Geosteranes: Identification and synthesis of a novel series of 3-substituted steranes

A novel series of 3-alkyl and 3-carboxyalkyl-5β(H)-steranes 7–10 along with a full homologous series of carboxyalkyl-sterane (C₁ to C₆) 4–6 with 3α(H)5α(H) configuration have been identified in marine-evaporitic oils from Fazenda Belém, Potiguar Basin (Brazil) on the basis of mass spectral interpretation. The synthesis of enantiomerically pure 3α-alkyl-5β(H)-cholestane and 3β-alkyl-5α(H)-cholestane standards and their coinjection with petroleum fractions confirmed the structural assignments.     

In situ anaerobic degradation of petroleum alkanes in marine sediments: preliminary results

The degradation of acyclic petroleum hydrocarbons was studied during a 24-month experiment in Mediterranean coastal sediments (Gulf of Fos). Sediment cores entirely contaminated with oil (Arabian Light Crude Oil) were incubated in situ. The use of conservative tracers of sediment's particles reworking (luminophores) allowed the distinction of the reworked layer from the anoxic deeper sediments. Using the 17α,21βC₃₀ hopane (C₃₀H) as an inert internal reference, we could demonstrate that, after 24 months of experiment, acyclic petroleum hydrocarbons can be degraded under natural anaerobic conditions. The reactivity of individual alkanes appeared to depend on their chemical structure. To cite this article: D. Massias et al., C. R. Geoscience 335 (2003).To cite this article: D. Massias et al., C. R. Geoscience 335 (2003).     

Structural identification of the C25 highly branched isoprenoid pentaene in the marine diatom Rhizosolenia setigera

2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadeca-2,5E,9E,13-tetraene I possessing a C₂₅ highly branched isoprenoid skeleton has been isolated from the marine diatom Rhizosolenia setigera and identified by ¹H and ¹³C NMR spectroscopy.     

Composition, lattice parameters, and room temperature elastic constants of natural single crystal xenotime from Novo Horizonte

The composition, lattice parameters, and elastic constants of natural single crystal YPO₄ xenotime from Novo Horizonte (Brazil) were determined using EPMA, XRD, and the pulse-echo technique. The composition indicates a 24% substitution of Y sites with other rare-earth elements. The lattice parameters of the studied crystal deviated only slightly from those reported for synthetic YPO₄ and were in a good agreement with trends obeyed by other orthophosphates with the xenotime structure. The measured elastic constants C ₁₁, C ₃₃, C ₄₄, and C ₆₆ were consistent with synthetic crystals when porosity was accounted for. C ₁₂ and C ₁₃ constants were evaluated based on the comparison with other materials with xenotime structure. The elastic constants could be rationalized using interionic force constants and bond energies.

A study of rare earth element (REE)–SiO2 variations in felsic liquids generated by basalt fractionation and amphibolite melting: a potential test for discriminating between the two different processes

The origin of felsic magmas (>63% SiO₂) in intra-oceanic arc settings is still a matter of debate. Two very different processes are currently invoked to explain their origin. These include fractional crystallization of basaltic magma and partial melting of lower crustal amphibolite. Because both fractionation and melting can lead to similar major element, trace element and isotopic characteristics in felsic magmas, such lines of evidence have been generally unsuccessful in discriminating between the two processes. A commonly under-appreciated aspect of rare earth element (REE) solid–liquid partitioning behavior is that D _REE for most common igneous minerals (especially hornblende) increase significantly with increasing liquid SiO₂ contents. For some minerals (e.g., hornblende and augite), REE partitioning can change from incomptatible (D < 1) at low liquid SiO₂ to compatible (D > 1) at high liquid SiO₂. When this behavior is incorporated into carefully constrained mass-balance models for mafic (basaltic) amphibolite melting, intermediate (andesitic) amphibolite melting, lower or mid to upper crustal hornblende-present basalt fractionation, and mid to upper crustal hornblende-absent basalt fractionation the following general predictions emerge for felsic magmas (e.g., ∼63 to 76% SiO₂). Partial melting of either mafic or intermediate amphibolite should, regardless of the type of melting (equilibrium, fractional, accumulated fractional) yield REE abundances that remain essentially constant and then decrease, or steadily decrease with increasing liquid SiO₂ content. At high liquid SiO₂ contents LREE abundances should be slightly enriched to slightly depleted (i.e., C _l/C _o ∼ 2 to 0.2) while HREE abundances should be slightly depleted (C _l/C _o ∼ 1 to 0.2). Lower crustal hornblende-bearing basalt fractionation should yield roughly constant REE abundances with increasing liquid SiO₂ and exhibit only slight enrichment (C _l/C _o ∼ 1.2). Mid to upper crustal hornblende-bearing basalt fractionation should yield steadily increasing LREE abundances but constant and then decreasing HREE abundances. At high liquid SiO₂ contents LREE abundances may range from non-enriched to highly enriched (C _l/C _o ∼ 1 to 5) while HREE abundances are generally non-enriched to only slightly enriched (C _l/C _o ∼ 1 to 2). Hornblende-absent basalt fractionation should yield steadily increasing REE abundances with increasing liquid SiO₂ contents. At high SiO₂ contents both LREE and HREE are highly enriched (C _l/C _o ∼ 3 to 4). It is proposed that these model predictions constitute a viable test for determining a fractionation or amphibolite melting origin for felsic magmas in intra-oceanic arc environments where continental crust is absent. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

