Application of sulfur and carbon isotopes to oil–source rock correlation: A case study from the Tazhong area,Tarim Basin,China

Up until now, it has been assumed that oil in the Palaeozoic reservoirs of the Tazhong Uplift was derived from Upper Ordovician source rocks. Oils recently produced from the Middle and Lower Cambrian in wells ZS1 and ZS5 provide clues concerning the source rocks of the oils in the Tazhong Uplift, Tarim Basin, China. For this study, molecular composition, bulk and individual n-alkane δ¹³C and individual alkyl-dibenzothiophene δ³⁴S values were determined for the potential source rocks and for oils from Cambrian and Ordovician reservoirs to determine the sources of the oils and to address whether δ¹³C and δ³⁴S values can be used effectively for oil–source rock correlation purposes. The ZS1 and ZS5 Cambrian oils, and six other oils from Ordovician reservoirs, were not significantly altered by TSR. The ZS1 oils and most of the other oils, have a “V” shape in the distribution of C₂₇–C₂₉ steranes, bulk and individual n-alkane δ¹³C values predominantly between −31‰ to −35‰ VPDB, and bulk and individual alkyldibenzothiophene δ³⁴S values between 15‰ to 23‰ VCDT. These characteristics are similar to those for some Cambrian source rocks with kerogen δ¹³C values between −34.1‰ and −35.3‰ and δ³⁴S values between 10.4‰ and 21.6‰. The oil produced from the Lower Ordovician in well YM2 has similar features to the ZS1 Cambrian oils. These new lines of evidence indicate that most of the oils in the Tazhong Uplift, contrary to previous interpretations, were probably derived from the Cambrian source rocks, and not from the Upper Ordovician. Conversely, the δ¹³C and δ³⁴S values of ZS1C Cambrian oils have been shown to shift to more positive values due to thermochemical sulfate reduction (TSR). Thus, δ¹³C and δ³⁴S values can be used as effective tools to demonstrate oil–source rock correlation, but only because there has been little or no TSR in this part of the section.     

Geochemistry and origins of natural gases in the central Junggar Basin,northwest China

The petroliferous central Junggar Basin in northwest China is predominantly an oil exploration region. However, its gas exploration also might have good prospects. Thus to assist in gas exploration, the geochemistry and origins of gases are discussed in this paper based on relatively comprehensive analyses of compositions, carbon isotopes and light hydrocarbons of gases. Based on the results, the gas genetic types are grouped into families and combined with the geological setting (e.g., biomarkers of retrograde condensates and source rock characteristics). We show that there are four representative genetic types of gases. The first consists of gases derived from Permian lacustrine mudstones with type I–II kerogen and type III kerogen sources in the Penyijingxi sag. Their representative geochemical feature is δ¹³C₂ ranging from −31.4‰ to −24.7‰. The second is gas sourced from Carboniferous tufaceous mudstones of type III kerogen in the Dishuiquan sag, whose representative geochemical feature is the heaviest values of δ¹³C₁ in the studied samples, ranging from −32.0‰ to −30.4‰. The third consists of gases sourced from Jurassic coals and mudstones in the Shawan–Fukang sag. The light hydrocarbon fingerprints of these gases are similar to those of gases and oils typically derived from Jurassic source rocks in the southern Junggar Basin. The fourth is gas most likely generated from the degradation of crude oil. It is mainly found in the Luliang area and has dryness values as much as 0.999 and δ¹³C₁ ranging from −54.8‰ to −43.2‰. Among these four types of gases, the first (mainly sourced from the Permian lacustrine mudstones in the Penyijingxi sag) is the predominant type.     

