Alkaline permanganate oxidation of kerogens from Cretaceous black shales thermally altered by diabase intrusions and laboratory simulations

Potassium permanganate oxidative degradations were conducted for kerogens isolated from Cretaceous black shales (DSDP Leg 41, Site 368), thermally altered during the Miocene by diabase intrusions and from unaltered samples heated under laboratory conditions (250–500°C).Degradation products of less altered kerogens are dominated by normal C₄–C₁₅ α,ω-dicarboxylic acids, with lesser amounts of n-C₁₆ and n-C₁₈ monocarboxylic acids, and benzene mono-to-tetracarboxylic acids. On the other hand, thermally altered kerogens show benzene di-to-tetracarboxylic acids as dominant degradation products, with lesser or no amounts (variable depending on the degree of thermal alteration) of α,ω-dicarboxylic acids. Essentially no differences between the oxidative degradation products of naturally- and artificially-altered kerogens are observed.As a result of this study, five indices of aromatization (total aromatic acids/kerogen; apparent aromaticity; benzenetetracarboxylic acids/total aromatic acids; benzene-1,2-dicarboxylic acid/benzenedicarboxylic acids; benzene-1,2,3-tricarboxylic acid/benzenetricarboxylic acids) and two indices of aliphatic character (Total aliphatic acids/kerogen; Aliphaticity) are proposed to characterize the degree of thermal alteration of kerogens.Furthermore, a good correlation is observed between apparent aromaticity estimated by the present KMnO₄ oxidation method and that from the ¹³C NMR method (DENNIS et al., 1982).     

Simulation of the fluorescence evolution of “live” oils from kerogens in a diamond anvil cell: Application to inclusion oils in terms of maturity and source

The evolution of fluorescence has been measured for “live” oils generated from 14 oil-prone kerogens or coals from varying depositional environments during closed system pyrolysis in a diamond anvil cell at three heating rates (3, 8, and 25 °C/min), and temperatures up to 600 °C. The measured fluorescence intensities of the samples, employing using violet excitation at 405 nm, increases significantly during maturation intervals within the oil window, while the fluorescence spectra of oils generated from all studied kerogens exhibit progressive blue-shift of peak wavelengths (λ_max) and red/green quotients (I₆₅₀/I₅₀₀) upon increasing maturity. The observed trend is consistent with a maturity dependence of the spectral shift, which is widely recognized in natural hydrocarbon inclusions and crude oils using ultraviolet (UV) excitation (365 nm). The data presented herein suggest that the λ_max of spectra for inclusion oils shift in similar direction despite differences in composition or source kerogen. This implies that the reverse or anomalous trends reported for inclusion oils in nature may be attributed to other processes, which significantly alters the fluorescence properties of oils subsequent to their generation. Oils with the similar color (λ_max or I₆₅₀/I₅₀₀) can be derived from diverse kerogens with maturities that vary by ±0.3% Ro, suggesting that the fluorescent colors of crude and inclusion oils are both maturity- and source-dependent, and therefore cannot be used as universal maturity indicators. In addition, the blue-shifts observed for cumulative oils generated from all kerogens approaches similar minima λ_max values around the green-yellow wavelength (564 nm) and at I₆₅₀/I₅₀₀ values around 0.6, at maturities close to the middle or late stage of oil generation. This suggests that most late-stage cumulative oils will exhibit similar colors. Oils generated during late-stage maturity intervals, however, can exhibit colors with shorter wavelengths.     

