Organic geochemical characterization and depositional paleoenvironments of the Devonian strata in Guizhong region,Guangxi, China

Devonian strata in the Guizhong region, Guangxi, China consist mainly of marine deposition carbonates (limestones and dolomites) as well as restricted clastic rocks. The strata thickness is measured as 2041m and classified into 14 formations. An integrated field geological, petrographic, sedimentological, palaeobiological, and geochemical study of these strata reveals that the depositional paleoenvironments were carbonate tidal flat, restricted platform, open platform, shelf, platform margin slop, and algal herm. The total organic carbon (TOC) of sedimentary organic matter ranges between 0.07% and 1.96% with average of 0.21%. The organic matter types are Type I and Type III. The vitrinite reflectance (R ₀) of kerogen ranges between 0.99% and 2.03%, indicating the maturated and highly maturated stages. The analytical results of the representative samples collected from shelf and subtidal facies show that the differences of organic matter type and biological diagnostic compounds are related to the types of source rocks deposited in different paleoenvironments.

     

Comprehensive geochemical identification of highly evolved marine carbonate rocks as hydrocarbon-source rocks as exemplified by the Ordos Basin

China’s widespread marine carbonate rock series are mostly characterized by intensive thermal evolu- tion and low abundance of organic matter, especially the Lower Paleozoic carbonate rocks have experienced multi-episodes of tectonics and prolonged history of thermal evolution, thus making it more complicatedethe development and distribution of hydrocar- bon-source rocks reflected in the sedimentary, bio- logical and geochemical facies. Consequently, it seems much less powerful to assess the …     

Genesis and organic geochemical characteristics of the carbonaceous rock stratabound gold deposits, South China

The organic matter of three different chronological major carbonaceous rock gold-bearing formations of South China (Middle Proterozoic Shangqiaoshan group of northeastern Jiangxi, Lower Cambrian Shuikou group of northern Guangxi and Devonian Shetianqiao group of eastern Hunan) and related carbonaceous stratabound gold deposits such as Jinshan, Longshui and Shixia deposits, respectively, has been characterized by organic geochemical techniques. These organic geochemical results show that the average total organic carbon (TOC) content of the three chronological carbonaceous rock gold-bearing formations of South China ranges from 0.15% to 1.56%. The thermal maturity of the organic matter of host rocks in the three gold-bearing formations is high. The micro-component of the organic matter of the host rocks consists primarily of solid bitumen and graphite. The organic carbon and gold of the host rocks appear to syndepositin situ during the formation of the gold-bearing formations. The organic carbon played a certain role in controlling the geochemical environment of the gold-bearing formations. The metallogenetic mechanism of the carbonaceous rock stratabound gold deposits of South China is closely associated in genesis with the sedimentation, diagenesis and thermal evolution history of the organic matter of host rocks in the gold-bearing formations.     

Eocene volcanism during the incipient stage of Izu–Ogasawara Arc: Geology and petrology of the Mukojima Island Group,the Ogasawara Islands

The Ogasawara Islands mainly comprise Eocene volcanic strata formed when the Izu–Ogasawara–Mariana Arc began. We present the first detailed volcanic geology, petrography and geochemistry of the Mukojima Island Group, northernmost of the Ogasawara Islands, and show that the volcanic stratigraphy consists of arc tholeiitic rocks, ultra‐depleted boninite‐series rocks, and less‐depleted boninitic andesites, which are correlatable to the Maruberiwan, Asahiyama and Mikazukiyama Formations on the Chichijima Island Group to the south. On Chichijima, a short hiatus is identified between the Maruberiwan (boninite, bronzite andesite, and dacite) and Asahiyama Formation (quartz dacite and rhyolite). In contrast, these lithologies are interbedded on Nakodojima of the Mukojima Island Group. The stratigraphically lower portion of Mukojima is mainly composed of pillow lava, which is overlain by reworked volcaniclastic rocks in the middle, whereas the upper portion is dominated by pyroclastic rocks. This suggests that volcanic activity now preserved in the Mukojima Island Group records growth of one or more volcanoes, beginning with quiet extrusion of lava under relatively deep water followed by volcaniclastic deposition. These then changed into moderately explosive eruptions that took place in shallow water or above sea level. This is consistent with the uplift of the entire Ogasawara Ridge during the Eocene. Boninites from the Mukojima Island Group are divided into three types on the basis of geochemistry. Type 1 boninites have high SiO₂ (>57.0 wt.%) and Zr/Ti (>0.022) and are the most abundant type in both Mukojima and Chichijima Island Groups. Type 2 boninites have low SiO₂ (<57.1 wt.%) and Zr/Ti (<0.014). Type 3 boninites have 57.6–60.7 wt.% SiO₂ and are characterized by high CaO/Al₂O₃ (0.9–1.1). Both type 2 and 3 boninites are common on Mukojima but are rare in the Chichijima Island Group.     

