Identification and significance of 3β-alkyl steranes in the Eogene lacustrine sediments and petroleum of China

The study on steranes with substitution at C-2, C-3 or C-4 positions in ring-A has become a hot field for their specific precursors in recent years[1-15]. Summons et al. firstly identified C28-C30 3β-methyl steranes in Michigan Basin and found that the p…     

Molecular fossils and oil-source rock correlations in Tarim Basin, NW China

The distribution of "molecular fossils" (bio-markers) of steroid compounds in extracts from some spe-cific geologic age in the Tarim Basin have been analyzed andare used as the fingerprints for the oil-source rock correla-tion. Having been affected by maturation, migration, phasefractionation and biodegradation, not any molecular fossilsrelated to source and environment can be used as the finger-prints for oil-source rock correlation. Some special bio-markers widely existed in the extracts from Cambrian andOrdovician rocks in the Tarim Basin and showed obviousdifference in each stratum, including dinosteranes (C₃₀),4-methyl-24-ethyl-cholestanes (C₃₀) and their aromatizedsteroids, C₂₄-norcholestanes and C₂₈ steranes originated fromdinoflagellates and diatom. Few oils such as the heavy oildrilled in the Cambrian reservoir from Tadong 2 well (TD2)correlated well with the extracts from the Cambrian. Theamazing similarity of the relative contents of these com-pounds between the marine oils produced in Tazhong andTabei uplifts and the extracts from the Upper Ordoviciansuggests that the Middle-Upper Ordovician is the very likelymain source for the marine oils.     

Relationship between climatic conditions and the relative abundance of modern C3 and C4 plants in three regions around the North Pacific

Using −24‰ and −14‰ as the endpoints of stable carbon isotopic composition of total organic carbon (δ¹³C_TOC) of surface soil under pure C₃ and C₄ vegetation, and surface soil δ¹³C_TOC data from eastern China, Australia and the Great Plains of North Amer- ica, we estimate the relative abundance of C₃/C₄ plants (i.e., the ratio of C₃ or C₄ biomass to local primary production) in modern vegetation for each region. The relative abundance of modern C₃/C₄ vegetation from each region is compared to the corresponding climatic parameters (mean annual temperature and precipitation) to explore the relationship between relative C₄ abundance and climate. The results indicate that temperature controls the growth of C₄ plants. However, even where temperature is high enough for the growth of C₄ plants, they will only dominate the landscape when precipitation declines as temperatures increase. Our results are consistent with those of other investigations of the geographic distribution of modern C₄ plant species. Therefore, our results provide an important reference for interpretation of past C₃/C₄ relative abundance records in these three regions.     

Molecular organic geochemical peculiarities of lacustrine core sediments in Fildes Peninsula,King George Island,Antarctica

The peculiarities of lipids in lacustrine core sediments from Fildes Peninsula, King George Island, Antarctica, are reported. The origin of high-molecular-weight n-alkanes, alkenes, fatty acids, alcohols Is supposed to be related to the abundant moss occurring in the sediments. Abundantiso- andanteiso-fatty acids that occur in the sediments indicate a relatively strong bacterial activity in non-ice-cover zone than in other regions in Antarctica. The C_18:2 unsaturated fatty acids mainly originate from autochthonous algae in the sediment pool, and the C_18:2C_18:0 ratios in the profile are controlled more by paleotemperatures than source assemblies. Long-chain alkenones were identified in freshwater lacustrine sediment in Antarctica for the first time, of whichPrymnesiophyceae is assumed to be the source organism. High relative abundance of C_37:4 homologue in long-chain alkenone well correlates to the severely cold climate in Antarctica.

     

内蒙古乌达地区上古生界不同沉积环境煤系烃源岩分子地球化学特征及意义

内蒙古乌达地区上古生界煤系烃源岩主要在海陆过渡相和陆相沉积环境的控制下沉积而成。精选出五种沉积环境下形成的煤系烃源岩样品,深入研究其分子地球化学特征。研究表明,泛滥平原形成的烃源岩以高氧芴、高姥鲛烷、高C29规则甾烷、高C19三环萜烷和高C24四环萜烷为主要特征;河控三角洲除氧芴含量稍低,具有和泛滥平原沉积环境相似的地化特征;潮坪和泻湖相的烃源岩中伽马蜡烷、植烷含量均较高,C26+三环萜烷丰度相对较高,三环萜烷绝对浓度大,规则甾烷以C27占优势,硫芴含量相对较丰富;潮控三角洲除Pr/Ph较高外,其余生物标志物特征与潮坪、泻湖环境下的烃源岩相似。     