Evaluation of the petroleum composition and quality with increasing thermal maturity as simulated by hydrous pyrolysis: A case study using a Brazilian source rock with Type I kerogen

Hydrous pyrolysis (HP) experiments were used to investigate the petroleum composition and quality of petroleum generated from a Brazilian lacustrine source rock containing Type I kerogen with increasing thermal maturity. The tested sample was of Aptian age from the Araripe Basin (NE-Brazil). The temperatures (280–360 °C) and times (12–132 h) employed in the experiments simulated petroleum generation and expulsion (i.e., oil window) prior to secondary gas generation from the cracking of oil. Results show that similar to other oil prone source rocks, kerogen initially decomposes in part to a polar rich bitumen, which decomposes in part to hydrocarbon rich oil. These two overall reactions overlap with one another and have been recognized in oil shale retorting and natural petroleum generation. During bitumen decomposition to oil, some of the bitumen is converted to pyrobitumen, which results in an increase in the apparent kerogen (i.e., insoluble carbon) content with increasing maturation.The petroleum composition and its quality (i.e., API gravity, gas/oil ratio, C₁₅₊ fractions, alkane distribution, and sulfur content) are affected by thermal maturation within the oil window. API gravity, C₁₅₊ fractions and gas/oil ratios generated by HP are similar to those of natural petroleum considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. API gravity of the HP expelled oils shows a complex relationship with increasing thermal maturation that is most influenced by the expulsion of asphaltenes. C₁₅₊ fractions (i.e., saturates, aromatics, resins and asphaltenes) show that expelled oils and bitumen are compositionally separate organic phases with no overlap in composition. Gas/oil ratios (GOR) initially decrease from 508–131 m³/m³ during bitumen generation and remain essentially constant (81–84 m³/m³) to the end of oil generation. This constancy in GOR is different from the continuous increase through the oil window observed in anhydrous pyrolysis experiments. Alkane distributions of the HP expelled oils are similar to those of natural crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. Isoprenoid and n-alkane ratios (i.e., pristane/n-C₁₇ and phytane/n-C₁₈) decrease with increasing thermal maturity as observed in natural crude oils. Pristane/phytane ratios remain constant with increasing thermal maturity through the oil window, with ratios being slightly higher in the expelled oils relative to those in the bitumen. Generated hydrocarbon gases are similar to natural gases associated with crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous, with the exception of elevated ethane contents. The general overall agreement in composition of natural and hydrous pyrolysis petroleum of lacustrine source rocks observed in this study supports the utility of HP to better characterize petroleum systems and the effects of maturation and expulsion on petroleum composition and quality.     

塔里木盆地西部阿克莫木气田形成初探

塔里木盆地西部阿克莫木气田天然气为非烃组份含量较高的干气,干燥系数高达99.7%;天然气δ¹³C₁和δ¹³C₂值明显偏重,δ¹³C₁为- 25.2‰～-21.9,δ¹³C₂为-21.2～-20.2‰,如果按传统的观点该天然气应为过成熟煤成气。但是综合气源对比研究表明阿克莫木气田天然气主要源自石炭系Ⅱ型烃源岩,成藏过程研究表明该气田主要聚集了石炭系烃源岩在Ro为1.5%～1.8%之后生成的天然气,具有晚期阶段聚气的特征,这是造成阿克1井天然气组份很“干”、碳同位素很重的主要原因。     

Different source of n-alkanes and n-alkan-2-ones in a 6000 cal. yr BP Sphagnum-rich temperate peat bog (Roñanzas,N Spain)

The most widely accepted origin of n-alkan-2-ones in peats is the microbial oxidation of the related n-alkanes and/or oxidative decarboxylation of fatty acids derived from plant input. The distributions of n-alkanes and n-alkan-2-ones in 48 samples from the Roñanzas 6000 cal. yr BP peat bog profile (N Spain) do not justify a single source. The n-alkan-2-ones typically dominate the n-alkanes, maximizing at C₁₉ or C₂₅/C_27, whereas the n-alkanes maximized either at C₂₃ or at C₃₁/C₃₃. The averaged δ¹³C values of the n-alkanes ranged from −32.3‰ to −33.1‰, but those of the n-alkan-2-ones were consistently higher (−29.2‰ to −29.9‰), suggesting a different, probably bacterial, source for the ketones.     

