Terrigenous Organic Matter Research Articles

Rhyolitic Ash Promoting Organic Matter Enrichment in a Shallow Carbonate Platform: A Case Study of the Maokou Formation in Eastern Sichuan Basin

Volcanic ash improves the amount of nutritive elements in the lake and ocean, but it is not fully understood how volcanic ash influences the total organic matter content (TOC) and bio-precursor, and the scope. A volcanic layer, measuring 5 cm in thickness, was examined using electron microscopy scanning, TOC/TS, rock pyrolytic, and inorganic geochemical analyses. The study shows that the TOC content in the overlying shales increases by 18 wt.% on average, and the vertical scope of influence on shales is twice as thick as the volcanic layer. Organic matter enrichment is attributed to the fact that the volcanic layer provides nutritive elements and meanwhile changes the oxidation-reduction condition. Large variations of hydrogen index (HI) and oxygen index (OI) in the lower and upper parts of the volcanic layer is related to terrestrial organic matter vanishing, due to volcanic eruption and subsequently more terrigenous organic matter migrating into the ocean. Little change of the bio-precursor in the overlying and underlying shales may be related to the fast restoration within the ocean and land ecosystems after a volcanic eruption. The decrease in terrigenous sediments indicated by Rb/Sr, Zr/Y, and ∑REE after a volcanic eruption indirectly reflects fast restoration of terrestrial vegetation boom. This study could decipher the influence of volcanic ash on the qualities of the source rock, which can provide a better understanding on discovering more economically petroleum reservoirs in igneous rocks.

Spatial distribution, compositional pattern and source apportionment of n-alkanes in surface sediments of the Bohai Sea, Yellow Sea, and East China Sea and implications of carbon sink

China's marginal seas (CMSs, including the Bohai Sea, Yellow Sea, East China Sea) are a significant sink for both terrestrial organic matter (OM) and marine OM, and they play an important role in the global biogeochemical carbon cycle. The spatial distribution and origin of organic matter based on n-alkanes in the surface sediments of CMSs and the implications of carbon sinks were comparatively analyzed. The n-alkane content in surface sediment from the Bohai Sea was higher than that of the Yellow Sea and the East China Sea. The spatial distribution trends of marine and terrestrial organic matter are obviously different in the surface sediments of China's marginal seas. The n-alkanes in the sediments of the Yellow Sea and the East China Sea were mainly derived from terrestrial higher plants, and land-based influence gradually decreased from the near shore to the open sea. Higher concentration of terrigenous OM are concentrated nearshore, especially near estuaries, such as the Yellow River Estuary, the Old Yellow River Estuary and the Yangtze River Estuary. The input of n-alkanes from woody plants in the Bohai Sea area was slightly higher than that of herbaceous plants, and the input of herbaceous plants in the Yellow Sea and East China Sea was slightly dominant. The distribution of marine organic matter is controlled by marine productivity and the sedimentary environment. Due to climate change, the decomposition and enrichment of organic matter also show the climate effect of carbon molecular combinations. As a semiclosed sea area, the Bohai Sea was beneficial to the growth and reproduction of marine phytoplankton. From the perspective of petroleum pollution, the Bohai Sea was the most serious, followed by the Yellow Sea, and the East China Sea was the lightest. The carbon burial amount of terrestrial organic matter accounts for approximately 7% of the terrestrial organic matter burial amount of global marginal sea sediments, indicating that China's marginal sea plays an important role in the global carbon cycle. The result provide a basis for further understanding the source pattern and burial preservation of sedimentary organic matter in this sea area.

Geochemical and petrological characteristics of the Middle Jurassic organic-rich siliciclastic sediments from the central part of the Polish Basin

Organic-rich claystones and mudstones can be very good source rocks for hydrocarbons. Middle Jurassic organic-rich siliciclastic horizons are known from the Polish Basin area, however, there are no hydrocarbon accumulations associated with these horizons. The main scientific goal of this paper is to propose why these horizons did not produce significant hydrocarbons quantities. To deal with it, we characterised organic matter origin, maturity level, and hydrocarbon potential of Middle Jurassic strata. Palaeoenvironmental conditions of the Middle Jurassic Polish Basin were also described. The samples were examined by geochemical (Rock-Eval and GC–MS) and petrological (macerals content, vitrinite, and fusinite reflectance values) analyses. The analyses revealed that the Middle Jurassic strata at the study area, in general, contain mixed terrigenous/aquatic organic matter with a significant content of pyrogenic origin polycyclic aromatic hydrocarbons (PAHs). Inertinite and liptinite group macerals dominate in maceral composition. This suggests that analysed samples contain a mixture of type IV and II kerogens. Most of the analysed samples contain immature or early-mature organic matter. In general, the maturity level is higher in the southern part of the study area, where higher subsidence of Polish Through allowed to reach greater burial depths. Sediments were deposited in anoxic basement conditions of the epicontinental sea with the serious influence of deltaic/estuarine systems. The water and air transport are responsible for significant input of terrigenous organic matter, mostly reworked in frequent palaeo-wildfire events. The high content of charred organic matter is the main reason why despite high total organic carbon (TOC) values, the hydrocarbon potential of analysed horizons is poor/fair. Fire maceral index (FMI) was proposed and juxtaposed with pyrolytic input ratio (PIR) to propose that PIR is sensitive mostly to higher-temperature palaeo-wildfires products.