ENVIRONMENTAL RESPONSE OF THE PERMIAN VOLCANISM IN LUCAOGOU FORMATION IN JIMSAR SAG,JUNGGAR BASIN,NORTHWEST CHINA

The lithological features, the types of organic matter and its occurrence and carbon and oxygen isotopic value were clarified by combining core observation, thin slice authentication, X ray diffraction analysis, kerogen type identification and carbon and oxygen isotope analysis. The characteristic of strata, the distribution of volcanoes of Junggar Basin were also taken into consideration. A comprehensive analysis was conducted to evaluate environmental response of volcanism in Lucaogou formation in Jimsar sag. The results show that rocks is a mixed sedimentation of effusive rock and carbonate rocks, volcanic materials is widely developed in Lucaogou formation and origins from the edge of sag or distant source volcano activity. Organic matter is predominantly unstructural algae and asphaltene. The carbon isotopic value of carbonates ranges between 6.8‰ and 9.7‰ with an average of 8.3‰, featured in high positive excursions, while oxygen isotopic value varies from -11.9‰ to -4.3‰ with an average of -6.2‰. During the period of volcanic activity, the volcanic material released high amounts of nutrient to the lake basin, which is beneficial to the algae and other organic organisms. In the poor oxygen and calm water environments, the organic matter is distributed in the laminar algal and the carbon isotope value is high positive drift. During the intermittent period of volcanic activity, the lake level decreased and the lake bottom water changed to the oxygen-enriched environments. The organic matter is locally enriched or dispersed in local layers, and the carbon isotope values decreased slightly. The frequent volcanic activity promoted the organism boom, which lead carbon isotope value to have high positive characteristics and change trends.     

Selective chemical degradation of kerogen from Nenjiang Formation of the southern Songliao Basin

A sequential selective chemical degradation has been performed on the kerogen from the Nenjiang Formation of the southern Songliao Basin by using a series of mild chemical degradations (alkaline hydrolysis, cleavage of ether-bonds and sulfur-bonds, and ruthenium tetroxide (RuO4) oxidation). Subsequently, the GC-MS analyses are carried out on different degradation products. The results show that chemical degradations can release a great number of GC/MS-determinable biomarkers from insoluble kerogen, such as, alkaline hydrolysis products mainly comprise n-alkanes, fatty acids and alkanols; thiophene compounds are predominantly ether-bound to kerogen matrix; the products from the cleavage of sulfur-sulfur and sulfur-carbon bonds in the kerogen include fatty acids, alkanols and some n-alkanes with high carbon numbers; RuO4 oxidation products are predominantly monocarboxylic acids and α,ω-dicarboxylic acids. The distributions of main degradation products indicate that organic matter in this kerogen is predominantly derived from algae and bacteria, and that small amounts of high plant-derived organic matter are possibly combined into kerogen matrix at the late stage by sulfur bonds and other means. This study will provide an important approach for further discussing sources of organic matter in source rocks and their depositional paleoenvironments.     

