three oils which were probably altered by TSR, indicating that no positive relationship existed between TSR and DBTs in this study. The sulfur compounds in the Tazhong-4 oils are quite similar to those in the majority of Lower Ordovician oils characterized by high concentrations of DBTs and dominant DBE = 9 S₁ species with only small amounts of sulfur compounds with low thermal stability (DBE = 0–8), suggesting only a small proportion of sulfur compounds were derived from TSR. It is thermal maturity rather than TSR that has caused the unusually high DBT concentrations in most of the Lower Ordovician oils. We suggest that the unusually high DBT oils in the Tazhong-4 Oilfield are caused by oil mixing from the later charged Lower Ordovician (or perhaps even deeper), with high DBT abundances from the earlier less mature oils, which was supported by our oil mixing experiments and previous relevant investigations as well as abundant authigenic pyrite of hydrothermal origin. We believe that TSR should have occurred in the Tazhong Uplift based on our FT-ICR MS results. However, normal sulfur compounds were detected in most oils and no increase of δ¹³C₂H₆–δ¹³C₄H₁₀ was observed for the gas hydrocarbons, suggesting only a slight alteration of the oils by TSR currently and/or recently. We suspect that the abnormal sulfur compounds in the Lower Ordovician oils might also be a result of deep oil mixing, which might imply a deeper petroliferous horizon, i.e., Cambrian, with a high petroleum potential. This study is important to further deep petroleum exploration in the area.     

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 MgSO₄ + 1-pentanethiol (C₅H₁₁SH) + 1-octene (C₈H₁₆), MgSO₄ + 1-octene (C₈H₁₆), MgSO₄ + 1-pentanethiol (C₅H₁₁SH), 1-pentanethiol (C₅H₁₁SH)+H₂O, and MgSO₄ + 1-pentanethiol (C₅H₁₁SH) + ZnBr₂ 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 ZnBr₂, 1-pentanethiol can be oxidized by sulfate to CO₂ 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.     

Proto-solid bitumen in petroleum altered by thermochemical sulfate reduction

Thermochemical sulfate reduction (TSR) involves a complex series of redox reactions whereby petroleum is oxidized by sulfate forming H₂S and CO_2. A highly aromatic, sulfur rich carbonaceous residue, which we define as TSR-solid bitumen, is commonly seen in reservoir rocks where TSR has occurred. Using atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance (FTICR) mass spectrometry, we find that TSR altered oils and condensates contain highly condensed polynuclear aromatic and naphthenoaromatic species with 0–3 S atoms. These chemical species are not present in petroleum fluids of equivalent maturity that have not experienced TSR and must have been formed by the TSR process. We call these species proto-solid bitumen as they represent the type of organic compounds that could easily precipitate from the TSR altered oils with slight chemical alteration or changes in reservoir conditions.     

Geochemical and isotopic approach to maturity/source/mixing estimations for natural gas and associated condensates in the Thrace Basin,NW Turkey

The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the 11 different gas fields along the NW–SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH₄, thermogenic C₂₊ gases, and associated condensates were evaluated separately.Methane appears to have 3 origins: Group-1 CH₄ is bacteriogenic (Calculated δ¹³C_C1–C = −61.48‰; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH₄. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (δ¹³C_C1–C = −35.80‰; Hamitabat Field) and Group-3 (δ¹³C_1–C = −49.10‰; Değirmenköy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C₂₊ gases. The Group-2 C₂₊ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (δ¹³C = −21‰) present in the Eocene deltaic Hamitabat shales. The Group-3 C₂₊ gases, produced from one field, were generated from isotopically light marine kerogen (δ¹³C = −29‰). Lower Oligoce ne Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases.The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C₂₊ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases.Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (δ¹³C = −21‰; −29‰) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (R_eq = 0.55–0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin.     

The impact of severe biodegradation on the molecular and stable (C,H, N,S) isotopic compositions of oils in the Alberta Basin,Canada

Although the effects of biodegradation on the composition and physical properties of crude oil have been well studied, effects of in-reservoir petroleum biodegradation on molecular and isotopic compositions of crude oils are not yet clearly understood. The Alberta Basin, in western Canada, is one of the world’s largest petroleum accumulations and constitutes an ideal example of a natural suite of sequentially biodegraded oils. The basin hosts moderately to severely biodegraded petroleum, regionally distributed and in single, more or less continuous, oil columns. In this study, a series of oil samples from the Alberta heavy oil and oil sands provinces, with varying degrees of biodegradation, were analyzed to assess the impact of progressive biodegradation on the molecular and C, H, N, and S isotopic compositions of oils. The results of the molecular characterization of the hydrocarbon fraction of the studied oils show that the oils have suffered biodegradation levels from 2 to 10⁺ (toward the Alberta–Saskatchewan border) on the Peters and Moldowan scale of biodegradation (abbreviated PM 2 to PM 10) and from tens to hundreds on the Manco scale. Within single reservoirs, increasing biodegradation was observed from top to bottom of the oil columns at all sites studied. The whole oil stable isotopic compositions of the samples varied in the ranges δ¹³C = −31.2‰ to −29.0‰, δ²H = −147‰ to −133‰, δ¹⁵N = 0.3–4.7‰ and δ³⁴S = 0.4–6.4‰. The maximum differences between δ values of samples (Δ) within single oil columns were Δ¹³C = 1.4‰, Δ²H = 7‰, Δ¹⁵N = 1.7‰ and Δ³⁴S = 1.0‰. Regional variations in the isotopic compositions of oil samples from different wells (averaged values from top to bottom) were 1.2‰ for δ¹³C, 12‰ for δ²H, 4.1‰ for δ¹⁵N and 5.5‰ for δ³⁴S and hence generally significantly larger variations were seen than variations observed within single oil columns, especially for N and S. It appears that even severe levels of biodegradation do not cause observable systematic variations in carbon, nitrogen or sulfur isotope composition of whole oils. This indicates that sulfur and nitrogen isotopic compositions may be used in very degraded oils as indicators for oil charge from different source rock facies.     