Submarine Geomorphology and Glacimarine Sedimentary Processes Associated to Deglaciation in Europa Fjord, Chilean Patagonia

In a period of high rates of glacier retreat, increasing meltwater discharge from tidewater glaciers can influence marine fjord ecosystems due to increase sediment delivery and accumulation rates in the proglacial environment. Glacier variations and associated changes in glacimarine processes are recorded in the sedimentary record and submarine geomorphology of fjords. In October-November 2017, CIMAR23 Cruise surveyed several fjords adjacent to the Southern Patagonian Icefield (SPI). In this study, we show detailed results of the multibeam bathymetry, backscatter and sub-bottom seismic profiles, and sediment core samples from Europa Fjord, located west of the central SPI area, to improve the analysis of the fjord glacial and sedimentary evolution and to connect its behavior with the other fjords in the region. For the cores 210Pb chronology, magnetic susceptibility, organic carbon, total nitrogen, and stable isotopes (δ13Corg, δ15N) were performed. The seafloor geomorphology showed that the most prominent submarine landforms are transverse morainal ridges, which indicate past stillstand glacier positions; these features formed during the deglaciation of the fjord (latest Pleistocene), and some, perhaps more recently during short episodic advance/retreat events. In the central and mouth fjord areas, erosional features such as lineations are found on the rocky bottom and interpreted as formed by past grounded glaciers. Several submarine moraines are generally well preserved, but some parts are crumbled by slope failures and erosion. Most of the seismic sections are interpreted as the expected subglacial-ice proximal-ice distal succession of facies, characteristic of single retreat environments. The sedimentary record includes bioturbed muds with muddy laminations and variable amounts of coarse sediments interpreted as ice rafted debris, suggesting a proglacial environment, punctuated by calving. From the beginning of the twentieth century there is a tendency to an increase in the terrigenous organic material content, including an abrupt increase during the 80–90s, and a decline in the last decade. This behavior is indicating possibly a general increase of surface terrestrial runoff and, for the last decade, an increase of glacial meltwater input which would be caused by an accelerated deglaciation.

Biodegradation of Terrigenous Organic Matter in a Stratified Large-Volume Water Column: Implications of the Removal of Terrigenous Organic Matter in the Coastal Ocean.

Large amounts of terrigenous organic matter (TOM) are delivered to the ocean every year. However, removal processes of TOM in the ocean are still poorly constrained. Here, we report results from a 339-day dark incubation experiment with a unique system holding a vertically stratified freshwater-seawater column. The quality and quantity of dissolved organic matter (DOM), RNA-based size-fraction microbial communities, and environmental factors were high-frequency-monitored. Microbial processes impacted TOM composition, including an increased DOM photobleaching rate with incubation time. The mixed layer had changed the bacterial community structure, diversity, and higher oxygen consumption rate. A two-end member modeling analysis suggested that estimated nutrient concentrations and prokaryotic abundance were lower, and total dissolved organic carbon was higher than that of the measured values. These results imply that DOM biodegradation was stimulated during freshwater-seawater mixing. In the bottom layer, fluorescent DOM components increased with the incubation time and were significantly positively related to highly unsaturated, oxygenated, and presumably aromatic compound molecular formulas. These results suggest that surfaced-derived TOM sinking leads to increased DOM transformation and likely results in carbon storage in the bottom water. Overall, these results suggest that microbial transforming TOM plays more important biogeochemical roles in estuaries and coastal oceans than what we know before.