Magnetic and mineralogical characteristics of hydrocarbon microseepage above oil/gas reservoirs of Tuoku region, northern Tarim Basin, China

The Tuoku region in northern Tarim Basin of China is a key area for studying oil/gas reservoir rocks. The magnetic and mineralogical parameters of well cuttings from two wells, well S₇, situated on oil/gas field, and well S₆, at an oil/water interface, were measured. The two wells are located in the same structure with similar strata and types of lithology, but well S₆ is a showing well of oil and gas 5 km northwest of well S₇. The purpose of this paper is to evaluate the possibility and distribution of secondary magnetic alteration that may have occurred due to hydrocarbon migration above an oil/gas accumulation. It is concluded that the magnetism of well cuttings from major strata in well S₇, including source rocks, oil reservoir rocks and cap rocks, and in Quaternary (Q) soil is higher than that from well S₆. The Cambrian oil-bearing strata and cap rocks have even higher magnetism in well S₇. The shape and parameters of magnetic hysteresis loops indicate that soft (H _c<20 mT,H _s<0.3 T) ferrimagnetic components dominate the magnetic carriers within the strongly magnetic strata of well S₇, whereas a mixed paramagnetic and ferrimagnetic distribution occurs in well S₆ (for example, low coecivityH _c and nonsaturating magnetized character). Analysis of heavy minerals shows that the contents of iron oxide (magnetite, maghemite and hematite) in well S₇ are often higher than those in well S₆. The magnetite content in samples of cuttings from Cambrian rocks can reach 9.7% in oil-bearing strata in well S₇, and in strata Ekm and N₁j are 1.215% and1.498%, respectively. Typical spherical magnetite grains are found within the main source rocks and the soils in well S₇. By analysis of surface microtexture and of trace element contents, we infer that the spherical magnetite is composed of aggregates of ultrafine particles that are probably authigenic magnetite formed in a hydrocarbon halo background. Project supported by the National Natural Science Foundation of China and the Geological Industry Foundation of China (Grant No. 49374216) and Foundation of the State Key Laboratory of Oil/Gas Reservoir Geology and Exploitation of China (Grant No. 9608).     

The relationship between methylated chromans and maturity of organic matter in the source rocks from Jianghan hypersaline basin

The distribution, the relative composition, and their variation of methylated chromans (MTTC) with maturation of organic matter in the source rocks from Mingjia 1 well, Jianghan hypersaline basin, have been analyzed and studied. There is a close relationship between the distribution of methylatd chromans and paleosalinity of depositional environment. In the Eq ₁–Eq ₃ source rocks deposited under hypersaline environment with lower Pr/Ph ratio, higher gammacerane index and abundant organic sulfur-containing compounds, methyl MTTC and dimethyl MTTC are main components, but in the Eq ₄ source rocks deposited non-hypersaline environment with relatively higher Pr/Ph ratio, lower gammacerane index and organic sulfur-containing compounds, trimethyl MTTC is a major compound in methylate chromans. Obviously, the distributions and the compositions of methylated chromans in the source rocks are in harmony with biomarker indicators indicating the paleaosalinity and redox of depositional environment. The relationship between the ratio of 5,8-dimethyl MTTC to 7,8-dimethyl MTTC (dimethyl MTTC ratio) and the maturity of organic matter in the source rocks from Mingjia 1 well has been discussed. The results show that dimethyl MTTC ratio increased with the burial depth and the maturation of organic matter in the source rocks, especially in immature level. It is noteworthy that this ratio is very susceptible to small variation in the maturity of organic matter in the source rocks when vitrinite reflectance R _o is less than 0.65%. It may imply that this ratio is a good and susceptible aromatic maturity indicator for the relative maturity of organic matter in immature source rocks.     

Geochemical constraints on the depositional environment of the 1.84 Ga Embury Lake Formation,Flin Flon Belt,Canada