The impact of thermal maturity level on the composition of crude oils,assessed using ultra-high resolution mass spectrometry

We have examined, using a 12 Tesla FTICR-MS instrument, the impact of varying thermal maturity level on a suite of 9 related crude oils charged from source rocks covering most of the liquid petroleum generating portion of the oil window (0.68–1.11% vitrinite reflectance equivalent (%Re)). The sample suite was analyzed as whole oils under three different conditions, electrospray ionization (ESI) in positive and negative ion mode to analyze basic and acidic components, respectively, and atmospheric pressure photoionization (APPI) in positive ion mode, for sulfur and hydrocarbon species.Increasing oil maturity level had a strong influence on the composition of all compound classes in the oils with several major observations evident:The relative apparent abundances of all heteroatom containing compound classes detected in this study, using all ionization modes, decrease systematically with increasing oil maturation levels. Both aromatic hydrocarbons, detectable in APPI mode, and NSO compound classes (detectable in both ESI and APPI modes), as broad classes, are becoming more aromatic (shift to a greater predominance of higher DBE group members) and dealkylated (decreasing average molecular mass of individual compound groups), with increasing maturation level in the oil suite. Several putative oil maturity level dependent, molecular ratios were identified in the study. Of particular note, the relative abundance ratios of heteroatom compound classes tentatively identified as alkylated carbazoles, quinolines and benzothiophenes, compared to their benzannulated homologues are very sensitive to maturation level. Several groups of compounds show interesting and specific carbon number distributions, suggesting there may be hints of specific molecular markers in the FTICR-MS data. One observation of note is the strong increase in the relative abundance of protonated hydrocarbon components with DBE 5. We speculate this might reflect the presence of previously unreported higher molecular weight diamondoid (diamantane) species in oils with up to 40 carbon atoms or more, at advanced maturity levels. Such species may prove very valuable as molecular markers in highly mature fluids, such as those currently being produced from some shale reservoirs. Covariation of quantitative GC–MS data for alkylated hetero aromatic sulfur and nitrogen compounds in this oil suite, together with the corresponding FTICR-MS data from compounds believed to be, based on accurate mass, alkylated sulfur and alkylated nitrogen compounds, suggests that FTICR-MS already has some very rudimentary quantitation capabilities.     

Application of the monoterpane ratio (MTR) to distinguish marine oils from terrigenous oils and infer depositional environment in northern Tarim Basin,China

Six monoterpenoid hydrocarbons including two acyclic, two monocyclic and two aromatic components were identified and quantified in 49 oil and condensate samples using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC–TOFMS). Among them, 2,6-dimethyloctane (2,6-DMO) and 2-methyl-3-ethylheptane (2-M-3-EH) are the most abundant compounds and geochemically significant. Used in conjunction with other geochemical parameters such as pristane/phytane (Pr/Ph) and dibenzothiophene/phenanthrene (DBT/P) ratios, the 2-M-3-EH/2,6-DMO ratio (monoterpane ratio, MTR) provides a useful tool for distinguishing marine oils and condensates from those of terrigenous origin. Oils and condensates derived from marine source rocks have high MTR (> 0.4), low Pr/Ph and high DBT/P values, whereas those generated from terrigenous source rocks have lower MTR (< 0.3), higher Pr/Ph and lower DBT/P values. Differences in MTR values are mainly caused by variations in the concentrations of 2-M-3-EH as the concentrations of 2,6-DMO are in the same range in different sample groups. The 2-M-3-EH is enriched in marine oils and condensates from highly anoxic depositional environments, but depleted in terrigenous oils and condensates from oxic/sub-oxic depositional environments. The MTR may serve as a new parameter for assigning the source rock depositional environment. MTR values higher than 0.4 may indicate reducing conditions while MTR values lower than 0.3 may suggest an oxic/sub-oxic condition.     