Molecular and isotopic evidence for the origin of light oils and associated gases in the onshore northeast Nile Delta

The Nile Delta is a prolific hydrocarbon province in Egypt and the eastern Mediterranean region, particularly for gas resources. However, the origin of discovered hydrocarbons from the onshore Nile Delta has not been comprehensively studied. Fourteen condensates and 10 natural gas samples retrieved from Oligocene–Pliocene pay zones in the onshore northeast Nile Delta were studied for their molecular and isotopic composition to infer origin, source lithology, organic facies, depositional environment, thermal maturity, and reservoir secondary alteration. The isotopic compositions of the analyzed condensates indicate non-marine waxy oils. Results show that these condensates have high Pr/Ph ratios (1.86–6.46), abundant C19 and C20 tricyclic terpane contents relative to the C23 homologue, elevated oleananes, paucity of gammacerane, high hopane/sterane ratios, very low abundance of homohopanes, low dibenzothiophene/phenanthrene ratios and high C29/C27 sterane ratios. Clay-rich source rocks with abundant Type-III terrigenous organic materials deposited in an oxic fluvio-deltaic setting are suggested for these condensates. Light hydrocarbon (C7) compositions indicate that all investigated condensates, except the Allium-1 sample, suffered evaporative fractionation within their reservoirs. Chemometric analysis based on 14 biomarker and isotopic results suggests 3 genetic oil families for these condensates. These oil families have geochemical characteristics that indicate various contributions of terrigenous and marine organic matter and different levels of thermal maturity. The molecular and isotopic results indicate that the onshore Nile Delta natural gases are wet-thermogenic in origin with no signs of microbial biodegradation. These gases were generated by secondary cracking of associated oils derived from Type-III and Type-II/III or Type-II kerogen. The condensate and associated gas samples from Oligocene pay zones have different geochemical signatures than those from Miocene reservoirs, suggesting derivation from different source rocks with variable levels of thermal maturity or the presence of multiple charge systems from a common source in the onshore Nile Delta.

Multi-Biomarker Analysis for Identifying Organic Matter Sources in Small Mountainous River Watersheds: A Case Study of the Yuba River Watershed

Organic matter in soils and sediments derives from a mixture of biological origins, often making it difficult to determine inputs from individual sources. Complicating the determination of source inputs to soil and sedimentary organic matter (OM) is the fact that physical and microbial processes have likely modified the initial composition of these sources. This study focused on identifying the composition of watershed-derived OM to better understand inputs to inland waters and improve our ability to resolve between terrigenous and aquatic sources in downstream systems, such as estuaries and coasts. We surveyed OM sources from the Yuba River watershed in northern California to identify specific biomarkers that represent aquatic and terrigenous OM sources. Multiple classes of organic proxies—including sterols, fatty acids (FA), lignin phenols and stable carbon and nitrogen isotope values (δ13C, δ15N)—were measured in soils, vegetation, charcoal, and freshwater plankton to characterize representative source endmembers. Sterols—including 27-nor-24-cholesta-5,22-dien-3β-ol, cholesta-5,22-dien-3β-ol, 24-methylcholesta-5,22-dien-3β-ol and cholesta-5-en-3β-ol, and positive δ15N values—were associated with aquatic OM (plankton, suspended particulate OM), whereas lignin phenols, long chain FA, and diacids characterized terrigenous sources (soils, charcoal, vegetation). Trends in organic carbon and biomarker signatures in soil samples showed a response to environmental disturbance (i.e., mining, agriculture) through an inverse relationship between OM content and land use. Results from this study demonstrate the utility of multi-biomarker studies for distinguishing between OM from different sources and land uses, offering new insights for biogeochemical studies in aquatic systems.

Multi-Biomarker Analysis for Identifying Organic Matter Sources in Small Mountainous River Watersheds: A Case Study of the Yuba River Watershed

Organic matter in soils and sediments derives from a mixture of biological origins, often making it difficult to determine inputs from individual sources. Complicating the determination of source inputs to soil and sedimentary organic matter (OM) is the fact that physical and microbial processes have likely modified the initial composition of these sources. This study focused on identifying the composition of watershed-derived OM to better understand inputs to inland waters and improve our ability to resolve between terrigenous and aquatic sources in downstream systems, such as estuaries and coasts. We surveyed OM sources from the Yuba River watershed in northern California to identify specific biomarkers that represent aquatic and terrigenous OM sources. Multiple classes of organic proxies—including sterols, fatty acids (FA), lignin phenols and stable carbon and nitrogen isotope values (δ13C, δ15N)—were measured in soils, vegetation, charcoal, and freshwater plankton to characterize representative source endmembers. Sterols—including 27-nor-24-cholesta-5,22-dien-3β-ol, cholesta-5,22-dien-3β-ol, 24-methylcholesta-5,22-dien-3β-ol and cholesta-5-en-3β-ol, and positive δ15N values—were associated with aquatic OM (plankton, suspended particulate OM), whereas lignin phenols, long chain FA, and diacids characterized terrigenous sources (soils, charcoal, vegetation). Trends in organic carbon and biomarker signatures in soil samples showed a response to environmental disturbance (i.e., mining, agriculture) through an inverse relationship between OM content and land use. Results from this study demonstrate the utility of multi-biomarker studies for distinguishing between OM from different sources and land uses, offering new insights for biogeochemical studies in aquatic systems.