The source and paleoclimatic implication of hydrogen isotopic composition of n-alkanes in sediments from the Yixian Formation,western Liaoning Province,NE China

Hydrogen isotopic composition of n-alkanes was measured in sediments from an excavated profile of the Early Cretaceous Yixian Formation in Liaoning Province, NE China, aiming to assess the significance of the δD value of n-alkanes in ancient lacustrine sediments as the indicator for determining the source inputs of organic matters and paleoclimatic conditions. The δD values of n-alkanes are in the range of − 250‰ to − 85‰ and display an obvious three-stage variation pattern through the profile, which is consistent with the distribution of the dominated n-alkanes and the profile of their δ¹³C values. The δD and δ¹³C values of n-alkanes suggest that short-chain n-alkanes are primarily derived from photosynthetic bacteria and algae; n-C₂₉ and n-C₃₁ are mainly originated from terrestrial higher plants; n-C₂₈ and n-C₃₀ may be derived from the same precursor but via the different biological mechanism of hydrogen isotopic fractionation; while the source inputs of medium-chain n-alkanes are more complicated, with n-C₂₃ being derived from some specific algae or biosynthesized by various aquatic organisms. The paleoclimatic conditions are reconstructed via two approaches. The reconstructed hydrogen isotopic values of lake water and meteoric water (expressed as δD_LW and δD_MW, respectively) were at the intervals of − 51.8‰ to 17.0‰ and − 118.1‰ to − 43.5‰, respectively, indicating a general climate transition from semi-arid to arid. The calculated ΔδD_LW-MW values vary from 37.0‰ to 89.1‰ and display a similar but a significant large-scale variation trend with the ΔδD_{C23 ‐ long} (− 28.8‰ to 85.0‰; long represents long-chain n-alkanes) and ΔδD_mid-long (− 15.4‰ to 43.4‰; mid represents medium-chain n-alkanes) values. The discrepancy may be attributed to the source input overlap for n-alkanes and the uncertainties of ε_water/lipid values. The coupling of ΔδD_{C23 ‐ long}, ΔδD_mid-long and ΔδD_LW-MW values with the paleoclimatic evidence indicates that the δD values of n-alkanes could be more sensitive to the change of paleoclimatic conditions.     

Stable carbon isotopes of alkane gases from the Xujiahe coal measures and implication for gas-source correlation in the Sichuan Basin,SW China

The Upper Triassic Xujiahe Formation in the Sichuan Basin, SW China consists of a series of coal measures. The first, third and fifth members of this formation are dominated by gas prone dark mudstones and coals. The mudstones contain Type II and III kerogens with average organic carbon contents around 1.96%. These source rocks are mature in the central Sichuan and highly mature in the western Sichuan Basin, characterized by gas generation with subordinate amounts of light oil or condensate oils. The source rocks are intercalated with the sandstone dominated second, fourth and sixth members of the Xujiahe Formation, thus leading to three separate self contained petroleum systems in the region. The proven gas reserves in the Xujiahe Formation are only less than that of the Triassic Feixianguan Formation and the Xujiahe Formation has the second largest gas field (Guang’an gas field) in the basin. Gases derived from the Xujiahe Formation coals generally show a normal stable carbon isotopic trend for C₁–C₄ n-alkanes, with the highest δ¹³C₂ values among the nine gas pay zones in the basin (?20.7‰ to ?28.3‰), and δ¹³C₁ values as low as ?43.0‰ in the central Sichuan. Gas accumulations with an oil leg have also been found in the eastern and southern Sichuan where the thickness of the Xujiahe Formation is significantly reduced. Gases in these accumulations tend to show low δ¹³C₂ values (?30.0‰ to ?36.3‰), characteristic of oil prone source rocks.     