Organic geochemistry of the Vindhyan sediments: Implications for hydrocarbons

The organic geochemical methods of hydrocarbon prospecting involve the characterization of sedimentary organic matter in terms of its abundance, source and thermal maturity, which are essential prerequisites for a hydrocarbon source rock. In the present study, evaluation of organic matter in the outcrop shale samples from the Semri and Kaimur Groups of Vindhyan basin was carried out using Rock Eval pyrolysis. Also, the adsorbed low molecular weight hydrocarbons, methane, ethane, propane and butane, were investigated in the near surface soils to infer the generation of hydrocarbons in the Vindhyan basin. The Total Organic Carbon (TOC) content in shales ranges between 0.04% and 1.43%. The S₁ (thermally liberated free hydrocarbons) values range between 0.01–0.09 mgHC/gRock (milligram hydrocarbon per gram of rock sample), whereas the S₂ (hydrocarbons from cracking of kerogen) show the values between 0.01 and 0.14 mgHC/gRock. Based on the T_max (temperature at highest yield of S₂) and the hydrogen index (HI) correlations, the organic matter is characterized by Type III kerogen. The adsorbed soil gas, CH₄ (C₁), C₂H₆ (C₂), C₃H₈ (C₃) and nC₄H₁₀, (nC4), concentrations measured in the soil samples from the eastern part of Vindhyan basin (Son Valley) vary from 0 to 186 ppb, 0 to 4 ppb, 0 to 5 ppb, and 0 to 1 ppb, respectively. The stable carbon isotope values for the desorbed methane (δ¹³C₁) and ethane (δ¹³C₂) range between −45.7‰ to −25.2‰ and −35.3‰ to −20.19‰ (VPDB), respectively suggesting a thermogenic source for these hydrocarbons. High concentrations of thermogenic hydrocarbons are characteristic of areas around Sagar, Narsinghpur, Katni and Satna in the Son Valley. The light hydrocarbon concentrations (C₁–C₄) in near surface soils of the western Vindhyan basin around Chambal Valley have been reported to vary between 1–2547 ppb, 1–558 ppb, 1–181 ppb, 1–37 ppb and 1–32 ppb, respectively with high concentrations around Baran-Jhalawar-Bhanpur-Garot regions (Kumar et al., 2006). The light gaseous hydrocarbon anomalies are coincident with the wrench faults (Kota – Dholpur, Ratlam – Shivpuri, Kannod – Damoh, Son Banspur – Rewa wrench) in the Vindhyan basin, which may provide conducive pathways for the migration of the hydrocarbons towards the near surface soils.     

The role of labile sulfur compounds in thermochemical 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 CaSO₄ in the presence of reduced sulfur (e.g. H₂S, 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 H₂S 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 H₂S 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 H₂S. Second, we considered the generation of olefines in association with the elimination of H₂S 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 H₂S accumulation in petroleum reservoirs.     

Experimental study of the effects of thermochemical sulfate reduction on low molecular weight hydrocarbons in confined systems and its geochemical implications

Experimental studies of the effects of thermochemical sulfate reduction (TSR) on light hydrocarbons were conducted in sealed gold tubes for 72 h at 400 °C and 50 MPa. A variety of pyrolysis experiments were carried out, including anhydrous, hydrous without MgSO₄ (hydrous experiments) and hydrous with MgSO₄ (TSR experiments). Common reservoir minerals including montmorillonite, illite, calcite and quartz were added to various experiments. Measurements of the quantities of n-C₉₊ normal alkanes (high molecular weight, HMW), n-C_6-8 normal alkanes (low molecular weight, LMW), C_7-8 isoalkanes, C_6-7 cycloalkanes and C_6-9 monoaromatics and compound specific carbon isotope analyses were made. The results indicate that TSR decreases hydrocarbon thermal stability significantly as indicated by chemically lower concentrations and isotopically heavier LMW saturated hydrocarbons in the TSR experiments compared to the hydrous and anhydrous experiments. In the LMW saturated hydrocarbon fraction, cycloalkanes tend to be more resistant to TSR than n-alkanes and isoalkanes. TSR promotes aromatization reactions and favors the generation of monoaromatics, resulting in higher chemical concentrations and isotopically equivalent compositions of monoaromatics in the anhydrous, hydrous and TSR experiments. This indicates that LMW monoaromatics are thermally stable during the pyrolysis experiments. Acid rather than basic catalyzed ionic reactions probably play a major role in TSR. This is suggested by the promotion effects of acid-clay minerals including illite and particularly montmorillonite. The basic mineral calcite retards the destruction of n-C₉₊ normal alkanes within the TSR experiments. Furthermore, clay minerals have a minor influence on the generation of LMW monoaromatics and play a negative role in regulating the concentrations of LMW saturated hydrocarbons; calcite does not favor the generation of LMW monoaromatics and plays a positive role in controlling the concentrations of LMW saturates relative to clay minerals. Quartz has a negligible role in the TSR experiments.Due to their differential responses to TSR, LMW hydrocarbon parameters, such as Schaefer [Schaefer, R.G., Littke, R., 1988. Maturity-related compositional changes in the low-molecular-weight hydrocarbon fraction of Toarcian Shale. Organic Geochemistry 13, 887-892], Thompson [Thompson, K.F.M., 1988. Gas-condensate migration and oil fractionation in deltaic systems. Marine and Petroleum Geology 5, 237-246], Halpern [Halpern, H., 1995. Development and application of light-hydrocarbon-based star diagrams. American Association of Petroleum Geologists Bulletin 79, 801-815] and Mango [Mango, F.D., 1997. The light hydrocarbons in petroleum: a critical review. Organic Geochemistry 26, 417-440] parameters and stable carbon isotopic compositions of individual LMW saturated hydrocarbons in TSR affected oils should be used with caution. In addition, water promotes thermal cracking of n-C₉₊ normal alkanes and favors the generation of LMW cycloalkanes and monoaromatics. The result is lower concentrations of n-C₉₊ HMW normal alkanes and higher concentrations of LMW cycloalkanes and monoaromatics in hydrous experiments relative to anhydrous experiments with or without minerals.This investigation provides a better understanding of the effects of TSR on LMW hydrocarbons and the influence of reservoir minerals on TSR in natural systems. The paper shows how LMW hydrocarbon indicators in TSR altered oils improve understanding of the processes of hydrocarbon generation, migration and secondary alteration in subsurface petroleum reservoirs.     