Stable carbon isotope compositions of isoprenoid chromans in Cenozoic saline lacustrine source rocks from the Western Qaidam Basin,NW China: Source implications

Although the distribution of mono-,di-and trimethylated 2-methyl-2-(4,8,12-Trimethyltrideeyl) chromans(MTTCs) is well understood as an indicator of water salinity,their origin and formation mechanism are still ambiguous and under debate.In this paper,abnormally high levels of MTTCs were detected in Cenozoic saline lacustrine source rocks from the Western Qaidam Basin.Using a two-step column chromatography method,the MTTCs and naphthalenes were separated from other aromatic compounds and concentrated in one fraction,so that the stable carbon isotope compositions of these compounds could be accurately measured.Similar carbon isotope ratios for the mono-,di-and trimethylated 2-methyl-2-(4,8,12-Trimethyltrideeyl) chromans in a given sample suggest the MTTCs may share the same biological source(s).The MTTCs from the Western Qaidam Basin have similar carbon isotope compositions to primary producer-derived pristane and phytane.However,the ? 13C values for the MTTCs showed significant differences with other primary producers-derived biomarkers(e.g.C27 and C29 steranes),probably indicating a distinct microalgae source from specific niches for the MTTC compounds.We speculate that the MTTCs distribution patterns may be controlled by the water chemistry dynamics in niches,which,in turn,is affected by hypersaline bottom water.     

High molecular weight (C35 +) n-alkanes of Neogene heavily biodegraded oil in the Qianmiqiao region, North China

With wax content of 1.62%, heavy oil has been produced from the sandstone reservoirs of Neogene Guantao Formation (Ng1^Ⅲ). In the GC and GC-MS RIC profiles of its aliphatic fraction, n-alkanes are totally lost, which shows the result of heavy biodegradedation. However, the remaining trace C₁₃－C₃₆ n-alkanes can be still seen from its m/z 85 mass chromatogram. In addition, a complete series of C₃₅－C₇₃ high molecular weight (HMW) n-alkanes was detected by high-temperature gas chromatography (HTGC). The HMW n-alkane series shows a normal distribution pattern, a major peak at nC₄₃, obvious odd-carbon-number predominance, CP_I37—55 and OEP_45—49 values up to 1.17 and 1.16—1.20 respectively. The present study not only has conformed the strong resistibility of HMW n-alkanes to biodegradation in crude oils as concluded by previous researchers, but also has provided some significant information on source input and maturity for the heavily biodegraded oil in the Qianmiqiao region.     

Comparison of the carbon isotope composition of total organic carbon and long-chain <Emphasis Type="Italic">n</Emphasis>-alkanes from surface soils in eastern China and their significance

Surface soil samples collected over a high spatial resolution in eastern China were analyzed for carbon isotope composition （δ^13C） of total organic carbon （TOC） and higher plant-derived long-chain n-alkanes, with the latter reported as weighted mean values. The two sets of δ^13C values are significantly correlated and show similar trends in spatial variation. The spatial distribution of δ^13C shows less negative values in the mid-latitudes between 31°N and 40°N and more negative ones at higher and lower latitudes. This is consistent with previously reported carbon isotope data from surface soil phytoliths in the same region and suggests that the mid-latitude area provides relatively favorable growing condi- tions for C4 plants. Furthermore, δ^13C values of both TOC and long-chain n-alkanes from 12 surface soil samples collected from a small grassland in north China displayed similar carbon isotope values and the difference between paired δ^13C of a soil samples remains relatively constant. Our data demonstrate that in eastern China, soil δ^13C composition of both TOC and long-chain n-alkanes is effective indicators of C3/C4 ratios of the prevailing vegetation. This work suggests that -22‰ and -32‰ are good es- timated end members for the weighted mean δ^13C values of long-chain n-alkanes （C27, C29 and C31 n-alkanes） from soils under dominant C4 or C3 vegetation, allowing us to reconstruct paleovegetation trends.     