Geochemical comparison between gas in fluid inclusions and gas produced from the Upper Triassic Xujiahe Formation,Sichuan Basin,SW China

Natural gas in the Xujiahe Formation of the Sichuan Basin is dominated by hydrocarbon (HC) gas, with 78–79% methane and 2–19% C₂₊ HC. Its dryness coefficient (C₁/C_1–5) is mostly < 0.95. The gas in fluid inclusions, which has low contents of CH₄ and heavy hydrocarbons (C₂₊) and higher contents of non-hydrocarbons (e.g. CO₂), is a typical wet gas produced by thermal degradation of kerogen. Gas produced from the Upper Triassic Xujiahe Formation (here denoted field gas) has light carbon isotope values for methane (δ¹³C₁: −45‰ to −36‰) and heavier values for ethane (δ¹³C₂: −30‰ to −25‰). The case is similar for gas in fluid inclusions, but δ¹³C₁ = −36‰ to −45‰ and δ¹³C₂ = −24.8‰ to −28.1‰, suggesting that the gas experienced weak isotopic fractionation due to migration and water washing. The field gas has δ¹³C_CO2 values of −15.6‰ to −5.6‰, while the gas in fluid inclusions has δ¹³C_CO2 values of −16.6‰ to −9‰, indicating its organic origin. Geochemical comparison shows that CO₂ captured in fluid inclusions mainly originated from source rock organic matter, with little contribution from abiogenic CO₂. Fluid inclusions originate in a relatively closed system without fluid exchange with the outside following the gas capture process, so that there is no isotopic fractionation. They thus present the original state of gas generated from the source rocks. These research results can provide a theoretical basis for gas generation, evolution, migration and accumulation in the basin.     

Kinetic Model of Gaseous Alkanes Formed from Coal in a Confined System and Its Application to Gas Filling History in Kuqa Depression, Tarim Basin, Northwest China

Based on the pyrolysis products for the Jurassic low-mature coal under programmed temperature,and chemical and carbon isotopic compositions of natural gas from the Kuqa Depression, the genetic origin of natural gas was determined,and then a gas filling model was established,in combination with the geological background of the Kuqa Depression.The active energy of CH_4,C_2H_6 and C_3H_8 was gotten after the data of pyrolysis gas products under different heating rates(2℃/h and 20℃/h)were fitted by the Gas O...     

Carbon isotope composition of long chain leaf wax n-alkanes in lake sediments: A dual indicator of paleoenvironment in the Qinghai-Tibet Plateau

The carbon isotope composition (δ¹³C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C₃ and C₄ photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting δ¹³C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C₂₇ and C₂₉) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. −26.7‰ to −15.7‰ for C₂₉) similar to that of terrestrial C₄ plants. Thus, the use of δ¹³C values of sedimentary C₂₇ and C₂₉ n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C₄ plants. For sedimentary C₃₁, however, the contribution from submerged plants was minor, so that the δ¹³C values for C₃₁ n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the δ¹³C values of sedimentary C₂₇ and C₂₉ n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C₄ vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the δ¹³C values of sedimentary long chain n-alkanes (C₂₇, C₂₉ and C₃₁) may carry different environmental signals. While the δ¹³C values of C₃₁ were a reliable proxy for C₄/C₃ terrestrial vegetation composition, the δ¹³C values of C₂₇ and C₂₉ n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth.     

Origin and Accumulation of Natural Gases in the Upper Paleozoic Strata of the Ordos Basin in Central China

The natural gases in the Upper Paleozoic strata of the Ordos basin are characterized by relatively heavy C isotope of gaseous alkanes with δ 13C1 and δ13C2 values ranging mainly from ?35‰ to ?30‰ and ?27‰ to ?22‰, respectively, high δ13C excursions (round 10) between ethane and methane and predominant methane in hydrocarbon gases with most C1/(C1-C5) ratios in excess of 0.95, suggesting an origin of coal-derived gas. The gases exhibit different carbon isotopic profiles for C1-C4 alkanes with those of the natural gases found in the Lower Paleozoic of this basin, and believed to be originated from Carboniferous-Permian coal measures. The occurrence of regionally pervasive gas accumulation is distinct in the gently southward-dipping Shanbei slope of the central basin. It is noted that molecular and isotopic composition changes of the gases in various gas reservoirs are associated with the thermal maturities of gas source rocks. The abundances and δ13C values of methane generally decline northwards and from the basin center to its margins, and the effects of hydrocarbon migration on compositional modification seem insignificant. However, C isotopes of autogenetic calcites in the vertical and lateral section of reservoirs show a regular variation, and are as a whole depleted upwards and towards basin margins. Combination with gas maturity gradient, the analysis could be considered to be a useful tool for gas migration.     