Enrichment of nitrogen and 13C of methane in natural gases from the Khuff Formation,Saudi Arabia,caused by thermochemical sulfate reduction

Permian Khuff reservoirs along the east coast of Saudi Arabia and in the Arabian Gulf produce dry sour gas with highly variable nitrogen concentrations. Rough correlations between N₂/CH₄, CO₂/CH₄ and H₂S/CH₄ suggest that non-hydrocarbon gas abundances are controlled by thermochemical sulfate reduction (TSR). In Khuff gases judged to be unaltered by TSR, methane δ¹³C generally falls between −40‰ and −35‰ VPDB and carbon dioxide δ¹³C between −3‰ and 0‰ VPDB. As H₂S/CH₄ increases, methane δ¹³C increases to as much as −3‰ and carbon dioxide δ¹³C decreases to as little as −28‰. These changes are interpreted to reflect the oxidation of methane to carbon dioxide.Khuff reservoir temperatures, which locally exceed 150 °C, appear high enough to drive the reduction of sulfate by methane. Anhydrite is abundant in the Khuff and fine grained nodules are commonly rimmed with secondary calcite cement. Some cores contain abundant pyrite, sphalerite and galena. Assuming that nitrogen is inert, loss of methane by TSR should increase N₂/CH₄ of the residual gas and leave δ¹⁵N unaltered. δ¹⁵N of Paleozoic gases in Saudi Arabia varies from −7‰ to 1‰ vs. air and supports the TSR hypothesis. N₂/CH₄ in gases from stacked Khuff reservoirs varies by a factor of 19 yet the variation in δ¹⁵N (0.3–0.5‰) is trivial.Because the relative abundance of hydrogen sulfide is not a fully reliable extent of reaction parameter, we have attempted to assess the extent of TSR using plots of methane δ¹³C versus log(N₂/CH₄). Observed variations in these parameters can be fitted using simple Rayleigh models with kinetic carbon isotope fractionation factors between 0.98 and 0.99. We calculate that TSR may have destroyed more than 90% of the original methane charge in the most extreme instance. The possibility that methane may be completely destroyed by TSR has important implications for deep gas exploration and the origin of gases rich in nitrogen as well as hydrogen sulfide.     

Sulfur cycling at the Mid-Atlantic Ridge: A multiple sulfur isotope approach

The role of sulfur in two hydrothermal vent systems, the Logatchev hydrothermal field at 14°45′N/44°58′W and several different vent sites along the southern Mid-Atlantic Ridge (SMAR) between 4°48′S and 9°33′S and between 12°22′W and 13°12′W, is examined by utilizing multiple sulfur isotope and sulfur concentration data. Isotope compositions for sulfide minerals and vent H₂S from different SMAR sites range from + 1.5 to + 8.9‰ in δ³⁴S and from + 0.001 to + 0.051‰ in Δ³³S. These data indicate mixing of mantle sulfur with sulfur from seawater sulfate. Combined δ³⁴S and Δ³³S systematics reveal that vent sulfide from SMAR is characterized by a sulfur contribution from seawater sulfate between 25 and 33%. This higher contribution, compared with EPR sulfide, indicates increased seawater sulfate reduction at MAR, because of a deeper seated magma chamber and longer fluid upflow path length, and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid-ocean ridges.Additionally, isotope data uncover non-equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 °C. δ³⁴S values between + 0.2 to + 8.8‰ for dissolved and precipitated sulfide from Logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at SMAR. δ³⁴S values between ? 24.5 and + 6.5‰ and Δ³³S values between + 0.001 and + 0.125‰ for sulfide-bearing sediments and mafic/ultramafic host rocks from drill cores taken in the region of Logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction. Basalts and basaltic glass from SMAR sites with Δ³³S = ? 0.008‰ reveal lower Δ³³S lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials.We conclude that the combined use of both δ³⁴S and Δ³³S provides a more detailed picture of the sulfur cycling in hydrothermal systems at the Mid-Atlantic Ridge and uncovers systematic differences to hydrothermal sites at different mid-ocean ridge sites. Multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow. Furthermore, based on Δ³³S we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when δ³⁴S were identical.     