Depositional and diagenetic constraints on the quality of shale‐gas reservoirs: A case study from the Late Palaeocene of the Potwar Basin (Pakistan, Eastern Tethys)

Unconventional shale‐gas reservoir assessment concurs numerous exploration challenges, often related to their complex lithology, which reflects secular changes in depositional environment(s) as well as spatially variable diagenetic overprinting. This study combines a range of methods (sedimentological, petrographical, CL‐microscopy, X‐ray diffraction, TOC (total organic carbon), TON (total organic nitrogen) and δ13Corg and δ15Norg stable isotopes) to address the controls of lithofacies, palaeo‐depositional environment(s) and diagenesis on the shale‐gas reservoir potential of the Late Palaeocene Patala Formation in the Potwar Basin of Pakistan. This formation records sediment accumulation in a shallow, mixed siliciclastic‐carbonate shelf environment. Sedimentological and X‐ray diffraction (XRD) analyses show that the formation is primarily composed of an alternation of carbonaceous, siliceous, calcareous and argillaceous mudstone lithofacies. Detrital assemblages, including grains of quartz, apatite, calcite, chlorite, as well as organic matter (OM) and clay minerals with auxiliary (authigenic) pyrite, dominate the formation. High terrigenous influx is represented by the abundance of siliciclastics and terrigenous OM. The δ13Corg and δ15Norg proxies reflect dysoxic to anoxic palaeo‐environmental conditions, which promoted preservation of mixed marine and terrigenous OM, a setting considered to be indicative for high shale‐gas potential. The organic‐rich siliceous and carbonaceous mudstone lithofacies are considered to be the most prospective intervals, while calcareous and argillaceous mudstones are considered least promising in terms of reservoir quality. Spatially variable eo‐, meso‐ and telo‐diagenetic features such as compaction, cementation, stylolitization, dissolution and re‐precipitation are superimposed upon the depositional fabric, which locally affected the reservoir quality of the Patala Formation. The results of this study strongly suggest that the organic‐ and quartz‐rich mudstone lithofacies constitute “sweet spots” whose occurrence in space and time may be traced to design a strategy for shale‐gas exploration in the Potwar Basin of Pakistan and in analogous settings elsewhere.

Depositional environment and petroleum source rock potential of Mesozoic lacustrine sedimentary rocks in central Mongolia

Middle Jurassic (five samples from one location) and Lower Cretaceous (thirty-two samples from eight locations) lacustrine sedimentary rocks of central Mongolia were studied to determine their depositional environments and petroleum source rock potential. The Middle Jurassic shale contains sufficient organic matter (OM) with Type II/III kerogen to represent a mixed oil and gas potential source rock. Tmax and biomarker maturity indices suggest a thermally immature level. Pristane/phytane and the gammacerane index indicate that the shale accumulated in an oxic, freshwater environment. Terrigenous OM with input from algae and macrophytes was a major contributor as indicated by long-chain n-alkanes and the abundance of C29 regular steranes. The stable carbon and nitrogen isotope compositions and kerogen types agree with land plant derived OM. Lower Cretaceous shales are highly oil-prone but thermally immature, reflected by Rock-Eval data and biomarker parameters. Carbon preference index (CPI), n-alkane distributions, and C27/C29 ratios suggest that OM in Lower Cretaceous shales is a mixture of phytoplankton, macrophyte, and land plant. The contribution from algal OM is variable. Lower Cretaceous shales were deposited in stratified lakes with anoxic bottom waters and the salinity of paleolakes was variable, indicated by Pr/Ph and the gammacerane index, respectively. Organic geochemical characteristics and depositional environments of Middle Jurassic and Lower Cretaceous sedimentary rocks studied herein are similar to those of Jurassic source rocks in western Chinese basins and Lower Cretaceous source rocks in eastern Mongolian basins, respectively.