The occurence of unusual C27 and C29 sterane predominances in two types of Oman crude oil

Two types of Oman crude oils reveal unusual sterane distributions. Type “A”, which is the more common (74 examples), is characterised by a predominance of C₂₉ iso- and normal-steranes and generally none or only very low relative concentrations of rearranged-steranes. The triterpanes are characterised by the predominance of the C₂₉ 17αH, 21βH norhopane over the C₃₀ 17αH, 21βH hopane and non-predominant C₂₀–C₃₀ tricyclic terpanes. The C₂₉ steranes of this type of crude were not derived from the C₂₉ sterols of land-plant origin (frequently proposed as the source of C₂₉ steranes) since there is good geological evidence that these crudes were generated from a pre-Cambrian source rock, a geological period when land-plants did not exist.The type “B” crude oil (11 known examples) is characterised by a strong predominance of C₂₇ iso-, normal- and rearranged-steranes, relatively lower concentrations of 17αH, 21βH hopanes and relatively high concentrations of C₂₀–C₃₀ tricyclic terpanes.The remarkably different biomarker characteristics of these crude oils imply that the organisms active in the depositional environment of the respective source rocks were significantly different.     

Chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100°C to 550°C: Case study in the Liassic black shale formation of Central Swiss Alps

Hydrocarbon distributions and stable isotope ratios of carbonates (δ¹³C_car, δ¹⁸O_car), kerogen (δ¹³C_ker), extractable organic matter (δ¹³C_EOM) and individual hydrocarbons of Liassic black shale samples from a prograde metamorphic sequence in the Swiss Alps were used to identify the major organic reactions with increasing metamorphic grade. The studied samples range from the diagenetic zone (<100°C) to amphibolite facies (∼550°C). The samples within the diagenetic zones (<100 and 150°C) are characterized by the dominance of C_<20n-alkanes, suggesting an origin related with marine and/or bacterial inputs. The metamorphic samples (200 to 550°C) have distributions significantly dominated by C₁₂ and C₁₃n-alkanes, C₁₄, C₁₆ and C₁₈n-alkylcyclopentanes and to a lesser extend C₁₅, C₁₇ and C₂₁n-alkylcyclohexanes. The progressive ¹³C-enrichment (up to 3.9‰) with metamorphism of the C_>17n-alkanes suggests the occurrence of cracking reactions of high molecular weight compounds. The isotopically heavier (up to 5.6‰) C_<17n-alkanes in metamorphic samples are likely originated by thermal degradation of long-chain homologous with preferential release of isotopically light C₁ and C₂ radicals. The dominance of specific even C-number n-alkylcyclopentanes suggests an origin related to direct cyclization mechanism (without decarboxylation step) of algal or bacterial fatty acids occurring in reducing aqueous metamorphic fluid conditions. The regular increase of the concentrations of n-alkylcycloalkanes vs. C_>13n-alkanes with metamorphism suggests progressive thermal release of kerogen-linked fatty acid precursors and degradation of n-alkanes. Changes of the steroid and terpenoid distributions are clearly related to increasing metamorphic temperatures. The absence of 18α(H)-22,29,30-trisnorneohopane (Ts), the occurrence of 17β(H)-trisnorhopane, 17β(H), 21α(H)-hopanes in the C₂₉ to C₃₁ range and 5α(H),14α(H),17α(H)-20R C₂₇, C₂₉ steranes in the low diagenetic samples (<100°C) are characteristic of immature bitumens. The higher thermal stress within the upper diagenetic zone (150°C) is marked by the presence of Ts, the disappearance of 