硫酸盐热化学还原作用的启动机制研究

硫酸盐热化学还原作用(TSR)是导致高含硫化氢天然气生成和聚集、碳酸岩盐储层酸化和溶蚀的重要因素,是地质盆地内烃-水-岩三者之间的复杂反应。本文利用黄金管热模拟实验,对TSR反应的可能启动机制及控制因素进行了研究。通过不同盐溶液与原油的热解实验,证实了硫酸盐的存在是启动TSR反应的必要因素,MgSO4比CaSO4和Na2SO4更容易启动TSR反应,体系中盐度的增加会加速H2S的生成。实验结果表明,不同水介质条件下,TSR反应的程度与溶液的离子强度呈正相关,弱酸性环境并不足以启动TSR反应;原油中不稳定含硫化合物的含量越高越有利于TSR反应的发生,饱和链烷烃比原油中其它组分更容易引发TSR反应,且大分子烷烃比小分子烷烃更容易被硫酸盐氧化。     

硫酸盐热化学还原作用对原油裂解成气和碳酸盐岩储层改造的影响及作用机制

硫酸盐热化学还原作用(Thermochemical sulfate reduction, TSR)是发生在油气藏中复杂的有机-无机相互作用,它不仅会引起含H₂S天然气的富集,其产生的酸性气体对碳酸盐岩储层还具有明显的溶蚀改造作用。本文基于黄金管热模拟实验,研究了TSR反应对原油裂解气的生成的影响,发现这种氧化还原反应的存在能明显降低原油的稳定性,促进具高干燥系数的含H₂S天然气的生成。结合原位激光拉曼实验结果,证实了实际油藏中启动TSR反应的最可行的氧化剂应该是硫酸盐接触离子对(CIP)。全面探讨了影响TSR反应的地质和地球化学因素,提出除了初始原油的组分特征、不稳定含硫化合物(LSC)的含量外,地层水的含盐类型及盐度同样是控制TSR反应的关键因素。同时,基于大量地质分析,发现TSR对碳酸盐岩储层具有明显的溶蚀改造作用。结合溶蚀模拟实验,提出了酸性流体对碳酸盐储层溶蚀改造的机制,且深层碳酸盐岩层存在一个由TSR作用形成的次生孔隙发育带。研究认为,烃类与硫酸盐矿物的氧化还原反应与其产物对碳酸盐岩储层的改造是TSR作用的两个不可分割的部分,它们相互依存和制约。     

Geology,mineralization, and fluid inclusion characteristics of the Chorukh-Dairon W–Mo–Cu skarn deposit in the Middle Tien Shan,Northern Tajikistan

The Chorukh-Dairon deposit is part of the metallogenic belt of WMo, CuMo, AuW, and Au deposits along the Late Paleozoic active continental margin of the Tien Shan. It is related to the Late Carboniferous multiphase pluton, with successive intrusive phases of early monzogabbro through monzonite-quartz monzonite to monzogranite and leucogranite, and the latest lamprophyre dikes. The deposit is an example of complex W–Mo–Cu magmatic-hydrothermal system related to magnetite-series shoshonitic igneous suite. It contains zones of W–Cu–Mo oxidized prograde and retrograde skarns, with abundant scapolite, plagioclase, K-feldspar, andradite garnet, magnetite, as well as molybdoscheelite and minor chalcopyrite, and molybdenite. Skarns are overprinted by hydrosilicate alteration assemblages, with amphibole, chlorite, epidote, quartz, calcite, scapolite, albite, scheelite, and chalcopyrite, and are cut by quartz-carbonate-barite-fluorite-sulfide veins.The fluid evolution included a release of high temperature (~ 400–500 °C), high pressure (900–1100 to 700–800 bars), high salinity magmatic-hydrothermal aqueous chloride fluid, with its direct separation from crystallizing magma and formation of prograde and retrograde skarns. Fluid enrichment in Ca (up to 15–22 wt.% CaCl₂) at the retrograde skarn stage was possibly related to magmatic differentiation and provided intense molybdoscheelite deposition from a homogenous fluid. In contrast, hydrosilicate alteration assemblages were formed at lower temperatures (~ 350–400 °C) initially from a homogenous and then from a boiling Ca-rich (20–22 wt.% CaCl₂) magmatic-hydrothermal fluid, with the latter contributing to the most intense scheelite deposition. The stable isotope data (δ¹³C_CO2 = − 3.0 ± 0.5‰ and δ¹⁸О_H2O = + 6.5 ± 0.5‰, δ³⁴S = + 7.5 to + 7.7‰) obtained for the hydrosilicate stage minerals suggest significant fluid sourcing from magmatic and meteoric waters as well as from sedimentary rocks enriched in seawater sulfate, possibly evaporites, although a strongly homogenous character of the isotopic composition reveals intense isotope homogenization in a magmatic chamber. Some light sulfur isotope enrichment of sulfides from the quartz-carbonate-barite-fluorite-sulfide veins (δ³⁴S = + 6.0 to + 6.1‰) may be linked to the evolution of the magmatic source toward more mantle-related sulfur species, as these veins were formed after emplacement of the late mafic (lamprophyre) dikes.     

