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.     

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.     

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.     

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.     

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