The Shahejie Formation in the Dongpu Depression, Bohai Bay Basin, China: Geochemical investigation of the origin, deposition and preservation of organic matter in a saline lacustrine environment during the Middle Eocene

The Eocene Shahejie Formation in the Bohai Bay, Eastern China is known to contain important petroleum source rocks. Here, a new, large set of geochemical, petrographical and mineralogical data obtained on 41 samples from 9 wells within the North Dongpu Depression is presented in order to investigate organic matter precursor material, thermal maturity as well as paleoenvironment during deposition of the Es3U, Es3L, Es3M and Es4U members within the Eocene Shahejie Formation. These shales and marlstones from the four sub-members show low to moderate TOC contents (up to 5%) with highest values in the Es3U member; quartz content is at about 10% and carbonate content is variable, partly reaching more than 40%. A range of different maturity parameters was determined indicating early mature to peak oil mature stages, and maturity increase with burial depth. Throughout, alginite is the predominant maceral, whereas both vitrinite and inertinite are rare. Hydrogen index values are highly variable, ranging from about 100 to 700. Distinctly different biomarker assemblages indicate variable depositional environments and organic matter inputs between the four intervals. The Eocene Shahejie shales and marlstones in the study area were deposited under episodic photic zone anoxia as revealed by the presence of aryl isoprenoids. Bottom water conditions were anoxic during most of the time, except for Es3M, as indicated by several parameters, e.g. low pristane/phytane ratios. In north Dongpu Depression Es4U displays a fairly uniform biomarker composition, which indicates relatively stable hypersaline lake conditions with a shallow chemocline based on the high Gammacerane Index > 0.6 and abundant aryl isoprenoids. Water salinity changed during deposition of Es3L according to variable C24 tetracyclic terpanes related parameters and evaporite minerals content. The aquatic algal organisms, i.e. haptophyte algae, diatoms and bryophytes which are reflected by abundant nC37–39 alkanes, a variable sterane pattern and other biomarkers, were flourishing during Es3L. Additionally, specific PAHs, i.e. retene, cadalene, fluoranthene, indeno[1,2,3-cd]pyrene and benzo[ghi]perylene also have been identified in small concentration in Es3L, which may be attributed to limited terrigenous organic matter input. Es3M was deposited in a normal lacustrine environment under rather low salinity conditions with substantial input of organic matter derived from algae. During Es3U, saline lake conditions were re-established accompanied by input of both algal and microbial organic matter, especially cyanobacteria as deduced from MMAs and 2α-MeHs, methanogens as deduced from abnormally abundant phytane, PMI, squalane, and methanotrophs from 3β-MeHs.

Late Neogene chronostratigraphy and integrated paleoecological trends in the southwestern Caribbean Sea

Palynology of the Neogene marine stratigraphic sequences in tropical latitudes is poorly studied Most studies focus on lower to middle Miocene deposits in the Caribbean, emphasizing the biostratigraphic value of dinoflagellate cysts. The palynological and calcareous nannofossil analysis of 50 samples from DSDP Hole 502A and their integration with the paleomagnetic and planktonic foraminifera data reveal a detailed and reliable chronostratigraphic framework, and the interpretation of the paleobathymetric and paleoenvironmental evolution of the sequence. Fossil content in a continuous late Tortonian to late Gelasian (~7.56-1.80 Ma) sequence of deep-marine sediments serves to calibrate ages of dinoflagellate cysts, and comparison with high latitude records reveals synchronous (Operculodinium janduchenei, Selenopemphix dionaeacysta, Barssidinium taxandrianum, and Lejeunecysta interrupta) and asynchronous events (Quadrina? condita) with biostratigraphic potential for the southwestern Caribbean Sea. Our results highlight the importance of analyzing continental palynomorphs in deep-marine deposits to increase the temporal resolution of future palynostratigraphic schemes. Micropaleontogical data indicate a lower bathyal–abyssal environment and four paleoceanographic settings defined by changes in surface water productivity. These intervals fluctuate from mesotrophic-oligotrophic (~7.56–6.31 Ma) to eutrophic (~6.18–4.98 Ma), oligotrophic (~4.74–3.41 Ma), and mesotrophic surface water conditions (~3.26–1.80 Ma). Changes in trophic conditions are strongly related to the paleoceanographic reorganization associated with the constriction and complete closure of the Central American Seaway, seasonal upwelling episodes, and the input of terrigenous organic matter probably transported from southern Central America and northern South America by turbidity currents.