Pregnanes as molecular indicators for depositional environments of sediments and petroleum source rocks

Steroids with unconventional side chains have increasingly been applied as diagnostic markers for geological source and age assessments. However, one of the most distinctive characteristics, the abnormal abundance of pregnane and homopregnane in ancient sediments and petroleum, remains unresolved. Higher pregnane and homopregnane, as well as C₂₃–C₂₆ 20-n-alkylpregnanes, relative to the regular steranes were observed in samples collected from different petroleum basins in China. These included Precambrian marine carbonate-derived petroleum (NW Sichuan Basin), Lower Paleozoic marine marl derived crude oils (Tarim Basin), and Eocene hypersaline lacustrine carbonate source rocks and associated petroleum (Bohai Bay Basin). However, all of the samples have many common biomarker characteristics, such as pristane/phytane ratios < 1, low amounts of diasteranes and high C₂₉/C₃₀ hopane (∼0.6–1), C₃₅/C₃₄ hopane (mostly ⩾ 1) and dibenzothiophene/phenanthrene (DBT/PHEN, mostly 0.5–1) ratios revealing a contribution from anoxic carbonate/marl source rocks deposited in restricted, clastic-starved settings. We suggest that 5α,l4β,l7β-pregnane and homopregnane, as well as their higher C₂₃–C₂₆ homologues, are geological products derived from steroids bound to the kerogen by a sulfurized side chain. Carbon or carbonate minerals are considered to be natural catalysts for this cracking reaction via preferential cleavage of the bond between C-20 and C-22. Similar distributions occur in the short chain analogues of 4-methylsterane, triaromatic steroid and methyltriaromatic steroid hydrocarbons, providing circumstantial evidence for this proposal. The ratio of pregnane and homopregnane to the total regular steranes and the ratio of C₂₇ diasteranes to cholestanes can be sensitive indicators of sedimentary environments and facies. In general, high diasteranes and low pregnanes (with homologues) indicate an oxic water column or significant input of terrigenous organic matter in clay rich source rocks and some organic lean carbonate rocks. Low diasteranes with high pregnanes implies restricted, sulfur rich conditions, typical of anoxic carbonate source rocks. Furthermore, the two ratios may be useful to assess the variation of mineralogy and openness of source rock depositional settings.     

Genetic investigation and comparison of Kartaldağ and Madendağ epithermal gold deposits in Çanakkale,NW Turkey

Two epithermal gold deposits (Kartaldağ and Madendağ) located in NW Turkey have been characterized through the detailed examinations involving geologic, mineralogical, fluid inclusion, stable isotope, whole-rock geochemistry, and geochronology data.The Kartaldağ deposit (0.01–17.65 ppm Au), hosted by Eocene dacite porphyry, is associated with four main alteration types with characteristic assemblage of: i) chlorite/smectite–illite ± kaolinite, ii) quartz–kaolinite, iii) quartz–alunite–pyrophyllite, iv) quartz–pyrite, the last being characterized by three distinct quartz generations comprising massive/vuggy (early), fine–medium grained, vug-lining (early), and banded, colloform, comb (late) textures. Observed sulfide minerals are pyrite, covellite, and sphalerite. Oxygen and sulfur isotope analyses, performed on quartz (δ¹⁸O_(quartz): 7.93 to 8.95‰ and calculated δ¹⁸O_(H2O): − 7.95 to 1.49‰) and pyrite (δ³⁴S_(pyrite): − 4.8‰ and calculated δ³⁴S_(H2S): − 6.08 to − 7.20‰) separates, suggest a meteoric water source for water in the hydrothermal fluid, and an igneous source for the sulfur dissolved in ore-related fluids. Microthermometric analyses of primary fluid inclusion assemblages performed on quartz (late quartz generation) yield temperatures (Th) dominantly in the range of 245–285 °C, and generally low salinity values at 0 to 1.7 wt.% NaCl eq. Based on the quartz textures and the associated base metal concentrations, along with fluid inclusion petrography, the early vug-lining quartz is considered to have been associated with the mineralization possibly through a boiling and a late mixing process at > 285 °C.The Madendağ deposit (0.27–20.60 ppm Au), hosted by Paleozoic mica schists, is associated with two main alteration types: sericite–illite–kaolinite, and quartz–pyrite dominated by two distinct quartz generations i) early colloform, comb and banded quartz and ii) late quartz, forming the cement in hydrothermal breccia. Whereas oxygen isotope analyses of quartz (δ¹⁸O_(quartz): 9.55 to 18.19‰ and calculated δ¹⁸O_(H2O): − 2.97 to 5.54‰) suggest varying proportions of meteoric and magmatic sources for the ore bearing fluid, sulfur isotope ratios (δ³⁴S_(pyrite): − 2.2‰ and calculated δ³⁴S_(H2S): (− 3.63) to (− 3.75) ‰) point to an essentially magmatic source for sulfur with or without contribution from sedimentary sources. Microthermometric analysis carried out on primary fluid inclusion populations of a brecciated sample (early quartz), give a temperature (Th) range of 235–255 °C and 0.0 to 0.7 wt.% NaCl eq. salinity. Based on the textural relationship, base metal and high gold contents, the ore precipitation stage is associated with late stage quartz formation via a possible boiling process.The presence of alunite, pyrophyllite and kaolinite, vuggy quartz and covellite suggest a high-sulfidation type of epithermal deposit for Kartaldağ. On the other hand, Madendağ is identified as an adularia-sericite type owing to the presence of significant sericite, neutral pH clays (mostly illite, chlorite/smectite, and kaolinite), low temperature quartz textures (e.g., colloform, comb, and banded quartz), and limited sulfide minerals.Given the geographical proximity of Kartaldağ and Madendağ deposits, the similar temperature and salinity ranges obtained from their fluid inclusions, and the similar ages of igneous rocks in both deposits (Kartaldağ: 40.80 ± 0.36 to 42.19 ± 0.45 Ma, Madendağ: 43.34 ± 0.85 Ma) the mineralizing systems in both deposits are considered to be genetically related.     

