Source Kerogen Research Articles

Signals of watershed change preserved in organic carbon buried on the continental margin seaward of the Waipaoa River, New Zealand

Abstract Holocene sediments buried on the continental shelf seaward of the Waipaoa River, northeastern New Zealand, preserve a stratigraphic record of terrestrial environmental change. The well characterized, long-term record of storms, volcanism, and human disturbance in this region provides an opportunity to examine how such changes are reflected in the character of organic carbon (OC) buried on the continental margin. Complimentary evidence obtained from analyses of the 14 C content and the elemental and stable carbon isotopic composition of different sedimentary fractions, including charcoal, wood, and clay-sized isolates, indicates that these perturbations led to mobilization of OC components with variable storage histories in the watershed. Charcoal transported in the modern Waipaoa River and buried in offshore depocenters includes a highly aged component that has apparently been slowly released from storage in soil or alluvial terraces. The charcoal fraction ages become dramatically younger in sediments deposited after the Taupo volcanic eruption (1717 cal yr BP) and then Polynesian settlement (ca. 700 cal yr BP), both signaling biomass burning. The mean age of woody plant fragments and clay-bound OC deposited on the shelf also varies over time, with older material having accumulated in the middle Holocene and since human occupation. Deposition of older-than-average wood fragments and clay-bound OC between about 5 and 3.6 kyr BP correlates with evidence for a period of increased storm frequency in the region and may reflect the enhanced delivery of aged soil and sedimentary rock mobilized from hill slopes via earthflows and/or deep seated landslides. Similarly, the deposition of older wood and clay-bound OC commensurate with the first anthropogenic disturbance ca. 700 years ago is consistent with accelerated mass wasting due to deforestation. At the same time, a change in the elemental and stable isotopic composition of bulk and clay-bound OC buried on the shelf may reflect increased marine primary productivity and/or mobilization of OC from deep levels in soil profiles. Deforestation of the Waipaoa headwaters by European settlers beginning in the middle 19th century is evinced by a sharper rise in the age of clay associated OC buried offshore. Today, deep gully incision into fractured sedimentary bedrock is a major source of sediment and kerogen to the river, and this process has left its mark on the age of sedimentary OC delivered to the adjacent margin.

Quantification, morphology and source of humic acid, kerogen and black carbon in offshore marine sediments from Xiamen Gulf, China

Three types of macromolecular organic matters (MOMs), i.e. humic acid (HA), kerogen+black carbon (KB), and black carbon (BC) were extracted from marine sediments of Xiamen Gulf, southeast of China. The chemical composition, morphological property and source of the three extractions were characterized by elemental analyzer/isotope ratio mass spectrometry (EA/IRMS) and scanning electron microscope (SEM). The results showed that KB was the predominant fraction in MOMs, which accounted for 61.79%-89.15% of the total organic content (TOC), while HA consisted less than 5%. The relative high contents of kerogen and BC, and low contents of HA in the samples indicated that anthropogenic input might be the major source of organic matter in marine sediments near the industrial regions. The characterization of SEM, not only revealed morphological properties of the three fractions, but also allowed a better understanding of the source of MOMs. The δ13C values of the three fractions suggested that materials from terrestrial C3 plants were predominant. Furthermore, the anthropogenic activities, such as the discharge of sewage, coal and biomass combustion from industry nearby and agricultural practices within drainage basin of the Jiulong River, were remarkably contributed to the variations in δ13C values of MOMs in the offshore marine sediments.