Characterization of organic matter in marine sediments to estimate age offset of bulk radiocarbon dating

Radiocarbon dating of Arctic marine sediment is often challenging due to the low availability of calcareous fossils. Consequently, bulk organic matter dating has at times been used to establish sediment core chronologies. Yet, radiocarbon dates based on bulk organic matter often appear to deviate vastly from dates based on fossils, mainly caused by input of allochthounous carbon, including terrigenous organic matter. In this study, we aim to examine the link between the composition of the bulk organic matter and the age offsets between the bulk radiocarbon dates and those obtained from calcareous foraminiferal tests. All samples are taken from the marine sediment core AMD14-204C from offshore Upernavik (eastern Baffin Bay). The radiocarbon dates for bulk organic matter are on average ∼3000 years older than the radiocarbon dates based on foraminifera, but with changing age offsets throughout the record. To investigate the cause of this age offset and its variations over time, we applied core scanning, X-ray Fluorescence analysis, stable isotopes, organic pyrolysis and microscopic organic petrology to examine the distribution and characterization of the organic matter. The results show that the older organic matter includes clastic input of reworked sedimentary rocks potentially originating from West Greenland and/or the Canadian Arctic Archipelago. Changes in the input of contemporary marine algal produced organic matter versus both terrigenous input and reworked ancient organic matter appear to control the age offsets between the bulk and foraminifera dates. A low Hydrogen Index and low δ13Corg values together with a high Oxygen Index, indicative of high influence of terrigenous organic matter, seem to correspond to samples with the largest age offsets; 1000–2000 years greater than in other samples. To examine the cause of the variations in the age offsets, a new quantification of the autochthonous organic matter as a fraction of the TOC was calculated. This shows that samples with the largest age offsets contained the lowest fraction (as low as ∼12%) of autochthonous organic matter in the TOC.

Terrigenous organic matter and formation of siderite in the Bathonian Ore-Bearing Clay Formation at Gnaszyn, Poland – a petrochemical study

The Bathonian Ore-Bearing Clay Formation, outcropping in the Gnaszyn open-pit mine at Częstochowa (Poland), includes several horizons of abundant iron carbonate concretions . The cores of the concretionary bodies commonly contain organic matter (OM), dominated by fragments of wood. These organic particles usually display well-preserved primary structures and occur rarely as more deformed and/or completely degraded fragments. Their original structures are frequently replaced by or filled with secondary mineralization, mostly represented by pyrite. The maceral composition of the OM of the wood fragments is dominated by huminite with subordinate inertinite and resinite. Vitrinite reflectance analyses revealed values lower than 0.45%. The total organic carbon content (TOC) displayed variable results between 2% and 18%. Rock-Eval analyses revealed low amounts of hydrogen (< 45 mg HC/g TOC) and relatively high amounts of oxygen (up to 136 mg CO 2 /g TOC). Analysed samples contained small quantities of free hydrocarbons (S1 peak < 0.26 mg HC/g rock) as well as hydrocarbons, generated during pyrolysis (S2 peak < 7.05 mg HC/g rock). These features are characteristic for immature type IV kerogen of terrigenous origin. However, the maceral composition and frequent occurrence of siderite affecting the Rock-Eval parameters may indicate that the original kerogen belonged to type III. According to previous authors, the OM of terrigenous origin was delivered to well-oxygenated water of the palaeo-basin in the Częstochowa area. The present data indicate that intensive biodegradation of this OM at shallow burial depleted the oxygen supply within the sediment, driving the pore water into dys- or anoxic conditions. The activity of microorganisms in reducing iron and/or sulphates became the dominant biodegradation reaction, introducing Fe 2+ and HCO 3 - ions into the system. Negative δ 13 C values in the cortex of the concretions analysed indicate that the bicarbonate consumed in siderite precipitation was supplied by this microbial activity. The reducing microenvironments developed in the sediment and wood fragments acted as nucleation sites for siderite precipitation.

BIOMARKER GEOCHEMISTRY OF PALEOGENE CRUDE OILS AND SOURCE ROCKS FROM THE RAOYANG SAG, BOHAI BAY BASIN, NE CHINA: AN OIL – SOURCE ROCK CORRELATION STUDY