The carbon isotopic composition of catalytic gas: a comparative analysis with natural gas

The idea that natural gas is the thermal product of organic decomposition has persisted for over half a century. Crude oil is thought to be an important source of gas, cracking to wet gas above 150°C, and dry gas above 200°C. But there is little evidence to support this view. For example, crude oil is proving to be more stable than previously thought and projected to remain intact over geologic time at typical reservoir temperatures. Moreover, when oil does crack, the products do not resemble natural gas. Oil to gas could be catalytic, however, promoted by the transition metals in carbonaceous sediments. This would explain the low temperatures at which natural gas forms, and the high amounts of methane. This idea gained support recently when the natural progression of oil to dry gas was duplicated in the laboratory catalytically. We report here the isotopic composition of catalytic gas generated from crude oil and pure hydrocarbons between 150 and 200°C. δ¹³C for C₁ through C₅ was linear with 1/n (n = carbon number) in accordance with theory and typically seen in natural gases. Over extended reaction, isobutane and isopentane remained lighter than their respective normal isomers and the isotopic differentials were constant as all isomers became heavier over time. Catalytic methane, initially −51.87‰ (oil = −22.5‰), progressed to a final composition of −26.94‰, similar to the maturity trend seen in natural gases: −50‰ to −20‰. Catalytic gas is thus identical to natural gas in molecular and isotopic composition adding further support to the view that catalysis by transition metals may be a significant source of natural gas.     

A molecular and carbon isotope biogeochemical study of biomarkers and kerogen pyrolysates of the Kimmeridge Clay Facies: palaeoenvironmental implications

Structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite, a bituminous shale and an oil shale of the Kimmeridge Clay Formation (KCF) in Dorset were studied in order to gain insight into (i) the type and extent of water column anoxia and (ii) changes in the concentration and isotopic composition of dissolved inorganic carbon (DIC) in the palaeowater column. The samples studied fit into the curve of increasing δ¹³C of the kerogen (δ¹³C_TOC) with increasing TOC, reported by Huc et al. (1992). Their hypothesis, that the positive correlation between TOC and δ¹³C_TOC is the result of differing degrees of organic matter (OM) mineralisation in the water column, was tested by measuring the δ¹³C values of primary production markers. These δ¹³C values were found to differ on average by only 1‰ among the samples, implying that differences in the extent of OM mineralisation cannot fully account for the 3‰ difference in δ¹³C_TOC. The extractable OM in the oil shale differs from that in the other sediments due to both differences in maturity, and differences in the planktonic community. These differences, however, are not likely to have significantly influenced δ¹³C_TOC either. All three sediments contain abundant derivatives of isorenieratene, indicating that periodically euxinia was extending into the photic zone. The sediments are rich in organic sulfur, as revealed by the abundant sulfur compounds in the pyrolysates. The prominence of C₁-C₃ alkylated thiophenes over n-alkanes and n-alkenes is most pronounced in the pyrolysate of the sediment richest in TOC. This suggests that sulfurisation of OM may have played an important role in determining the TOC-δ¹³C_TOC relationship reported by Huc et al. (1992).     

Kerogen based characterization of major gas shales: Effects of kerogen fractionation

Research into the origin and the mode of entrapment and expulsion of natural gas from unconventional plays requires the isolation and separation of kerogen in its purest and most intact form from the rock matrix. This study expands on the comparative analysis of the effects that isolation methods, conservative closed system versus conventional open system, have on kerogen’s elemental, isotopic and physical properties. Four major gas shales, including the Barnett, the Marcellus, the Haynesville and a Polish gas shale, were chosen. In addition, the Monterey shale, though not strictly a gas shale, was included to address the effects on sulfur rich, Type II-S kerogen.Results indicate that the kerogen residues from the conventional open system method showed lower recovery and higher mineral content than those from the conservative closed system method. Differences were manifested in the elemental analysis data, where kerogens isolated using the open system method showed a significant deficit in the organic C, H, O, S and N material balance. Furthermore, the recovered residues show different sulfur content and δ³⁴S composition, most likely attributable to differences in pyrite content. Nevertheless, the relative abundances of the various macerals in the kerogen residues from the same parent shale are not very different; neither was the bulk δ¹³C composition of the recovered residues. This is not particularly surprising, considering that in all the five cases examined in this study, the organic matter was fairly homogeneous.     