17β(H)-trisnorhopane and thermodynamic equilibrium of the 22S/(22S + 22R) homohopane ratios. The increase of the ααα-sterane 20S/(20S + 20R) and 20R ββ/(ββ + αα) ratios (from 0.0 to 0.55 and from 0.0 to 0.40, respectively) in the upper diagenetic zone indicates the occurrence of isomerization reactions already at <150°C. However, the isomerization at C-20 (R → S) reaches thermodynamic equilibrium values already at the upper diagenesis (∼150°C) whereas the epimerisation at C-14 and C-17 (αα → ββ) arrives to constant values in the lower anchizone (∼200°C). The ratios Ts vs. 17α(H)-22,29,30-trisnorneohopane [(Ts/(Ts + Tm)] and 18α(H)-30-norneohopane (C₂₉Ts) vs. 17α(H),21β(H)-30-norhopane [C₂₉Ts/(C₂₉Ts + C₂₉)] increase until the medium anchizone (200 to 250°C) from 0.0 to 0.96 and from 0.0 to 0.44, respectively. An opposite trend towards lower values is observed in the higher metamorphic samples.The occurrence of specific hydrocarbons (e.g., n-alkylcyclopentanes, cadalene, hydrogenated aromatic compounds) in metamorphic samples points to kerogen degradation reactions most probably occurring in the presence of water and under reducing conditions. The changes of hydrocarbon distributions and carbon isotopic compositions of n-alkanes related to metamorphism suggest that the organic geochemistry may help to evaluate the lowest grades of prograde metamorphism.     

Electronic absorption spectra of Cr3+ ions in natural clinopyroxenes

The polarized (E‖a′, E‖b and E‖c) electronic absorption spectra of five natural chromium-containing clinopyroxenes with compositions close to chromdiopside, omphacite, ureyite-jadeite (12.8% Cr₂O₃), jadeite, and spodumene (hiddenite) were studied. The polarization dependence of the intensities of the Cr³⁺ bands in the clinopyroxene spectra cannot be explained by the selection rules for the point groups C ₂ or C _2v but can be accounted for satisfactorily with the help of the higher order pseudosymmetry model, i.e. with selection rules for the point symmetry group C _3v. The trigonal axis of the pseudosymmetry crystal field forms an angle of 20.5° with the crystallographic direction c in the (010) plane. D _q increases from diopside (1542 cm^?1) through omphacite (1552 cm^?1), jadeite (1574 cm^?1) to spodumene (1592 cm^?1). The parameter B which is a measure of covalency for Cr³⁺-O bonds at M1 sites in clinopyroxene depends on the Cr³⁺ concentration and the cations at M2 sites.     

Selective enrichment of steroid and triterpenoid hydrocarbons from crude oil using gel permeation chromatography for stable carbon isotope analysis

Gel permeation chromatography (GPC) using a high performance liquid chromatography (HPLC) system was studied for the separation and enrichment of steroid and hopanoid hydrocarbons from crude oil for stable carbon isotope analysis. A crude oil sample was pretreated using silica gel chromatography and 5A molecular sieve to remove polycyclic aromatic hydrocarbons and n-alkanes. The GPC behavior of both the pretreated saturated hydrocarbon fraction of the oil and standard steroid [5α(H), 14α(H), 17α(H) C₂₇–C₂₉ steranes], hopanoid [17α(H) C₂₇ trisnorhopane, 17α(H), 21β(H) C₂₉–C₃₂ hopanes] and triterpenoid [18α(H)-oleanane, gammacerane] mixtures were examined. The results indicate that 17α(H), 21β(H) hopanes as well as steranes could be enriched efficiently using GPC and that they could be obtained without removing n-alkanes from the oil saturated hydrocarbon fraction. The GPC behavior of steroid and triterpenoid hydrocarbons was controlled by molecular size and shape.     