Spontaneous transformation of C60Br24 to C60 in solvents

It was found that C₆₀Br₂₄ transforms to C₆₀ spontaneously in some solvents. The intermediate, C₆₀Br₈, can be separated in some cases. When the bromine trap, phenol, was added,p-bromophenol and other brominated product were obtained and identified. The influences of time, the kind of solvent, etc were investigated. Possible mechanism was suggested. Supported by the National Natural Science Foundation of China Liu Ying: born in 1969, Ph. D     

Distribution of C4 plants along the Northeast China transect and its correlation to the environmental factors

The distribution of C₄ plants and the C₄/C₃ ratio along Northeast China transect (NECT) have been studied. A model to quantify their relation with environmental factors has been set up. The ratio of the number of C₄ plants to that of C₃ plants along NECT shows two low and two high trends from east to west, and their distribution is mainly decided by annual average temperature and precipitation.     

The occurrence and significance of C25HBI in Cenozoic saline lacustrine source rocks from the Western Qaidam Basin,NW China

The saline lacustrine deposit of the Oligocene Lower Ganchaigou Formation is the main source rock for the Western Qaidam Basin,NW China.In this study,abundant highly branched isoprenoids with 25 carbon atoms (C25HBI) were detected in the upper section of the Lower Ganchaigou Formation.C25HBI is a biomarker for diatoms,and can provide information regarding biogeochemical processes during production and preservation of sedimentary organic matter.The carbon isotopic values of C25HBI in these source rocks were in the range of-18‰ to-20‰.The relative enrichment in 13C of C25HBI suggests that these isoprenoids were derived from diatom blooms.In this case,the diatoms used HCO3-as an additional carbon source to dissolved CO2 in water column due to their high biological productivity.Conversely,the diatom blooms indicated an abundant nutrient supply to the environment and high primary productivity.Thus,the occurrence of 13C-enriched C25HBI could be an ideal marker for good source rocks.The decreased concentration of CO2(aq) in the water column induced by diatom blooms can result in an enrichment in 13C of organic matter synthesized by primary producers.This may be a possible reason for the occurrence of 13C-enriched organic matter in the Cenozoic source rocks of the Western Qaidam Basin.Previous studies have shown that the high carbon number n-alkanes sourced from diatoms have no carbon preference.Hence,the sources of n-alkanes in regional source rocks are complex,and should be considered when discussing the sources of organic matter in future studies.     

Vegetation evolution on the central Chinese Loess Plateau since late Quaternary evidenced by elemental carbon isotopic composition

There are many controversial issues in loess studies such as natural vegetation types on the Chinese Loess Plateau during the historical periods and the spatial and temporal evolution of C3/C4 plants. Elemental carbon isotopic composition （δ^13Cec） in the loess section may offer new evidence for these problems. Elemental carbon （EC） is produced by incomplete combustion of vegetation, and its carbon isotopic composition has a very small difference from that of the formal vegetation, then δ^13Cec can be used as a record to recover the changes of vegetation. Elemental carbon was extracted by applying the oxidation method from the Ioess-paleosol sequence in the central Chinese Loess Plateau, and its car- bon isotope composition was analyzed by the isotope mass spectrometer. The results showed that the vegetation in this region was a mixed type of C3 and C4 plants, dominated with C3 plants in most of the time. Since late Quaternary, C3/C4 plants may not follow a simple glacial-interglacial cycle mode on the Chinese Loess Plateau, but showing fluctuations. C3 plants increased gradually in L4 period, and more C3 plants occurred during $3 period, and C4 plants increased again during L3-- L2 periods, after that, Cs plants dominated again during S1 --S0 periods. During periods of paleosol development, C3 plants were abundant in S3 and S1, and there were more Ca plants in S2 and SO. During periods of loess sedimen- tation, there were more C3 plants in L4 and L1, and there were more C4 plants in L3 and L2. On the orbital timescale, the vegetation variations revealed by δ^13Cec record are consistent with the results of pollen data and also similar to the results obtained by organic carbon isotopic composition since the last glacial period.     