On the depth,time and mechanism of origin of the heavy to medium-gravity naphthenic crude oils

Paraffinic crude oils are designated ‘primary’ because their composition is very close or identical to that of the hydrocarbons extracted from the corresponding oil source rocks. Heavy and medium-gravity naphthenic crude oils, on the other hand, typically are quite different compositionally from hydrocarbon mixtures in either mature or immature shales.The normal paraffin carbon number odd/even ratio 2C₂₉/(C₂₈ + C₃₀) of all the heavy to medium-gravity crude oils which could be analysed are in exactly the same range as is observed for the primary paraffinic crude oils, namely 0.95–1.42. The naphthene indices of the medium to heavy gravity naphthenic crude oils and of the primary paraffinic crude oils from the same area are identical or close. These facts are significant because both the n-paraffin carbon number odd/even ratio and the naphthene index of shale hydrocarbons are strongly depth and subsurface temperature dependent. The facts observed demonstrate beyond question that, in the same area, the paraffinic precursors of the heavy to medium-gravity naphthenic crude oils are generated and expelled in the identical depth range, and from the same mature relatively deep oil source beds as the primary paraffinic crude oils. Later, during and/or after a generally upward migration into oil reservoirs, the primary crude may be transformed compositionally into a naphthenic crude oil.In none of the five widely scattered oil basins studied are medium to heavy naphthenic crude oils found at temperatures greater than a limiting subsurface temperature. The abruptness of the temperature cutoff of the change in oil compositions in all five oil basins, as well as the average value of the cutoff temperature of 66°C (150°F), leaves no doubt that the mechanism of this crude oil transformation process is microbial.Optical activity, which was observed in narrow saturate hydrocarbon fractions of the 80–325°C range of all microbially transformed crude oils, but not in the primary untransformed oils, is strong additional evidence for the microbial nature of the crude oil transformation process. The observed optical activity is explained by the microbial digestion at different rates of optical antipodes present in the primary paraffinic crude oils.To gain perspective the vast scale of the microbial oil transformation process in nature is pointed out. Billions of tons of heavy to medium-gravity naphthenic crude oils, originating from the microbial transformation of primary paraffinic oils, are present in oil fields and tar sands all over the world.     

Light hydrocarbons in subsurface sediments

The major features and numerous compositional details of the indigenous C₂–C₇ hydrocarbon suites of argillaceous sediments are systematically temperature dependent. The relative concentrations of alicyclic compounds exhibit a consistent maximum at subsurface temperatures close to 170°F (77°C) without regard to the chemical nature of the bulk of the kerogen, whether rich or poor in hydrogen, though this strongly affects the specific yield. A continuous increase in relative alkane content follows at higher temperatures. Indices of paraffinicity may be devised. One such, termed the ‘heptane value’ (essentially the percentage of n-heptane in the b.p. range 80.7–100.9°C), possesses a linear association with temperature, provides an index of catagenesis, and frequently provides a means of appraising paleotemperatures. Regressions of heptane value on temperature are compared in two composite stratigraphic sections dominated by kerogens representing two extremes of composition. The regression coefficients differ by 7%. Yields of light hydrocarbons increase exponentially in these sections by more than three orders of magnitude along sub-parallel, temperature-dependent curves. These similarities infer universally similar generating reactions and compositionally similar suites of light hydrocarbons at given subsurface temperatures, regardless of kerogen type, particularly for sections which underwent burial and heating during the Tertiary period.     

Generation and expulsion of oils from Permian coals of the Sydney Basin,Australia

Organic geochemical and petrological assessment of coals/coaly shales and fine grained sediments, coupled with organic geochemical analyses of oil samples, all from Permo–Triassic sections of the Southern Sydney Basin (Australia), have enabled identification of the source for the widely distributed oil shows and oil seeps in this region. The Permian coals have higher hydrogen indices, higher liptinite contents, and much higher total organic matter extract yields than the fine grained sediments. A variety of source specific parameters obtained from n-alkanes, regular isoprenoids, terpanes, steranes and diasteranes indicate that the oil shows and seeps were generated and expelled predominantly from higher plant derived organic matter deposited in oxic environments. The source and maturity related biomarkers and aromatic hydrocarbon distributions of the oils are similar to those of the coals. The oil-coal relationship also is demonstrated by similarities in the carbon isotopic composition of the total oils, coal extracts, and their individual n-alkanes. Extracts from the Permo–Triassic fine grained sediments, on the other hand, have organic geochemical signatures indicative of mixed terrestrial and prokaryotic organic matter deposited in suboxic environments, which are significantly different from both the oils and coal extracts. The molecular signatures indicating the presence of prokaryotic organic matter in some of the coal extracts and oils may be due to thin sections of possibly calcareous lithologies interbedded within the coal measures. The genetic relationship between the oils and coals provides new evidence for the generation and expulsion of oils from the Permian coals and raises the possibility for commercial oil accumulations in the Permian and Early Triassic sandstones, potentially in the deeper offshore part of the Sydney Basin.     