Sulfur-poor intense acid hydrothermal alteration: A distinctive hydrothermal environment

A fundamentally distinct, sulfide-poor variant of intense acid (advanced argillic) alteration occurs at the highest structural levels in iron oxide-rich hydrothermal systems. Understanding the mineralogy, and geochemical conditions of formation in these sulfide-poor mineral assemblages have both genetic and environmental implications. New field observations and compilation of global occurrences of low-sulfur advanced argillic alteration demonstrates that in common with the sulfide-rich variants of advanced argillic alteration, sulfide-poor examples exhibit nearly complete removal of alkalis, leaving a residuum of aluminum-silicate + quartz. In contrast, the sulfur-poor variants lack the abundant pyrite ± other sulfides, hypogene alunite, Al-leached rocks (residual “vuggy” quartz) as well as the Au-Cu-Ag ± As-rich mineralization of some sulfur-rich occurrences. Associated mineralization is dominated by magnetite and/or hematite with accessory elements such as Cu, Au, REE, and P. These observations presented here indicate there must be distinct geologic processes that result in the formation of low-sulfur advanced argillic styles of alteration.Hydrolysis of magmatic SO₂ to sulfuric acid is the most commonly recognized mechanism for generating hypogene advanced argillic alteration, but is not requisite for its formation. Low sulfur iron-oxide copper-gold systems are known to contain abundant acid-styles of alteration (e.g. sericitic, chloritic), which locally reaches advanced argillic assemblages. A compilation of mapping in four districts in northern Chile and reconnaissance observations elsewhere show systematic zoning from near surface low-sulfide advanced argillic alteration through chlorite-sericite-albite and locally potassic alteration. The latter is commonly associated with specular hematite-chalcopyrite mineralization. Present at deeper structural levels are higher-temperature styles of sodic-calcic (oligoclase/scapolite – actinolite) alteration associated with magnetite ± chalcopyrite mineralization. These patterns are in contrast to the more sulfur-rich examples which generally zone to higher pyrite and locally alunite-bearing alteration.Fluid inclusion evidence from the systems in northern Chile shows that many fluids contain 25 to >50 wt% NaCl_eq with appreciable Ca, Fe, and K contents with trapping temperatures >300 °C. These geological and geochemical observations are consistent with the origin of the low-sulfur advanced argillic assemblages from HCl generated by precipitation of iron oxides from iron chloride complexes from a high-salinity fluid by reactions such as 3FeCl₂ + 4H₂O = Fe₃O₄ + 6HCl + H₂. Such HCl-rich (and relatively HSO₄⁼-poor) fluids can then account for the intense acid, Al-silicate-rich styles of alteration observed at high levels in some iron-oxide-coppe-gold (IOCG) systems. The geochemical differences between the presence of sulfide-rich and sulfur-poor examples of advanced argillic alteration are important to distinguishing between system types and the acid-producing capacity of the system, including in the modern weathering environment. They have fundamental implications for effective mineral exploration in low-sulfur systems and provide yet another vector of exposed alteration in the enigmatic IOCG clan of mineral deposits. Furthermore, understanding the geochemistry and mineralogy of this distinct geologic environment has applications to understanding the acid generating capacity and deleterious heavy metals associated with advanced argillic alteration.     