塔里木盆地喀什凹陷克拉托天然气来源分析及聚气特征

塔里木西南喀什凹陷的克拉托天然气主要表现为原油的溶解气或者湿气,甲烷含量为74.59%~85.58%,克4井和克30井天然气则为较干的湿气。克拉托天然气的δ¹³C₁值为-41.2‰~-40.6‰,δ¹³C₂值为-30.0‰~-27.4‰。气源对比表明克拉托天然气主要源自具有混源母质特征的中侏罗统湖相烃源岩,不同于源自石炭系烃源岩的阿克莫木天然气。喀什凹陷的中-下侏罗统烃源岩主要是由于新近系的巨厚沉积才从未成熟—低成熟阶段进入成熟—高成熟阶段,生成的油气在克拉托背斜圈闭中聚集,虽也属晚期成藏,却具有连续聚气的特征。上新世末期,喀什凹陷的周缘开始抬升,早期油气藏受到破坏,形成了现今的地表油气苗或油砂。     

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.     

Comparison of natural gases accumulated in Oligocene strata with hydrous pyrolysis gases from Menilite Shales of the Polish Outer Carpathians

This study examined the molecular and isotopic compositions of gases generated from different kerogen types (i.e., Types I/II, II, IIS and III) in Menilite Shales by sequential hydrous pyrolysis experiments. The experiments were designed to simulate gas generation from source rocks at pre-oil-cracking thermal maturities. Initially, rock samples were heated in the presence of liquid water at 330 °C for 72 h to simulate early gas generation dominated by the overall reaction of kerogen decomposition to bitumen. Generated gas and oil were quantitatively collected at the completion of the experiments and the reactor with its rock and water was resealed and heated at 355 °C for 72 h. This condition simulates late petroleum generation in which the dominant overall reaction is bitumen decomposition to oil. This final heating equates to a cumulative thermal maturity of 1.6% R_r, which represents pre-oil-cracking conditions. In addition to the generated gases from these two experiments being characterized individually, they are also summed to characterize a cumulative gas product. These results are compared with natural gases produced from sandstone reservoirs within or directly overlying the Menilite Shales. The experimentally generated gases show no molecular compositions that are distinct for the different kerogen types, but on a total organic carbon (TOC) basis, oil prone kerogens (i.e., Types I/II, II and IIS) generate more hydrocarbon gas than gas prone Type III kerogen. Although the proportionality of methane to ethane in the experimental gases is lower than that observed in the natural gases, the proportionality of ethane to propane and i-butane to n-butane are similar to those observed for the natural gases. δ¹³C values of the experimentally generated methane, ethane and propane show distinctions among the kerogen types. This distinction is related to the δ¹³C of the original kerogen, with ¹³C enriched kerogen generating more ¹³C enriched hydrocarbon gases than kerogen less enriched in ¹³C. The typically assumed linear trend for δ¹³C of methane, ethane and propane versus their reciprocal carbon number for a single sourced natural gas is not observed in the experimental gases. Instead, the so-called “dogleg” trend, exemplified by relatively ¹³C depleted methane and enriched propane as compared to ethane, is observed for all the kerogen types and at both experimental conditions. Three of the natural gases from the same thrust unit had similar “dogleg” trends indicative of Menilite source rocks with Type III kerogen. These natural gases also contained varying amounts of a microbial gas component that was approximated using the Δδ¹³C for methane and propane determined from the experiments. These approximations gave microbial methane components that ranged from 13–84%. The high input of microbial gas was reflected in the higher gas:oil ratios for Outer Carpathian production (115–1568 Nm³/t) compared with those determined from the experiments (65–302 Nm³/t). Two natural gas samples in the far western part of the study area had more linear trends that suggest a different organic facies of the Menilite Shales or a completely different source. This situation emphasizes the importance of conducting hydrous pyrolysis on samples representing the complete stratigraphic and lateral extent of potential source rocks in determining specific genetic gas correlations.     