Extended saturated and monoaromatic tricyclic terpenoid carboxylic acids found in Tasmanian tasmanite

A series of tricyclic terpenoid carboxylic acids (C₂₀–C₄₀) was found in the acidic fraction of Tasmanian tasmanite bitumen, occurring as a mixture of stereoisomers with mainly the 13β(H), 14α(H)-and 13α(H),14α(H)-configurations. These dominant acidic tricyclic constituents have the same carbon skeleton as the ubiquitous tricyclic terpane biomarkers. A novel series of ring-C monoaromatic tricyclic terpenoid carboxylic acids was also characterized. The series ranges from C₁₉ to C₃₉ and is the acidic counterpart of the recently described series of monoaromatic tricyclic terpanes.     

Bitumen from Coalport tar tunnel

The Coalport (Shropshire, U.K.) Tar Tunnel bitumen has been known since 1787 and the first geochemical data are reported here. The bitumen was analyzed for molecular markers useful for correlational studies. Gas chromatographic analysis of aliphatic and aromatic hydrocarbons failed to detect any specific major components normally used for genetic correlational and maturational studies. A search for minor and trace components by gas chromatographic-mass fragmentographic analysis showed the presence of triterpenoid hydrocarbons primarily of the 17α (H)-hopane series (C₂₇ to C₃₅, ex. C₂₈) and a C₂₆ to C₃₁ series of ring A/B demethylated hopanes. Two homologous sterane series (C₂₇ to C₂₉) of the 5α, 14β, 17β (H)-sterane and 13β, 17α (H)-diasterane type were also detected. Pophyrins of the DPEP and etio series (C₂₇ to C₄₁, DPEP/etio > 1) were also found. Characterization of their alkyl substitution pattern demonstrated C₁, C₂ and C₃ substituents on the pyrole moieties of the parent petroporphyrins.The molecular markers detected in this bitumen indicate its biogenic origin and show evidence of diagenetic and geothermal maturation processes. The overall geochemical characteristics of the Coalport Tar Tunnel bitumen suggest that it corresponds to a well matured crude oil, which was heavily altered by in-reservoir biodegradation or close to surface exposure.     

Molecular and stable carbon isotopic compositions of hopanoids in seep carbonates from the South China Sea continental slope

The lipid biomarkers of hopanoids in cold seep carbonates from the South China Sea continental slope were investigated by gas chromatography–mass spectrometer (GC–MS) and gas chromatography-isotope ratio-mass spectrometer (GC-ir-MS). The distribution of hopanes/hopenes shows a preference for the ‘biological’ 17β(H), 21β(H)-over the ‘geological’ 17α(H), 21β(H)-configuration. This interpretation is in agreement with the strong odd–even preference of long-chain n-alkanes in those samples, suggesting that the ββ hopanes may be the early diagenetic products of biohopanoids and the αβ, βα configurations of hopanes were mainly derived from allochthonous sources contributing to the organic matter of the carbonates. In terms of hopanoid acids, the C₃₀ to C₃₃ 17β(H), 21β(H)-hopanoid acids were detected with C₃₂ 17β(H), 21β(H)-hopanoid acid being the most abundant. However, there is a significant difference in stable carbon isotopic compostions of the C₃₂ 17β(H), 21β(H)-hopanoic acid among samples (−30.7‰ to −69.8‰). The δ¹³C values match well with the carbon isotopic compositions of SRB-derived iso-/anteiso-C_15:0 fatty acids in the samples, which strongly depend on the carbon utilization types by microbe. The most abundant compound of hopanols detected in the samples, C₃₀-17β(H), 21β(H)-hopanol, may be a good indicator of diagenetic product of type I methanotrophs. The molecular and carbon isotopic compositions of hopanoids demonstrate clearly that there is a combination contribution of both SRB and type I or type X methanotrophs to the source organism in the seep carbonates from the South China Sea continental slope.     