Using dual isotopic data to track the sources and behaviors of dissolved sulfate in the western North China Plain

This paper investigated the sources and behaviors of sulfate in groundwater of the western North China Plain using sulfur and oxygen isotopic ratios. The groundwaters can be categorized into karst groundwater (KGW), coal mine drainage (CMD) and pore water (subsurface saturated water in interstices of unconsolidated sediment). Pore water in alluvial plain sediments could be further classified into unconfined groundwater (UGW) with depth of less than 30 m and confined groundwater (CGW) with depth of more than 60 m. The isotopic compositions of KGW varied from 9.3‰ to 11.3‰ for δ³⁴S_SO4 with the median value of 10.3‰ (n = 4) and 7.9‰ to 15.6‰ for δ¹⁸O_SO4 with the median value of 14.3‰ (n = 4) respectively, indicating gypsum dissolution in karst aquifers. δ³⁴S_SO4 and δ¹⁸O_SO4 values of sulfate in CMD ranged from 10.8‰ to 12.4‰ and 4.8‰ to 8.7‰ respectively. On the basis of groundwater flow path and geomorphological setting, the pore water samples were divided as three groups: (1) alluvial–proluvial fan (II₁) group with high sulfate concentration (median values of 2.37 mM and 1.95 mM for UGW and CGW, respectively) and positive δ³⁴S_SO4 and δ¹⁸O_SO4 values (median values of 8.8‰ and 6.9‰ for UGW, 12.0‰ and 8.0‰ for CGW); (2) proluvial slope (II₂) group with low sulfate concentration (median values of 1.56 mM and 0.84 mM for UGW and CGW, respectively) and similar δ³⁴S_SO4 and δ¹⁸O_SO4 values (median values of 9.0‰ and 7.4‰ for UGW, 10.2‰ and 7.7‰ for CGW); and (3) low-lying zone (II₃) group with moderate sulfate concentration (median values of 2.13 mM and 1.17 mM for UGW and CGW, respectively) and more positive δ³⁴S_SO4 and δ¹⁸O_SO4 values (median values of 10.7‰ and 7.7‰ for UGW, 20.1‰ and 8.8‰ for CGW). In the present study, three major sources of sulfate could be differentiated as following: sulfate dissolved from Ordovician to Permian rocks (δ³⁴S_SO4 = 10–35‰ and δ¹⁸O_SO4 = 7–20‰), soil sulfate (δ³⁴S_SO4 = 5.9‰ and δ¹⁸O_SO4 = 5.8‰) and sewage water (δ³⁴S_SO4 = 10.0‰ and δ¹⁸O_SO4 = 7.6‰). Kinetic fractionations of sulfur and oxygen isotopes as a result of bacterial sulfate reduction (BSR) were found to be evident in the confined aquifer in stagnant zone (II₃), and enrichment factors of sulfate–sulfur and sulfate–oxygen isotopes calculated by Rayleigh equation were −12.1‰ and −4.7‰ respectively along the flow direction of groundwater at depths of 60–100 m. The results obtained in this study confirm that detailed hydrogeological settings and identification of anthropogenic sources are critical for elucidating evolution of δ³⁴S_SO4 and δ¹⁸O_SO4 values along with groundwater flow path, and this work also provides a useful framework for understanding sulfur cycling in alluvial plain aquifers.     

Mineral chemistry of hydrothermal biotite from the Kahang porphyry copper deposit (NE Isfahan), Central Province of Iran

The Kahang porphyry Cu deposit, located northeast of Isfahan city in central of Iran, is associated with a composite Miocene stock and ranges in composition from diorite through granodiorite to quartz-monzonite. Field observations and petrographic studies show that the emplacement of the Kahang stock occurred in several pulses, each associated with its related hydrothermal activity. Early hydrothermal alteration started with a potassic style in the central part of the system and produced a secondary biotite–K-feldspar–magnetite assemblage accompanied by chalcopyrite and pyrite mineralization. Propylitic alteration that took place at the same time as the potassic alteration occurred in the peripheral portions of the stock. Subsequent phyllic alteration overprinted earlier potassic and propylitic alterations. Biotite grains from the potassic and phyllic zones show distinct chemical compositions. The FeO, TiO₂, MnO, K₂O, and Na₂O concentrations in biotite from the phyllic alteration zone are lower than those from the potassic alteration zone. The F and Cl contents of biotite from the potassic alteration zone display relatively high positive correlation with the X_Mg. The fluorine intercept values [IV(F)] from the potassic and phyllic alteration zones are strongly correlated with the fluorine/chlorine intercept values [IV(F/Cl)]. Biotite geothermometry for the potassic and phyllic alteration zones, based on the biotite geothermometer of Beane (1974), yields a temperature range of 422° to 437 °C (mean = 430 °C) and 329° to 336 °C (mean = 333 °C), respectively. The position of data in log (X_F/X_OH) ratio vs. X_Mg and X_Fe diagram suggests that biotite formed under dissimilar composition and temperature conditions in the potassic and phyllic alteration zones. Calculated log fugacity ratios of (fH₂O/fHF), (fH₂O/fHCl), and (fHF/fHCl) show that hydrothermal fluids associated with the potassic alteration were distinctively different from those fluids associated with the phyllic alteration zone at Kahang porphyry Cu deposit. The results of this research indicate that the chemistry of biotite is related to the chemical composition of the magma and the prevailing physical conditions during crystallization.