This study presents a systematic geochemical analysis of Paleogene crude oils and source rocks from the Raoyang Sag in the Jizhong sub‐basin of the Bohai Bay Basin (NE China). The geochemical characteristics of fifty‐three oil samples from wells in four sub‐sags were analysed using gas chromatography (GC) and gas chromatography – mass spectrometry (GC‐MS). Twenty core samples of mudstones from Members 1 and 3 of the Eocene‐Oligocene Shahejie Formation were investigated for total organic carbon (TOC) content and by Rock‐Eval pyrolysis and GC‐MS to study their geochemistry and hydrocarbon generation potential. The oils were tentatively correlated to the source rocks.The results show that three groups of crude oils can be identified. Group I oils are characterized by high values of the gammacerane index and low values of the ratios of Pr/Ph, Ts/Tm, 20S/(20S+20R) C29 steranes, ββ/(ββ+αα) C29 steranes, C27 diasteranes/ C27 regular steranes and C27/C29 steranes. These oils have the lowest maturity and are interpreted to have originated from a source rock containing mixed organic matter deposited in an anoxic saline lacustrine environment. The biomarker parameter values of Group III oils are the opposite to those in Group I, and are interpreted to indicate a highly mature, terrigenous organic matter input into source rocks which were deposited in suboxic to anoxic freshwater lacustrine conditions. The parameter values of Group II oils are between those of the oils in Groups I and III, and are interpreted to indicate that the oils were generated from mixed organic matter in source rocks deposited in an anoxic brackish–saline or saline lacustrine environment.The results of the source rock analyses show that samples from Member 1 of the Shahejie Formation were deposited in an anoxic, brackish – saline or saline lacustrine environment with mixed organic matter input and are of low maturity. Source rocks in Member 3 of the Shahejie Formation were deposited in a suboxic to anoxic, brackish – saline or freshwater lacustrine environment with a terrigenous organic matter input and are of higher maturity. Correlation between rock samples and crude oils indicates that Group I oils were probably derived from Member 1 source rocks, while Group III oils were more likely generated by Member 3 source rocks. The Group II oils with transitional characteristics are likely to have a mixed source from both sets of source rocks.

Fingerprinting and biomarker analysis of oil from Akshabulak group for determining oil types

This article presents the results of fingerprinting of 128 oil samples derived from all production wells of three Akshabulak group fields to determine the types of oils. Based on the results of this analysis, four groups of oils were identified: The first group includes oils (red) of the lower hydrocarbon pay zones (III-IV-V) in Akshabulak Central, the upper zones of the I southern dome of Akshabulak Central and all pay zones of the northern dome at Akshabulak South. The second group of oils (green) was identified in the wells exploiting pay zone I in the northern dome of Akshabulak Central, while oils (blue) in the productive horizons of Akshabulak East and paleochannel sediments of Akshabulak Central form the third group. The fourth group includes one oil sample with different genesis from well № 37, which penetrates paleo-channel № 13 at South Akshabulak.
 Biomarker analysis was carried out for 39 oil samples, according to which the oils were formed in terrigenous (clayey) organic matter, deposited in lacustrine environment. In terms of thermal parameters, the oils of the Eastern Akshabulak are less mature than the oils of the Central Akshabulak and the South Akshabulak, despite their relative deeper depths. Based on the work performed, the conclusion presents the prospective hydrocarbon accumulation zones suitable for increasing hydrocarbon resources and reserves in the future. However it should be noted that the results of the studies performed provide valuable information only when integrated with the corresponding geological and seismic data.

Black shale formation environment and its control on shale oil enrichment in Triassic Chang 7 Member, Ordos Basin, NW China

Based on geochemical analysis results of core samples from the Triassic Chang 7 Member of Well Feng 75 drilled in the northwest margin of Ordos Basin, combined with geological characteristics of this region, the formation environment of the black shale and its control on shale oil enrichment are comprehensively studied. From the Chang 73 to Chang 71 Sub-members, the black shale have organic carbon contents decreasing from 5.70% to 3.55%, hydrogen indexes decreasing from 345 mg/g to 269 mg/g, while the oxygen indexes increasing gradually from 6 mg/g to 29 mg/g, indicating that the sedimentary environment during the depositional period of Chang 72 and Chang 73 Sub-members was anoxic. Biomarkers in the black shale change regularly, and have an obvious “inflection point” at the depth of 2753–2777 m in the Chang 73 Sub-member, indicating that the input of terrigenous organic matter increased. However, there is a negative drift about 2% of organic carbon isotopic composition near the “inflection point”, which is in conflict with the results of biomarker compounds. This is because the extreme thermal and anoxic events caused by continental volcanic activity in the ancient Qinling region caused negative drift of carbon isotopic composition of the black shale in the Ordos Basin. The volcanic activity caused rise of temperature, polluted air, extreme weathers, more floods and thus more input of terrigenous organic matter, and gave rise to extremely anoxic environment conducive to the preservation of organic matter. Terrigenous organic matter is more conducive to the formation of light oil than aquatic organic matter, so these sections in Yanchang Formation are major “sweet spots” for shale oil enrichment.