River-gulf system—the major location of marine source rock formation

Petroleum was generated from sedimentary rocks. The world’s oldest oil source rock so far was found in Proterozoic rocks. Since then, 73% to 81% of the earth’s surface has been covered with sedimentary rocks. However, only quite a limited area is rich in oil and gas. It is found that source rocks have controlled oil and gas distribution, and they are mainly formed in two systems: (1) river-lake systems and (2) river-gulf systems. Phytoplankton is an important source of kerogen, the blooming of which is strongly dependent on nutrients. Rivers are the major nutrient provider for basins. Rivers around lakes and an undercompensation (where the sedimentation rate is less than the rate of basin subsidence) environment provide favorable conditions for phytoplankton blooming in lakes. Gulfs are usually located at the estuary of big rivers, characterized by restricted current circulation and exchange with the open sea, which benefit maintaining the nutrient density, phytoplankton levels and organic matter preservation. The river-gulf system is the most favorable place for marine source rock development. Most of the world famous marine petroleum-rich provinces are developed from river-gulf systems in geological history, such as the Persian Gulf Basin, Siberian Basin, Caspian Basin, North Sea, Sirte Basin, Nigerian Basin, Kwanza Basin, Gulf of Mexico, Maracaibo Basin and the Eastern Venezuelan Basin.

Late Cretaceous paleoenvironment and lake level fluctuation in the Songliao Basin, northeastern China

AbstractStudy of Late Cretaceous lacustrine sedimentary strata in the eastern Songliao Basin, China revealed that the paleoclimate was relatively arid and hot during sedimentation of the upper Santonian of the Yaojia Formation, but became relatively humid and warm during deposition of the lower Campanian Nenjiang Formation. The upper Yaojia Formation was deposited in a freshwater lake environment, while the lower Nenjiang Formation was deposited in a slightly brackish to brackish environment. The average total organic carbon content in the upper Yaojia Formation is 0.15%, while the hydrogen index is 36 mgHC/gTOC, implying poor source rock for oil generation and the organic matter comprised of a mixture of woody and herbaceous organic matter. In contrast, the hydrogen index of oil shale and black shale of the lower Nenjiang Formation is 619 mgHC/gTOC, and total organic carbon content on average is 3.37%, indicating a mixed algae and herbaceous source of kerogen and an increase in aquatic bioproductivity. The black shale and oil shale have low Pristane/Phytane and C29 5α,14α,17α(H) − stigmastane 20R/(20R + 20S) ratios, with maximum concentration of n‐alkanes at n‐C23, implying an anoxic depositional environment with algae, bacteria and higher plants providing most of the organic matter. Relatively abundant gammacerane and a higher Sr/Ba ratio in the oil shales suggest the presence of brackish water and development of salinity stratification in the lake. During sedimentation of the upper Yaojia through the lower Nenjiang Formations, the level of Songliao lake increased and a deep‐lake environment was formed with bottom waters being oxygen depleted. Concomitantly, as the lake deepened bottom conditions were changing from oxic to anoxic, and the input of organic matter changed from predominantly higher plants to a mixture of bacteria, algae and higher plants providing favorable conditions for oil source rock accumulation.

The molecular and isotopic effects of hydrothermal alteration of organic matter in the Paleoproterozoic McArthur River Pb/Zn/Ag ore deposit

The molecular distribution and compound specific stable carbon and hydrogen isotope ratios were measured on solvent extractable hydrocarbons from the Late Paleoproterozoic McArthur River, or “Here's Your Chance” (HYC) Pb/Zn/Ag ore deposit in the Northern Territory of Australia. Five samples were collected from the McArthur River mine on a northeast–southwest transect in order to sample a gradient of hydrothermal alteration. One sample was taken from the unmineralized W-Fold Shale unit immediately below the HYC ore deposit. δD of n-alkanes, branched alkanes and bulk aromatic fractions were measured, and δ13C of n-alkanes, polynuclear aromatic hydrocarbons (PAHs) and bulk kerogen were measured to assess the isotopic effects of a varying degree of hydrothermal alteration on different components of HYC organic matter (OM). Relative to n-alkanes in Barney Creek Formation sediments that did not undergo mineralization, HYC n-alkanes are enriched in deuterium (D) by 50–60‰. This is likely to be a result of equilibrium hydrogen exchange during ore genesis with a highly D-enriched fluid that originated in an evaporitic basin. Trends with distance along the sample transect are ambiguous, but from the northernmost to southernmost point, n-alkanes are less D-enriched, and PAHs are less abundant and less 13C-enriched. This could be due to decreasing hydrothermal alteration effects, decreasing delivery of highly altered OM by the mineralizing fluid, or both. The carbon isotopic composition of HYC PAHs is inconsistent with a Barney Creek Fm source, but consistent with an origin in the underlying Wollogorang Formation. PAHs are 13C-depleted relative to n-alkanes, reflecting a kerogen source that was 13C-depleted compared to n-alkanes, typical for Precambrian sediments. PAHs are more 13C-depleted with increasing molecular weight and aromaticity, strengthening the case for a negative isotopic effect associated with aromatization in ancient sediments. Together, these data are consistent with a an ore deposition model in which fluids originated in an evaporitic deposit lower in the basin and interacted with metals and OM in the Tawallah Group at temperatures above 250°C and a depth of ~6km before ascending along a flower structure associated with the Emu Fault and cooling to 200±20°C before reaching Barney Creek sediments.