The specific carbon isotopic compositions of branched and cyclic hydrocarbons from Fushun oil shale

Various branched and cyclic hydrocarbons are isolated from the Fushun oil shale and their carbon isotopes are determined. The analytical results show that the branched and cyclic hydrocarbons are fully separated from n-alkanes by 5 A molecular-sieve adduction using long time and cold solvent. The branched and cyclic hydrocarbon fraction obtained by this method is able to satisfy the analytic requests of GC-IRMS. The carbon isotopic compositions of these branched and cyclic hydrocarbons obtained from the sample indicate that they are derived from photoautotrophic algae, chemoautotrophic bacteria (-33.4‰— -39.0‰) and methanotrophic bacteria (-38.4‰— -46.3‰). However the long-chain 2-methyl-branched alkanes indicate that their carbon isotopic compositions reflect biological origin from higher plants. The carbon isotopic composition of C30 4-methyl sterane (-22.1‰) is the heaviest in all studied steranes, showing that the carbon source or growth condition for its precursor, dinoflageilate, may be different from that of regular steranes. The variation trend of δ^13C ValUes between isomers of hopanes shows that ^13C-enriched precursors take precedence in process of their epimerization. Methanotrophic hopanes presented reveal the processes of strong transformation of organic matter and cycling of organic carbon in the water column and early diagenesis of oil shale.     

Ch4-rich fluid inclusions in the Yushigou mantle peridotite and their implications, North Qilian Mountains, China

Studies were carried out on the early phase of fluid inclusions which occur in residual olivines in harzburgite from the Yushigou ophiolitic mantle peridotite, the North Qilian Mountains. Components of these inclusions, analyzed by micro laser Raman spectroscopy, are dominantly CH, (70%–95%) with minor H₂, N₂, H₂S, CO₂,C₂H₄, C₂H₆, and C₃H₆ but there are no H₂O,CO and SO₂. The highly CH₄-rich fluid was probably derived from an ancient deep mantle. This discovery plays an important role in all-round understanding of the fluid property of the upper mantle, especially the ancient oceanic upper mantle.     

Formation of substitutional metallofullerenes by laser ablation of externally doped fullerenes C60M x (M=Sm, Pt, Ni and Rh)

Photofragmentation study of metal fullerides C₆₀M _x (M = Sm, Pt, Ni and Rh) by excimer laser ablation-TOF mass spectrometry shows that metallofullerenes C_2n M and C_2n+1M formed in both the positive and negative ionic modes. The isotopic distributions of the metallofullerenes C_2n M and C_2n+1M are consistent with the calculated spectra based on the natural abundance of isotopes of C and M, confirming the formation of metallofullerenes. The metal atom is supposed to be incorporated into the network of the fullerene cage to replace one carbon atom of the cage forming substitutional metallofullerene. Odd-numbered high carbon clusters are observed in our laser ablation study of all the metal fullerides in the negative ion channel. Evolution of mass spectrum with irradiation laser shots shows that the formation of the substitutionally doped fullerenes is closely related to the production of metal carbide (MC). The structures, as well as formation mechanism, of metallofullerenes C_2n+1M and C_2n M with even and odd numbers of total atoms respectively are discussed based on the structural optimization of the odd-numbered high carbon clusters.     

Spatial and temporal variations of C3/C4 relative abundance in global terrestrial ecosystem since the Last Glacial and its possible driving mechanisms

The primary factor controlling C 3 /C 4 relative abundance in terrestrial ecosystem since the Last Glacial has been widely debated. Now more and more researchers recognize that climate, rather than atmospheric CO 2 concentration, is the dominant factor. However, for a specific area, conflicting viewpoints regarding the more influential one between temperature and precipitation still exist. As temperature and precipitation in a specific area usually not only vary within limited ranges, but also covary with each other, it is difficult to get a clear understanding of the mechanism driving C 3 /C4 relative abundance. Therefore, systematic analysis on greater spatial scales may promote our understanding of the driving force. In this paper, records of C3/C4 relative abundance since the Last Glacial on a global scale have been reviewed, and we conclude that: except the Mediterranean climate zone, C3 plants predominated the high latitudes during both the Last Glacial and the Holocene; from the Last Glacial to the Holocene, C4 relative abundances increased in the middle latitudes, but decreased in the low latitudes. Combining with studies of modern process, we propose a simplified model to explain the variations of C3 /C4 relative abundance in global ecosystem since the Last Glacial. On the background of atmospheric CO2 concentration since the Last Glacial, temperature is the primary factor controlling C3/C4 relative abundance; when temperature is high enough, precipitation then exerts more influence. In detail, in low latitudes, temperature was high enough for the growth of C4 plants during both the Last Glacial and the Holocene; but increased precipitation in the Holocene inhibited the growth of C4 plants. In middle latitudes, rising temperature in the Holocene promoted the C4 expansion. In high latitudes, temperature was too low to favor the growth of C4 plants and the biomass was predominated by C3 plants since the Last Glacial. Our review would benefit interpretation of newly gained records of C3/C4 relative abundance from different areas and different periods, and has its significance in the understanding of the driving mechanisms of C3/C4 variations on longer timescales (e.g., since the late Miocene) with reliable records of temperature and atmospheric CO2 concentration.     