Determination of the stress conditions of the ductile-to-brittle regime along the Asuke Shear Zone,SW Japan

The stress conditions of the ductile-to-brittle regime have been assessed along the Asuke Shear Zone (ASZ), which strikes NE–SW in the Cretaceous Ryoke granite terrain in SW Japan. Along the ASZ, pseudotachylyte and mylonitized pseudotachylyte are locally developed together with cataclasite. The simultaneous operation of dislocation creep and grain-size-sensitive creep, as indicated by the coexistence of the Z-maximum and relatively random c-axis lattice preferred orientations as well as the sizes of dynamically recrystallized quartz grains (6.40–7.79 μm) in the mylonitized pseudotachylyte, suggest differential stresses of 110–130 MPa at ∼300 °C. The e-twin morphology, twinning ratio, and distribution of the glide direction on the e-twin plane of the twinned calcite in the amygdules of the pseudotachylyte suggest the stress conditions of the σ₁ and σ₃ axes trend 228° and 320° and plunge 55° and 1°, respectively, and indicate differential stresses of 40–80 MPa at 150–200 °C. Based on kinematic indicators in the fault rocks, the stress conditions estimated from calcite twins, and the cooling history of the granitic protolith, the ASZ is inferred to have been activated under a stress state that caused sinistral normal movements before and after pseudotachylyte formation at 70–50 Ma.     

Possible carotenoid-derived structures in fossil kerogens

The unique KMnO₄ degradation products of β-carotene, previously identified as 2,2-dimethyl succinic acid (C₆) and 2,2-dimethyl glutaric acid (C₇) have been found in the oxidation products of Green River shale (Eocene, 52 × 10⁶yr) and Tasmanian Tasmanite (Permian, 220−274 × 10⁶yr) kerogens. These two compounds were also detected in KMnO₄ degradation products of young kerogens from lacustrine and marine sediments. The results indicate that kerogens incorporated carotenoids (possibly β-carotene) at the time of kerogen formation in surface sediments. Both acids are useful markers to obtain information on biological precursors contributing to the formation of fossil kerogens.     

Dicarboxylic acids generated by thermal alteration of kerogen and humic acids

Significant amounts (up to 2% of organic geopolymers) of low molecular weight (LMW) dicarboxylic acids (C₂–C₁₀) have been detected during thermal alteration (270°C, 2 h) of kerogens and humic acids isolated from young or ancient lithified sediments. Their distribution is characterized by predominance of oxalic acid followed by succinic, fumaric and methylsuccinic acids. These acids are probably released by the breakdown of macromolecular structures, which have incorporated biogenic organic compounds, including diacids, during early diagenesis in sediments. Because of their reactivity, LMW diacids may play the following geochemically important roles under natural conditions: (1) the diacids dissolve carbonates and clay minerals to increase porosity and permeability, which enhances migration of oils and gas generated from catagenesis of kerogen dispersed in shale, and (2) the diacids may form organo-metal complexes, which are important for mobilization, transport and accumulation of trace metals in sedimentary basins.     