Thermodynamic stability of various alkylated, dealkylated and rearranged 17α- and 17β-hopane isomers using molecular mechanics calculations

The thermodynamic stability of selected alkylated, dealkylated and rearranged 17α- and 17β-hopane isomers in the C₂₇, C₂₈, C₂₉, C₃₀ and C₃₁ families were calculated using molecular mechanics (MM2) methods and, where possible, calculated equilibrium ratios of certain isomers were compared with observed ratios of isomers in thermally mature crude oil samples. Those calculated and observed ratios having similar values include: (1) the relative distributions among 17β(H)/17α(H) and 21β(H)/21α(H)-hopanes including the absence of the 17β(H),21β(H)- and 17α(H),21α(H)-hopanes; (2) the 22R/22S ratios in 30-methyl-17α-hopane and 30-methyl-17β-moretane; (3) the relative distributions among 17α(H)/17β(H)- and 21α(H)/21β(H)-28,30-bisnorhopanes and among 25,28,30-trisnorhopanes, including the relatively greater stability of 17β(H) isomers in contrast to the regular hopane series; and (4) the ratios of 28(18−17S)abeo hopanes with respect to their unrearranged counterparts including the C₂₇ compounds, Ts/Tm.     

Novel C26 tetracyclic degraded bicadinanes in oil sands from the Haitoubei–Maichen Sag of the Beibuwan Basin,South China Sea

Three C₂₆ tetracyclic terpanes were detected in the saturated hydrocarbon fractions of oil sand samples especially rich in bicadinanes W, T and R from the Oligocene Huizhou Formation in the XW-1 well in the Haitoubei–Maichen Sag of the Beibuwan Basin. Their mass spectra show molecular ions of the three tetracyclic terpanes at m/z 358, and their key fragment ions at m/z 343 (M⁺-15) and m/z 315 due to loss of an isopropyl group. These characteristics are very similar to that of C₃₀ cis–cis–trans (W) and trans–trans–trans (T) bicadinanes. In addition, the relative abundances of the C₂₆ tetracyclic terpanes show an excellent positive correlation with the C₃₀ bicadinanes W and T in 27 oil sand samples. Therefore, the three C₂₆ tetracyclic terpanes are tentatively identified as degraded bicadinanes, which like bicadinanes, may indicate input from dammar resins and can be used to discriminate the origin of crude oils.     

Geochemical characteristics and genetic types of crude oils from Qinjiatun and Qikeshu oilfields in the Lishu Fault Depression,Songliao Basin,northeastern China

The Qinjiatun and Qikeshu oilfields are new Mesozoic petroleum exploration targets in Lishu Fault Depression of Songliao Basin, northeastern China. Currently, researches on geochemistry of crude oils from Qinjiatun and Qikeshu oilfields have not been performed and the genesis of oils is still uncertain. Based on bulk analyses, the crude oils in the Qinjiatun and Qikeshu oilfields of Lishu Fault Depression from the Lower Cretaceous can be classified as three types. TypeⅠoils, from Quantou and Denglouku formations of Qikeshu oilfield, are characterized by high C24tetracyclic terpane/C26tricyclic terpanes ratios, low gammacerance/C30hopane ratios, tricyclic terpanes/hopanes ratios, C29Ts/C29norhopane ratios and 17α(H)-diahopane/17α(H)-hopane ratios, indicating a brackish lacustrine facies. TypeⅡoils, from Shahezi Formation of Qikeshu oilfield show low C24tetracyclic terpane/C26tricyclic terpanes, high gammacerance/C30hopane ratios, tricyclic terpanes/hopanes ratios, C29Ts/C29 norhopane and C30diahopane/C30hopane ratios, thus suggesting that they originated from source rocks deposited in a weak reducing brackish lacustrine environment, or clay-rich sediments. Type oilsⅢ, from some wells of Qikeshu oilfield have geochemical characteristics intermediate between those two types and may be mixture of typeⅠand Ⅱoils.     