The Flin Flon Belt of Canada contains Paleoproterozoic volcanic–sedimentary sequences that are related to the Trans‐Hudson Orogeny. The sequences include island arc volcanic and volcaniclastic rocks (Amisk Group) that are unconformably overlain by subaerial sedimentary rocks (Missi Group), and younger deep facies sediments. In the Flin Flon area, several north–south trending faults divide the sequences into blocks and obscure the depositional environment of the deep facies sediments. Locally, within the Flin Flon area, the Embury Lake Formation is in fault contact with island arc volcanic–sedimentary sequences of the Amisk and Missi Groups. To identify the depositional environment of the Embury Lake Formation, we used lithologic and geochemical approaches. Here, we report carbon isotopic values in organic matter (δ¹³C_org) and sulfur isotopes (δ³⁴S), as well as total organic carbon and total sulfur measurements for the black shale in the formation. Samples were taken from a drill core that contains alternating bands of sandstone and black shale. Pyrite in the black shale is divided into four textural types: euhedral, vein‐type, elliptical, and microcrystalline. Microcrystalline pyrite is typically generated by microbially mediated sulfate reduction. An extremely low S/C ratio (avg. = 0.04) is consistent with lacustrine deposition. The ranges of δ¹³C_org (?36 ‰ to ?27 ‰) and δ³⁴S (+3.0 ‰ to +7.7 ‰) values can be explained by bacterial photosynthesis that involved Calvin cycle and acetyl CoA pathways, and sulfate reduction in a low‐sulfate environment. Considering the depositional age reported in a previous study of < 1.84 Ga, the Embury Lake Formation was likely emplaced in a lacustrine setting during the Trans‐Hudson Orogeny.     

Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks,Japan, a possible high-pressure dehydration product of antigorite serpentinite

Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks (HMR), west-central Kyushu, Japan, may be formed by high-pressure dehydration of antigorite, and may indicate deep subduction of serpentinite reaching a pressure–temperature condition of 1.6 GPa and 740–750 °C. Three rock types have been identified based on mineral assemblage and rock texture: Type I (L) consisting of medium-grained (1–5 cm long) olivine + enstatite + chromite ±tremolite with secondary talc and anthophyllite that occurs in low-grade metamorphic rocks of the biotite zone, Type I (H) of coarse-grained (up to 10 cm long) olivine + enstatite (with clinoenstatite lamella) + chromite ±tremolite with secondary talc that occurs in high-grade metamorphic rocks of the garnet-cordierite zone, and Type II composed of Al-spinel + chlorite + olivine + apatite + ilmenite with minor sodic gedrite in the garnet-cordierite zone together with Type I (H). Olivines in all rock types are mostly serpentinized during exhumation. The chromite-olivine thermometer gives 560–690 °C for Type I (L) rocks, and the spinel-olivine thermometer gives 610–740 °C for Type II rocks. The peak metamorphic pressure will be higher than 1.6 GPa based on the location of the experimentally determined invariant point (P = 1.6 GPa and T = 670 °C) of antigorite + forsterite + enstatite + talc + H₂O. This estimate is consistent with the occurrence of chlorite in Type II rocks, which is stable up to 890 °C at 2.0 GPa. The spinifex-like textured metaperidotites occur as small bodies in the low P/T type gneisses, implying tectonic juxtaposition of them probably during exhumation of the HMR. Recent findings of medium pressure (0.9–1.2 GPa) granulites and gneisses from the HMR may indicate that the HMR has a deep root into the wedge mantle from which the spinifex-like textured metaperidotites have derived.     

Presence of carboxylate salts in marine carbonate strata of the Ordos Basin and their impact on hydrocarbon generation evaluation of low TOC, high maturity source rocks

The total organic carbon(TOC)in the marine source rock of the Ordos Basin mostly ranges from 0.2%to 0.5%.The industrial standard commonly states that the TOC value has to be no less than 0.5%(0.4%for high mature or over-mature source rock)to form large petroleum reservoirs.However,gas source correlation indicates that the natural gas in the Jingbian gas field does receive contribution from marine source rocks.In order to determine the effect of carboxylate salts(or called as organic acid salts)on TOC in highly mature source rocks with low TOC value,we sampled the Ordovician marine source rock and the Permian transitional facies source rock in one drilled well in the southern Ordos Basin and performed infrared and GC-MS analysis.It is found that both kerogen-derived organic acids and carboxylate salt-conversed organic acids exist in both marine and transitional facies source rocks.The carboxylate salt-conversed organic acids mainly come from the complete acidification of carboxylate salts,which confirms the presence of carboxylate salts in the marine source rocks.Although the C16:O peak is the main peak for the organic acids both before and after acidification,the carboxylate salt-conversed organic acids have much less relative abundance ahead of C16:O compared with that of the kerogen-based and free organic acids.This observation suggests that the kerogen-based and free organic acids mainly decarboxylate to form lower carboxylic acids,whereas the carboxylate salt-conversed organic acids mainly break down into paraffins.By using calcium hexadecanoate as the reference to quantify the kerogen-derived and carboxylate salt-conversed organic acids,the high TOC(>2.0%)marine source rocks have low carboxylate salt content and the low TOC(0.2%–0.5%)marine source rocks contain high content of carboxylate salt.Therefore,for the marine source rocks with 0.2%–0.5%TOC,the carboxylate salts may be a potential gas source at high maturity stage.     