The Tsagan earthquake of 1862 on Lake Baikal revisited: a study of secondary coseismic soft-sediment deformation

Coseismic soft-sediment deformation has been studied by structural and tectononophysical methods in the Selenga Delta area shaken by the devastating M ～ 7.5 Tsagan earthquake in 1862. Among the documented deformation structures (seismites), clastic dikes are the most reliable paleoseismic indicators. The dikes have their sizes and extent showing proximity to the primary coseismic rupture zone and are closely associated with faults of different hierarchic levels. The Tsagan event occurred under SW–NE extension as motion on a stepped system of normal faults dipping at 300°–350°, ∠45°–75°.The amount of vertical motion measured against a reference layer in a trench reached 2.83 m, and the maximum dip displacement measured in a single fracture was 0.5 m. The earthquake was generated by the Delta Fault that dips at 60° on average to the northwest.The distribution of quantitative parameters of brittle and brittle-plastic deformation has been analyzed along two profiles, and two new parameters were introduced: indices of mean intensity (I) of clastic dikes and microdikes; the new parameters were calculated by specially developed equations. Summation of significant peaks in all parameters (SUM_spp) allowed contouring the zone of most intense soft-sediment deformation near Dubinino Village.Deformation mostly propagated in the NE–SW and N–S directions. The location of the 1862 Tsagan earthquake at 52.35° N and 106.67° E was inferred from the SUM_spp value taking into account the dip of the causative fault plane and the average origin depth of earthquakes in the Baikal rift. The approach we used is applicable to locating preinstrumental events.The recurrence of large earthquakes in the area of Proval Bay (Lake Baikal) has been estimated to be 1120–1230 years proceeding from alternating deformed and undeformed sediments in the sections, their thicknesses and deposition rates according to radiocarbon dating. The seismic activity has been associated with the same fault which can generate M ≥ 7 events.     

Organic geochemical characteristics and oil generating potential of the Upper Jurassic Safer shale sediments in the Marib-Shabowah Basin,western Yemen

The organic rich Safer shales exposed in the north-central part of onshore Marib-Shabowah Basin are evaluated and their depositional environments are interpreted. Total organic carbon contents (TOC) of the shales range from 1.02–16.8 wt%, and yield hydrogen index (HI) values ranging from 130 to 820 mg HC/g TOC, consistent with mainly Type II with minor contributions from Type I and mixed Types II–III kerogens. The Safer shale samples have vitrinite reflectance values in the range of 0.5–1.0 R_o%, indicating early mature to peak mature stage for oil generation. T_max values range from 429–438 °C, which are in reasonably good agreement with vitrinite reflectance data. Kerogen microscopy shows that the Safer shales are characterized by high amounts of organic matter, consisting predominantly of yellow fluorescing amorphous organic matter and alginite of marine origin. This is supported by their high content of hydrogen rich Type II and I oil-prone kerogen.The biomarker distributions of the Upper Jurassic Safer extracts are characterized by dominant low to medium molecular weight compounds (n-C₁₄ to n-C₂₀), low Pr/Ph ratio (<1.0), high phytane/n-C₁₈ ratios (0.82–2.68), and predominant regular sterane C₂₇. All biomarker parameters clearly indicate that the organic matter was derived from marine algal inputs and deposited under anoxic (reducing) conditions. Hypersaline conditions also prevailed during deposition of these sediments, as indicated by the presence of gammacerane.     

Analysis of instantaneous spectra of a complete sample of flat-spectrum radio sources

The results of observations of a complete sample of radio sources with spectral indices α>?0.5 (S∝v ^α) are presented. The sample was selected from the Zelenchuk Survey at 3.9 GHz and contains all sources with declinations 4°–6°, Galactic latitudes |b|>10°, and 3.9-GHz fluxes >200 mJy. Spectra at 0.97–21.7 GHz were obtained for all 69 sample sources. The spectra were classified, and a correlation between variability amplitude and spectrum shape was found. The spectra were separated into extended and compact components. The distribution of spectral indices α for the extended components coincides with the distribution for sources with power-law spectra. The correlation between the luminosity and frequency of the peak flux density is confirmed. This correlation is due to the fact that the distribution of source linear dimensions does not depend on luminosity.     