Predictive crystal-chemical relations in Ti-silicates based on the TS block

In a group of minerals of reasonable complexity in which the structure topology is related but not identical, the general relation between structure topology and chemical composition is not known. This problem is of major significance. The structural hierarchy and stereochemistry are described for 27 titanium disilicate minerals that contain the TS (titanium-silicate) block, a central trioctahedral (O) sheet and two adjacent (H) sheets of [5]- and [6]-coordinated polyhedra and (Si₂O₇) groups and related delindeite. The TS block is characterized by a planar cell based on translation vectors, t ₁ and t ₂ , with t ₁ ～ 5.5 and t ₂ ～ 7 Å and t ₁ ∧ t ₂ close to 90°. The general formula of the TS block is A ₂ ^P B ₂ ^P M ₂ ^H M ₄ ^O (Si ₂ O ₇ ) ₂ X _{4 + n}, where M ₂ ^H and M ₄ ^O = cations of the H and O sheets; M^H = Ti (= Ti + Nb), Zr, Mn²⁺, Ca; M^O = Ti, Zr, Mn²⁺, Ca, Na; A ^P and B ^P are cations at the peripheral (P) sites = Na, Ca, Ba; X = anions = O, OH, F; n = 0, 2, 4; the core part of the TS block is shown in bold and is invariant. Cations in each sheet of the TS block form a close-packed layer and the three layers are cubic close packed.There are three topologically distinct TS blocks, depending on the type of linkage of two H sheets and the central O sheet. The H sheets of one TS block attach to the O sheet in the same manner. All structures consist of a TS block and an I (intermediate) block that comprises atoms between two TS blocks. Usually, the I block consists of alkali and alkaline-earth cations, (H₂O) groups and oxyanions (PO₄)^3?, (SO₄)^2? and (CO₃)^2?. These structures naturally fall into four groups, based on differences in topology and stereochemistry of the TS block. In Group I, Ti = 1 apfu Ti occurs in the O sheet, and (Si₂O₇) groups link to a Na polyhedron of the O sheet (linkage 1). In Group II, Ti = 2 apfu, Ti occurs in the H sheet, and (Si₂O₇) groups link to two M ²⁺ octahedra of the O sheet adjacent along t ₂ (linkage 2). In Group III, Ti = 3 apfu, Ti occurs in the O and H sheets, and (Si₂O₇) groups link to the Ti octahedron of the O sheet (linkage 1). In Group IV, Ti = 4 apfu (the maximum possible content of Ti in the TS block), Ti occurs in the O and H sheets, and (Si₂O₇) groups link to two Ti octahedra of the O sheet adjacent along t ₁ (linkage 3). The stability of the TS block is due to the ability of Ti (Nb) to have an extremely wide range in Ti (Nb)-anion bond lengths, 1.68–2.30 Å, which allows the chemical composition of the TS block to vary widely. In crystal structures so far known, only one type of TS block occurs in a structure. The TS block propagates close-packing of cations onto the I block. The general structural principles and the relation between structure topology and chemical composition are described for the TS-block minerals. These principles allow prediction of structural arrangements and possible chemical compositions, and testing whether or not all aspects of the structure and chemical formula of a mineral are correct. Here, I show how these principles work, and review recent results that show the effectiveness of these principles as a predictive technique.     

Geochemical Characteristics of Natural Gases in the Sichuan Basin—A Further Discussion

This paper is the continuation of the paper published in 1985, with the emphasis on the irregular components of natural gases from the Sichuan Basin—the isotopic compositions of C,H and S, the n-and iso-alkane predominance (C₆-C₁₃) of methane homologues, the compositional characteristics of benzene and methylbenzene, and the distribution and geochemical characteristics of organic sulfur-bearing compounds including thiophen, thio-alcohol, etc. It is considered that natural gases from the Sichuan Basin have different distributional characteristics in different layers and locations, suggesting that they are controlled by a combination of factors such as kerogen type, maturity of organic matter and wall-rock assemblage.

1. The type of source material (or precursor) is a main factor affecting the geochemical characteristics of natural gases. T_3x-h and P₂l produce coal-series gases with type-III source material dominant. The C and H isotopic compositions of natural gases are heavier, the contents of C₆H₆ and C₇H₈ are high, C₇H₈/C₆H₆ < 1. the content of C₄H₄S is low, and the predominance in isoalkane (C₆C₁₃) is obvious. J₁t. P₁y. Z₂b. etc. produce oil-series gases with type-I and -II kerogens dominant, indicating that the geochemical characteristics of natural gases are different from those described above.
2. The maturity of organic matter is an important factor affecting the composition of natural gases.With increasing maturity of organic matter, the C and H isotopic compositions tend to become light. C₁ and C₂ isotopic values are nearly equal or even inverse, and C₄H₄S decreases in content.
3. Wall-rock assemblage has apparent influence on the S isotopic composition. The Middle-Lower Triassic series belongs to sulfate-carbonate formations enriched in gypsum. Therefore, the (δ³⁴S values of natural gases are relatively high (< 20 %.) against the lower values for other layers due to the absence of gypsum layers.