Effect of depositional processes on the origin and composition of organic matter: Examples from the Pleistocene sediments in the Choshi core, Boso Peninsula

Abstract   Both marine and terrigenous organic matter are deposited in shelf and continental slope environments. In the present study, the relationship between environmental changes in the Choshi area and the sedimentation of organic matter was examined. The sediments of the Choshi core were deposited on a shelf environment and their lithology and ichnofacies, as well as the composition of the contained kerogen (insoluble organic matter) indicate a shallowing upward succession. The organic matter preserved in the sediments is of both marine and terrigenous origin, on the basis of C/N ratios (5.90–9.45), δ¹³C values (−21.6‰−24.6‰) and kerogen microscopy. The total organic carbon (TOC) content (0.39–1.08%) of the sediments shows a positive correlation with the increase of terrigenous organic matter before 500 ka, but decreases (0.26–0.61%) after 500 ka as the shelf environment becomes shallower because of dilution, caused by the input of terrigenous inorganic clasts, and oxidation. The variation in TOC contents was thus influenced by the increasing sedimentation rate of terrigenous materials, including both organic and inorganic particles as the basin filled.     

中尺度流域次降雨洪水过程模拟——以太湖上游西苕溪流域为例

古近系沙河街组是阳信洼陷最重要的生烃层段.其湖盆充填具阶段性和沉积演化的旋回性,形成了由沙四段、沙三段与沙一段组成的复合生烃系统和不同的地球化学特征.利用Rock-Eval生油岩评价仪、色谱-质谱仪等实验分析技术对不同层系样品进行了地球化学特征分析研究,其中生烃潜力指标包括有机碳含量(TOC)、残余生烃潜量(S1 S2)、氢指数IH、氯仿沥青"A";成熟度指标包括热解峰温Tmax、奇偶优势参数OEP、饱和烃轻重比(C21 C22)/(C28 C29)及∑C21-/∑C22 、镜质体反射率Ro、莫烷/藿烷及甾烷生物标志物参数C29ββ(ββ αα)、C29ααα20S(20S 20R);古环境指标包括异构烷烃参数Pr/Ph、Pr/n-C17、Ph/n-C18及伽马蜡烷等.结果表明,沙四段为弱还原-还原性的半深湖沉积,沉积了一套中等厚度、分布局限的烃源岩(TOC平均为1.5%),其较深位置的烃源岩基本进入成熟阶段,多形成成熟油;沙三段属弱还原-还原性的半深湖-深湖环境,其烃源岩中有机质丰度较高(TOC平均为3.5%),大部分烃源岩处于未成熟-低成熟状态,主要形成未熟油;沙一段为湖水咸化、还原性的半深湖相环境,其烃源岩中有机质丰度高(TOC平均为5%左右),但处于未成熟阶段,主要生成生物气.     

Deposition, mineralization, and storage of carbon and nitrogen in sediments of the far northern and northern Great Barrier Reef shelf