Vanadyl porphyrins in exploration: maturity indicators for source rocks and oils

The Porphyrin Maturity Parameter (PMP), which is derived from the vanadyl porphyrin distribution, is an excellent parameter for: (1) identifying the zone of hydrocarbon generation from marine source rock extracts; and (2) determining from oils the thermal maturity of their source rocks at expulsion.The PMP is measured using a methodology which is inexpensive, reliable and faster than earlier methods, allowing it to be used as a routine exploration tool. The PMP may be a more reliable maturity indicator for marine organic matter than some conventional methods such as vitrinite reflectance. Unlike most conventional maturity parameters guided by processes other than kerogen conversion, the reactions causing PMP evolution directly monitor the generation of bitumen and the concurrent thermal degradation of kerogen.Measurements on hydrous pyrolyzates from the Monterey Formation (offshore California), source rock bitumens from the Devonian-Mississippian Bakken Shale (Williston Basin), and Miocene Monterey equivalent source strata (San Joaquin Basin, California) illustrate the method. In all cases reviewed so far, PMP begins increasing at the onset of hydrocarbon generation and increases systematically and predictably as kerogen decomposition proceeds.In oils generated from high-S marine kerogens, PMP reflects the maturity of the source rock at the time of oil expulsion, provided that the oil does not undergo subsequent reservoior maturation or mixing with in-situ bitumen.     

P-T-t constraints on the metamorphic evolution of the Transangarian Yenisei Ridge: Geodynamic and petrological implications

Two metamorphic complexes of the Yenisei Ridge with contrasting composition are analyzed to unravel their tectonothermal evolution and geodynamic processes during the Riphean geologic history of the area. The structural, mineralogical, petrological, geochemical and geochronological data are used to distinguish two stages of the evolution with different ages, thermodynamic regimes, and metamorphic field gradients. Reaction textures, chemical zoning in minerals, shapes of the P-T paths, and isotope dates provide convincing evidence for a poly metamorphic history of the region. The first stage is marked by the formation of the ~ 970 Ma low-pressure zoned And-Sil rocks (P = 3.9-5.1 kbar, T = 510–640 °C) of the Teya aureole and a high metamorphic field gradient with dT/dH = 25–35 °C/km typical of many orogenic belts. At the second stage, these rocks experienced Late Riphean (853–849 Ma) collisional medium-pressure metamorphism of the kyanite-sillimanite type (P = 5.7-7.2 kbar, T = 660–700 °C) and a low metamorphic field gradient with dT/dH < 12 °C/km. This metamorphic event was almost coeval with the Late Riphean (862 Ma) contact metamorphism in the vicinity of the granitic plutons, which was accompanied by a high metamorphic field gradient with dT/dH > 100 °C/km. At the first stage, the deepest blocks of the Garevka complex in the vicinity of the Yenisei regional shear zone underwent high-pressure amphibolite-facies metamorphism within a narrow range of P = 7.1-8.7 kbar and T = 580–630 °C, suggesting the burial of rocks to mid-crustal depths at a metamorphic field gradient with dT/dH ~ 20–25 °C/km. At the second stage, these rocks experienced the Late Riphean (900–850 Ma) syn-exhumation dynamometamorphism under epidote-amphibolte facies conditions (P = 3.9-4.9 kbar, T = 460–550 °C) and a low gradient with dT/dH < 10 °C/km accompanied by the formation of blastomylonitic complexes in shear zones. All these deformation and metamorphic events identified on the western margin of the Siberian craton are correlated with the final episodes of the Late Grenville orogeny and provide supporting evidence for a close spatial connection between Siberia and Laurentia during early Neoproterozoic time, which is in good agreement with recent paleomagnetic reconstuctions.     