Distinguishing Cambrian from Upper Ordovician source rocks: Evidence from sulfur isotopes and biomarkers in the Tarim Basin

The reported source rocks for the abundant petroleum in the Tarim Basin, China range from Cambrian to Lower Ordovician and/or Upper Ordovician in age. However, the difference between the two groups of source rocks is not well characterized. In this study, pyrite was removed from eleven mature to over mature kerogen samples from source rocks using the method of CrCl₂ reduction and grinding. The kerogen and coexisting pyrite samples were then analyzed for δ³⁴S values. Results show that the kerogen samples from the Cambrian have δ³⁴S values between +10.4‰ and +19.4‰. The values are significantly higher than those from the Lower Ordovician kerogen (δ³⁴S of between +6.7‰ and +8.7‰), which in turn are generally higher than from the Upper Ordovician kerogen samples (δ³⁴S of between ?15.3 and +6.8‰). The associated pyrite shows a similar trend but with much lower δ³⁴S values. This stratigraphically controlled sulfur isotope variation parallels the evolving contemporary marine sulfate and dated oil δ³⁴S values from other basins, suggesting that seawater sulfate and source rock age have an important influence on kerogen and pyrite δ³⁴S values. The relatively high δ³⁴S values in the Cambrian to Lower Ordovician source rocks are associated with abundant aryl isoprenoids, gammacerane and C₃₅ homohopanes in the extractable organic matter, indicating that these source rocks were deposited in a bottom water euxinic environment with water stratification. Compared with the Upper Ordovician, the Cambrian to Lower Ordovician source rocks show abundance in C₂₈ 20R sterane, C₂₃ tricyclic terpanes, 4,23,24-trimethyl triaromatic dinosteroids and depletion in C₂₄ tetracyclic terpane, C₂₉ hopane. Thus, δ³⁴S values and biomarkers of source rock organic matter can be used for distinguishing the Cambrian and Upper Ordovician source rocks in the Tarim Basin.     

Salinity variations in the northern Coorong Lagoon,South Australia: Significant changes in the ecosystem following human alteration to the natural water regime

European settlement and drought have significantly impacted the hydrology of the Coorong, a shallow coastal lagoon complex in South Australia, which is part of a terminal wetland at the mouth of the River Murray. An increased salinity associated with lower water levels and progressive isolation from ocean flushes contributed to a severe decline in ecological diversity over the past decades. Here we have conducted a molecular and stable isotopic study of a sedimentary core from the northern Coorong Lagoon spanning more than 5000 years to investigate the recent palaeoenvironmental history of the ecosystem. Major alterations were evident in many biogeochemical parameters in sediments deposited after the 1950s coinciding with the beginning of intensified water regulations. The most prominent shift occurred in δ¹³C profiles of C₂₁–C₃₃ n-alkanes from average values of −23.5‰ to an average of −28.2‰. Further changes included decreases in carbon preference index (CPI) and average chain length (ACL) of the n-alkane series as well as significant increases in algal (e.g. C₂₀ HBI, long chain alkenes and C₂₉-alkadiene) and bacterial (e.g. ¹³C depleted short chain n-alkanes and hopanoids, δ¹³C: −35.9‰ to −30.1‰) derived hydrocarbons. Long chain n-alkanes with a strong odd/even predominance as observed here are typically attributed to terrigenous plants. In the Coorong however, terrigenous input to sedimentary OM is only minor. Therefore changes in the before mentioned parameters were attributed to a source transition from a major contribution of macrophytes towards predominantly microalgae and bacteria.δD values of C₂₁–C₃₃ n-alkanes showed a general trend towards more enriched values in younger sediments, indicating an overall rising salinity. However, the most pronounced positive shift in these profiles again occurred after the 1950s. Altogether this study demonstrates that the recent human induced changes of the Coorong hydrology, compounded by a severe drought led to an increase in salinity and alterations of primary production which have been much more significant than natural variations occurring throughout the Holocene over several thousands of years.