The flow of carbon and nitrogen in sediments of the far northern and northern sections of the Great Barrier Reef continental shelf was examined. Most of the organic carbon (81–94%) and total nitrogen (74–92%) depositing to the seabed was mineralized, with burial of carbon (6–19%) and nitrogen (8–20%) being proportionally less on this tropical shelf compared with other non-deltaic shelves. Differences in carbon and nitrogen mineralization among stations related best to water depth and proximity to river basins, with rates of mineralization based on net ∑CO₂ production ranging from 17 to 39 ( mean=23) mmol C m⁻² d⁻¹. The overall ratio of O₂:CO₂ flux was 1.3, close to the Redfield ratio, implying that most organic matter mineralized was algal. Sulfate reduction was estimated to account for ≈30% (range: 6–62%), and denitrification for ≈5% (range: 2–13%), of total C mineralization; there was no measurable CH₄ production. Discrepancies between ∑CO₂ production across the sediment–water interface and sediment incubations suggest that as much as 5 mmol m⁻² d⁻¹ (≈25% of ∑CO₂ flux) was involved in carbonate mineral formation. Most microbial activity was in the upper 20 cm of sediment. Rates of net NH₄⁺ production ranged from 1.6 to 2.7 mmol N m⁻² d⁻¹, with highly variable N₂ fixation rates contributing little to total N input. Ammonification and nitrification rates were sufficient to support rapid rates of denitrification (range: 0.1–12.4 mmol N m⁻² d⁻¹). On average, nearly 50% of total N input to the shelf sediment was denitrified. The average rates of sedimentation, mineralization, and burial of C and N were greater in the northern section of the shelf than in the far northern section, presumably due to higher rainfall and river discharge, as plankton production was similar between regions. The relative proportion of plankton primary production remineralized at the seafloor was in the range of 30–50% which is at the high end of the range found on other shelves. The highly reactive nature of these sediments is attributed to the deposition of high-quality organic material as well as to the shallowness of the shelf, warm temperatures year-round, and a variety of physical disturbances (cyclones, trawling) fostering physicochemical conditions favorable for maintaining rapid rates of microbial metabolism. The rapid and highly efficient recycling of nutrients on the inner and middle shelf may help to explain why the coral reefs on the outer shelf have remained unscathed from increased sediment delivery since European settlement.     

Subaquatic volcanic eruptions in continental facies and their influence on high quality source rocks shown by the volcanic rocks of a faulted depression in Northeast China

This paper reports the analysis on cores and rock slices,data on seismic and logging activities,characteristics of core samples,and the paleogeographic background of the Yingcheng Formation of the Xujiaweizi faulted depression in the Songliao Basin.The results show that some of the volcanic rocks were formed during subaquatic eruptions.These subaqueous volcanic rocks are further characterized by the interbedded black mudstone and tuffite,the presence of double-layer perlite enclosing aphyric or sparsely phyric rhyolite,the presence of a bentonite layer,and the coefficient of oxidation(Fe2O3/FeO).The types of rocks are volcanic breccia,lava breccias,perlite,rhyolite,tuff and sedimentary tuff.The subaquatic eruptions are distributed mainly in Wangjiatun,Shengping,Xuxi,Xuzhong,and Xudong.The XS-1 area is the most typical.The organic abundance of overburden mud rocks within the volcanic rocks of the Yingcheng Formation indicates that these rocks represent high-quality source rocks.The analysis also shows that continental subaquatic volcanic eruptions provide a rich supply of minerals and energies for the lake basin and increase the organic matter content in the water.Moreover,the water differentiation provides a good reducing environment for the conservation of organic matter,and is beneficial for the formation of high-quality source rocks.Finally,we propose a hypothesis to describe the mode of subaquatic eruptions and the formation of high-quality source rocks.     

Allochthonous and autochthonous organic matter in surface waters in Karelia

A relationship between indirect chemical indices of organic matter content of surface waters and light absorption in the visible and ultraviolet ranges is discussed. The allochthonous and autochthonous types of organic matter are found to essentially differ in the rate of light absorption. Techniques are proposed for the calculation of the concentrations of allochthonous and autochthonous organic matter by the integral rate of light consumption in the visible region and the bichromate oxidability of water, as well as by three indirect indices: chemical oxygen demand, permanganate oxidability, and water color index. The mean concentrations of allochthonous and autochthonous organic matter in large lakes (Ladoga, Onega, and Baikal) and in rivers and smaller water bodies in Karelia (>300 water objects) are analyzed. Allochthonous organic matter was found to predominate in most surface waters of Karelia (>80%), while allochthonous organic matter predominates only in lakes with a small specific catchment area (supposedly, <5) and in highly eutrophic lakes.     