The occurrence of ultra-deep heavy oils in the Tabei Uplift of the Tarim Basin,NW China

Deeply buried heavy oils from the Tabei Uplift of the Tarim Basin have been investigated for their source origin, charge and accumulation time, biodegradation, mixing and thermal cracking using biomarkers, carbon isotopic compositions of individual alkanes, fluid inclusion homogenization temperatures and authigenic illite K–Ar radiometric ages. Oil-source correlation suggests that these oils mainly originated from Middle–Upper Ordovician source rocks. Burial history, coupled with fluid inclusion temperatures and K–Ar radiometric ages, suggests that these oils were generated and accumulated in the Late Permian. Biodegradation is the main control on the formation of these heavy oils when they were elevated to shallow depths during the late Hercynian orogeny. A pronounced unresolved complex mixture (UCM) in the gas chromatograms together with the presence of both 25-norhopanes and demethylated tricyclic terpanes in the oils are obvious evidence of biodegradation. The mixing of biodegraded oil with non-biodegraded oil components was indicated by the coexistence of n-alkanes with demethylated terpanes. Such mixing is most likely from the same phase of generation, but with accumulation at slightly different burial depths, as evidenced by overall similar oil maturities regardless of biodegradation level and/or amount of n-alkanes. Although these Ordovician carbonate reservoirs are currently buried to over 6000 m with reservoir temperatures above 160 °C, no significant secondary hydrocarbon generation from source rocks or thermal cracking of reservoired heavy oil occur in the study area. This is because the deep burial occurred only within the last 5 Ma of the Neogene, and there has not been enough heating time for additional reactions within the Middle–Upper Ordovician source rocks and reservoired heavy oils.     

Hydrocarbon potential of the Sargelu Formation,North Iraq

Microscopic and chemical analysis of 85 rock samples from exploratory wells and outcrops in northern Iraq indicate that limestone, black shale and marl within the Middle Jurassic Sargelu Formation contain abundant oil-prone organic matter. For example, one 7-m (23-ft.)-thick section averages 442 mg?HC/g S2 and 439 °C Tmax (Rock-Eval pyrolysis analyses) and 16 wt.% TOC. The organic matter, comprised principally of brazinophyte algae, dinoflagellate cysts, spores, pollen, foraminiferal test linings and phytoclasts, was deposited in a distal, suboxic to anoxic basin and can be correlated with kerogens classified as type A and type B or, alternatively, as type II. The level of thermal maturity is within the oil window with TAI?=?3^? to 3⁺, based on microspore colour of light yellowish brown to brown. Accordingly, good hydrocarbon generation potential is predicted for this formation. Terpane and sterane biomarker distributions, as well as stable isotope values, were determined for oils and potential source rock extracts to determine valid oil-to-source rock correlations. Two subfamily carbonate oil types—one of Middle Jurassic age (Sargelu) carbonate rock and the other of Upper Jurassic/Cretaceous age—as well as a different oil family related to Triassic marls, were identified based on multivariate statistical analysis (HCA and PCA). Middle Jurassic subfamily A oils from Demir Dagh oil field correlate well with rich, marginally mature, Sargelu source rocks in well MK-2 near the city of Baiji. In contrast, subfamily B oils have a greater proportion of R₂₈ steranes, indicating they were generated from Upper Jurassic/Lower Cretaceous carbonates such as those at Gillabat oil field north of Mansuriyah Lake. Oils from Gillabat field thus indicate a lower degree of correlation with the Sargelu source rocks than do oils from Demir Dagh field. One-dimension petroleum system models of key wells were developed using IES PetroMod Software to evaluate burial-thermal history, source-rock maturity and the timing and extent of petroleum generation; interpreted well logs served as input to the models. The oil-generation potential of sulphur-rich Sargelu source rocks was simulated using closed system type II-S kerogen kinetics. Model results indicate that throughout northern Iraq, generation and expulsion of oil from the Sargelu began and ended in the late Miocene. At present, Jurassic source rocks might have generated and expelled between 70 % and 100 % of their total oil.     