Geochemistry of the Organic Matter of Ulug-Khem Basin Coals

Abstract —The composition of the organic matter (OM) of coals of the Ulug-Khem Basin has been studied. According to Rock-Eval pyrolysis data, this OM is a type III kerogen, sometimes significantly oxidized. The coal of low-grade metamorphism has a high hydrocarbon potential. Based on the gas–liquid chromatography and chromatography–mass spectrometry data on the distribution of n-alkanes, isoprenoids, polycyclic biomarkers, and aromatic hydrocarbons, we have established the composition of the primary OM of the coals and the grade of OM metamorphism. The primary OM of the coals consists mainly of remains of aquatic vegetation and terrigenous OM. The latter includes conifer remains, which are identified from the presence of 4β(H)-19-norisopimarane in the aliphatic fraction of bitumen and from the domination of retene over cadalene and 6-isopropyl-1-isohexyl-2-methylnaphthalene in the aromatic fraction.

Saturated biomarkers adsorbed and occluded by the asphaltenes of some Venezuelan crude oils: Limitations in geochemical assessment and interpretations

This paper presents the study of saturated biomarkers in maltenes and the compounds adsorbed and occluded by the asphaltenes of six Venezuelan crude oils. The study is intended to know to what extent geochemical information stored in the adsorbed and occluded compounds matches the information in maltenes when it comes to evaluating source input, depositional environment, and thermal maturity. Our findings suggest that geochemical information obtained from the adsorbed and occluded compounds agrees with the information provided by maltenes of unaltered crude oils such as Alturitas, Boscan, Guafita, and Silvestre. We confirmed a clear correlation of Alturitas and Boscan crude oils with a carbonate source rock sedimented under anoxic conditions and containing marine organic matter, while Guafita and Silvestre crude oils resulted from a marl-type source rock characterized by mixtures of marine and terrigenous organic matter inputs preserved under varying redox conditions (anoxic to sub-oxic). Regarding maturity, Alturitas and Silvestre resulted more mature than Boscan and Guafita. Despite the good correspondence between the geochemical information provided by the maltenes and the adsorbed and occluded compounds, we observed compound enrichments that caused limitations in the use of some geochemical indicators. The adsorbed compounds of the six studied crude oils were enriched in linear alkanes higher than n-C24, regular steranes (C27–C28), and pentacyclic terpanes, while in the occluded compounds pregnanes, tricyclic terpanes, and n-alkanes higher than n-C24 were more abundant. Thus, maturity relationships defined by the relative abundances of low and high molecular mass steranes and terpanes yielded values that confused when the adsorbed and occluded compounds were compared with the respective maltenes. However, when these relationships were used to compare the occluded compounds of two or more different crude oils, they yielded a reliable appraisal of the maturity stage of the respective source rock. Our results strongly suggest that the adsorbed compounds still preserve their geochemical information as they probably undergo a very low exchange with the surrounding maltenes. So, these compounds together with the occluded ones become a valuable source of geochemical information to assess altered crude oils such as Ayacucho and Junín that have lost most of their maltene biomarkers due to biodegradation. In these cases, the information provided by the adsorbed and occluded compounds confirmed that Ayacucho crude oil is more mature than Junín and that the former derives from a carbonate, marine-organic-matter source rock sedimented under anoxic conditions while the latter correlates with a marl-type source rock sedimented in anoxic conditions and containing mixtures of terrigenous and marine organic matter. Compounds such as oleanane, C27 25,28,30-trisnorhopane (TNH), C28 17α,21β,28,30-dinorhopane (DNH) and C29 17α(H),21β(H)-25-norhopane (NH) detected in the occluded material of the six studied oils, indicate that the major contributions to the occluded compounds probably came from the original material initially adsorbed and occluded in the parent kerogen.

Molecular evidence for the export of terrigenous organic matter to the north Gulf of Mexico by solid-state 13C NMR and Fourier transform ion cyclotron resonance mass spectrometry of humic acids

Sediment samples along a transect extending from the Mississippi River Birdsfoot Delta to the Mississippi Canyon on the Louisiana continental shelf were examined, by advanced analytical techniques, electrospray ionization coupled to a 12T Fourier transform ion cyclotron resonance mass spectrometer (ESI-FTICR-MS) and quantitative solid-state multiple cross polarization magic angle spinning (multi-CPMAS) 13C NMR, in an effort to understand the source and export of terrigenous organic matter to the Gulf of Mexico. Both NMR and mass spectral data indicate that condensed aromatics (CA) and carboxyl-containing aliphatic molecules (CCAM) are present at the mouth of the river, reflective of high contributions from terrigenous soil-like organic matter. Mass spectral peak magnitudes of CA diminish by 15%, with increasing distance offshore, and represent 30% of integrated NMR peak areas. In contrast, mass spectral and NMR CCAM peaks increase by 7 and 13%, respectively. These trends in humic acid extracts, provide novel molecular evidence of terrigenous organic matter deposition in offshore sediments.