Thermochemical sulphate reduction and Huayuan lead-zinc ore deposit in Hunan, China

In recent years, some arguments with regard to the organic mineralization of MVT lead-zinc ore deposit are focused on the thermochemical sulphate reduction in the presence of organic matter. Based on the research into the organic geochemistry and C, O, S isotopes of mineralized host rocks, mineral gas fluid inclusion and solid bitumen from Huayuan (W. Hunan, China) lead-zinc ore deposit formed in the algal limestones of Qingxudong formation, Lower Cambrian, the authors consider that a lot of organic matter occurred and participated in mineralization. The organic matter from different sources participated in the mineralization with two main forms: thermochemical sulphate reduction and thermal degradation which supplied abundant H₂S for the precipitation.     

Organic geochemical studies of sinking particulate material in China sea area(I)

Sinking particulate material collected from Nansha Yongshu reef lagoon and the continental shelf of the East China Sea by sediment traps has been analyzed and studied for the first time using organic geochemical method. The results show that about half of the sinking particulate organic matter in the two study areas are consumed before reaching the depth of 5 m to the sea floor and the degree of this consumption in Yongshu reef lagoon is larger than that in the continental shelf of the East China Sea. The distributions of hydrocarbons and fatty acids indicate that the minor difference of biological sources of sinking particulate organic matter exists between Yongshu reef lagoon and the continental shelf of the East China Sea, but they mainly come from marine plankton. Stronger biological and biochemical transformations of sinking particulate organic matter are also observed and the intensity of this transformation in Yongshu reef lagoon is greater than that in the continental shelf of the East China Sea. It is found that the occurrence of C₂₅ highly branched isoprenoid (HBI) diene may be related to the composition of diatom species.     

The distribution characteristic and its significance of compound specific isotopic composition of aromatic hydrocarbon from marine source rock and oil in the Tarim Basin,western China

Aromatic hydrocarbons are generally main distillation of crude oil and organic extract of source rocks. Bicyclic and tricyclic aromatic hydrocarbons can be purified by two-step method of chromatography on alumina. Carbon isotopic composition of individual aromatic hydrocarbons is affected not only by thermal maturity, but also by organic matter input, depositional environment, and hydrocarbon generation process based on the GC-IRMS analysis of Upper Ordovician, Lower Ordovician, and Cambrian source rocks in different areas in the Tarim Basin, western China. The subgroups of aromatic hydrocarbons as well as individual aromatic compound, such as 1-MP, 9-MP, and 2,6-DMP from Cambrian-Lower Ordovician section show more depleted 13 C distribution. The 13 C value difference between Cambrian-Lower Ordovician section and Upper Ordovician source rocks is up to 16.1‰ for subgroups and 14‰ for individual compounds. It can provide strong evidence for oil source correlation by combing the 13 C value and biomarker distribution of different oil and source rocks from different strata in the Tarim Basin. Most oils from Tazhong area have geochemical characteristics such as more negative 13C9-MP value, poor gammacerane, and abundant homohopanes, which indicate that Upper Ordovician source rock is the main source rock. In contrast, oils from Tadong area and some oils from Tazhong area have geochemical characteristics such as high 13C9-MP value, abundant gammacerane, and poor homohopanes, which suggest that the major contributor is Cambrian-Lower Ordovician source rock.     

Distribution and sources of sedimentary organic matter in a tropical estuary,south west coast of India (Cochin estuary): A baseline study

Surface sediments samples were collected from 9 stations of the Cochin estuary during the monsoon, post-monsoon and pre-monsoon seasons and were analyzed for grain size, total organic carbon (OC), total nitrogen (TN) and stable isotopic ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) to identify major sources of organic matter in surface sediments. Sediment grain size is found to be the key factor influencing the organic matter accumulation in surface sediments. The δ¹³C values ranges from ?27.5‰ to ?21.7‰ in surface sediments with a gradual increase from inner part of the estuary to the seaward side that suggest an increasing contribution of marine autogenous organic matter towards the seaward side. The δ¹⁵N value varies between 3.1‰ and 6.7‰ and it exhibits complex spatial and seasonal distributions in the study area. It is found that the dynamic cycling of nitrogen through various biogeochemical and organic matter degradation processes modifies the OC/TN ratios and δ¹⁵N to a considerable degree. The fraction of terrestrial organic matter in the total organic matter pool ranges from 13% to 74% in the surface sediments as estimated by δ¹³C based two end member mixing model.