Geochemistry,origin and correlation of crude oils in Lower Cretaceous sedimentary sequences of the southern Mesopotamian Basin,southern Iraq

Thirty one crude oil samples from Lower Cretaceous reservoirs in southern Iraq were analyzed using bulk property and molecular methods to determine their maturity and biomarker characteristics, as well as to obtain information on their respective source rocks. All the oils are unaltered, non-biodegraded, have high sulfur content and API gravity is in the range for light to heavy oil (19–40° API). They are characterized by low Pr/Ph values, even/odd predominance and front-end biased n-alkane distributions. Based on these parameters the oils were generated and expelled from a marine carbonate source rock bearing Type II-S kerogen. Compositional similarities of hopane and sterane biomarkers with those from potential source rocks allowed identification of the Upper Jurassic–Lower Cretaceous Sulaiy and Yamama carbonate succession as the effective source beds. A similar composition of normal and isoprenoid hydrocarbons among the oils suggests an origin from a common source rock. However, biomarker maturity ratios indicate a wide range of maturity. This appears to result from the type of burial history of the source rock, characterized by a slow passage through the liquid window interval during an extended period of geologic time.     

Liquid hydrocarbon generation potential from Tertiary Nyalau Formation coals in the onshore Sarawak, Eastern Malaysia

Tertiary coals exposed in the north-central part of onshore Sarawak are evaluated, and their depositional environments are interpreted. Total organic carbon contents (TOC) of the coals range from 58.1 to 80.9 wt. % and yield hydrogen index values ranging from 282 to 510 mg HC/g TOC with low oxygen index values, consistent with Type II and mixed Type II–III kerogens. The coal samples have vitrinite reflectance values in the range of 0.47–0.67 Ro %, indicating immature to early mature (initial oil window). T _max values range from 428 to 436 °C, which are good in agreement with vitrinite reflectance data. The Tertiary coals are humic and generally dominated by vitrinite, with significant amounts of liptinite and low amounts of inertinite macerals. Good liquid hydrocarbons generation potential can be expected from the coals with rich liptinitic content (>35 %). This is supported by their high hydrogen index of up to 300 mg HC/g TOC and Py-GC (S ₂) pyrograms with n-alkane/alkene doublets extending beyond C₃₀. The Tertiary coals are characterised by dominant odd carbon numbered n-alkanes (n-C₂₃ to n-C₃₃), high Pr/Ph ratio (6–8), high T _m /T _s ratio (8–16), and predominant regular sterane C₂₉. All biomarkers parameters clearly indicate that the organic matter was derived from terrestrial inputs and the deposited under oxic condition.     

Anomalous birefringence in andradite–grossular solid solutions: a quantum-mechanical approach

The static linear optical properties (refractive indices, birefringence and axial angle) of andradite–grossular (Ca₃Fe₂Si₃O₁₂–Ca₃Al₂Si₃O₁₂) solid solutions have been computed at the ab initio quantum-mechanical level through the Coupled Perturbed Kohn–Sham scheme, using an all-electron Gaussian-type basis set. Geometry relaxation after substitution of 1–8 Al for Fe atoms in the primitive cell of andradite yields 23 non-equivalent configurations ranging from cubic to triclinic symmetry. Refractive indices vary quite regularly between the andradite (1.860) and grossular (1.671) end-members; the birefringence δ and the axial angle 2V at intermediate compositions can be as large as 0.02° and 89°, respectively. Comparison with experiments suffers from inhomogeneities and impurities of natural samples; however, semi-quantitative agreement is observed.     

Changes in the composition of bitumen extracts and chemical structure of kerogen during hydrous pyrolysis

The paper presents data on the composition of biomarkers from bitumen extracts and the chemical structure of kerogen from C_org-rich sedimentary rocks before and after hydrothermal treatment in an autoclave at 300°C. Samples selected for this study are kukersite and Ordovician Dictyonema shale from the Baltics, Domanik oil shale from the Ukhta region, Upper Permian brown coal from the Pre-Ural foredeep, carbonaceous shale from the Oxfordian horizon of the Russian plate, and Upper Jurassic oil shales from the Sysola oil shale bearing region. The rocks contain type I, II, III, and II-S kerogens. The highest yield of extractable bitumen is achieved for Type II-S kerogen, whereas Type III kerogen produces the lowest amount of bitumen. The stages of organic matter thermal maturation achieved during the experiments correspond to a transition from PC_2–3 to MC_1–2. The ¹³C NMR data on kerogen indicate that the aromatic structures of geopolymers underwent significant changes.