The formation and stability of Si1-x C x alloys in Si implanted with carbon ions

Si_1-xC_x alloys of carbon (C) concentration between 0.6%—1.0% were grown in Si by ion implantation and high temperature annealing. The formation of Si_1-xC_x alloys under different ion doses and their stability during annealing were studied. If the implanted dose was less than that for amorphizing Si crystals, the implanted C atoms would like to combine with defects produced during implantation and it was difficult to form Si_1-xC_x alloys after being annealed at 850℃. With the increment of implanted C ion doses, the lattice damage increased and it was easier to form Si_1-xC_x alloys. But the lattice strain would become saturate and only part of implanted carbon atoms would occupy the substitutional positions to form Si_1-xC_x alloys as the implanted carbon dose increased to a certain degree. Once Si_1-xC_x alloys were formed, they were stable at 950℃, but part of their strain would release as the annealing temperature increased to 1 000℃. Stability of the alloys became worse with the increment of carbon concentration in the alloys.     

东濮凹陷北部地区古近系油型气成因类型及分布特征

以热压模拟实验模拟东濮凹陷干酪根热裂解和原油热裂解过程,分析干酪根热裂解气和原油裂解气组分特征的差异,以此建立干酪根热裂解成因气和原油裂解成因气的判识方法;结合东濮凹陷北部地区天然气碳同位素及天然气组分数据,对研究区油型气进行划分,进而探讨干酪根热裂解气与原油裂解气的分布特征。结果表明,研究区古近系油型气分为干酪根热裂解气与原油裂解气,干酪根热裂解气具有相对较高的C1/C2值和较低的C2/C3值特征,而原油裂解气则与之相反。干酪根热裂解气和原油裂解气的分布存在明显差异:前者分布范围更广,后者更近于洼陷中心分布,两者分布的差异性与其生成及成藏条件差异有关。根据干酪根热裂解气与原油裂解气分布特点,推测在邻近洼陷中心区具有良好的干酪根裂解成气及原油裂解成气的条件和油型气勘探前景。     

Study on sterols from brown algae （Sargassum muticum）

Significance of sterol compounds in studying bio- logical precursors, evolution and application of bio- markers in geological bodies and fossil fuels such as oil and coal resulted in a research upsurge during the 1970s―1980s, and a lot of papers were the…     

Sequential extraction and compositional analysis of oil-bearing fluid inclusions in reservoir rocks from Kuche Depression, Tarim Basin

The free oils, adsorbed oils and oil-bearing fluid inclusions have been extracted separately and analyzed by GC and GC-MS in reservoir rock samples collected from the Kuche Depression. The results demonstrate that the molecular compositions of oil-bearing fluid inclusions are significantly different from those of the free oils (the current oils). Compared with the current oil, the oil-bearing fluid inclusions are characterized by relatively high values of parameters Pr/nC17and Ph/nC18, low values of Pr/Ph, hopanes/steranes, C30-diahopane/C30-hopane and Ts/Tm, low content of C29Ts terpane and high maturities as indicated by C29 steranes 20S/(20R+20S). In addition, the oil-bearing fluid inclusions correlate very well with the oils in northern and central Tarim Basin, which were derived from Cambrian-Ordovician marine source rocks. The adsorbedoils appear to be an intermediate type between free oils and oil-bearing fluid inclusions. The above analytical data indicate that there are at least two oil-charging episodes for these reservoir rock samples. The early charging oils were derived from Cambrian-Ordovician marine source rocks, and the later charging oils, from Triassic-Jurassic terrestrial source rocks. The primary marine oils were overwhelmingly diluted by the following terrestrial oils.