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

The evolution of fluorescence has been measured for “live” oils generated from 14 oil-prone kerogens or coals from varying depositional environments during closed system pyrolysis in a diamond anvil cell at three heating rates (3, 8, and 25 °C/min), and temperatures up to 600 °C. The measured fluorescence intensities of the samples, employing using violet excitation at 405 nm, increases significantly during maturation intervals within the oil window, while the fluorescence spectra of oils generated from all studied kerogens exhibit progressive blue-shift of peak wavelengths ( λ max) and red/green quotients ( I 650/ I 500) upon increasing maturity. The observed trend is consistent with a maturity dependence of the spectral shift, which is widely recognized in natural hydrocarbon inclusions and crude oils using ultraviolet (UV) excitation (365 nm). The data presented herein suggest that the λ max of spectra for inclusion oils shift in similar direction despite differences in composition or source kerogen. This implies that the reverse or anomalous trends reported for inclusion oils in nature may be attributed to other processes, which significantly alters the fluorescence properties of oils subsequent to their generation. Oils with the similar color ( λ max or I 650/ I 500) can be derived from diverse kerogens with maturities that vary by ±0.3% Ro, suggesting that the fluorescent colors of crude and inclusion oils are both maturity- and source-dependent, and therefore cannot be used as universal maturity indicators. In addition, the blue-shifts observed for cumulative oils generated from all kerogens approaches similar minima λ max values around the green-yellow wavelength (564 nm) and at I 650/ I 500 values around 0.6, at maturities close to the middle or late stage of oil generation. This suggests that most late-stage cumulative oils will exhibit similar colors. Oils generated during late-stage maturity intervals, however, can exhibit colors with shorter wavelengths.

Active venting at the Isis mud volcano, offshore Egypt: Origin and migration of hydrocarbons

The Isis mud volcano is an active camembert-like mud expulsion structure in the eastern Nile deep-sea fan area. It has been investigated during the NAUTINIL (2003) and MIMES (2004) cruises carried out within the MEDIFLUX project. Active, gas-rich fluid venting occurs at the Isis mud volcano as indicated by enhanced gas content in the sediments, having a measured 7.5 mmol of methane per liter of wet sediment in the central area of the mud volcano. In addition, gas bubble expulsions have resulted in the formation of multiple hydrocarbon plumes in the water column, with methane concentrations as high as 1550 nmol/L, compared to ∼ 1 nmol/L for background levels in deep water. In view of the stable carbon and hydrogen isotopic composition of methane ( δ 13C = − 55.1‰ VPDB, δD = − 203‰ VSMOW) and of higher hydrocarbons ( δ 13C of − 30.3‰, − 26.1‰, − 28.4‰, and − 25.0‰ VPDB, for ethane, propane, i-butane and n-butane, respectively), their origin is mainly thermogenic. These isotopic data also show that the wet compounds are co-generated with oil and we suggest they predominantly originate from type II kerogen source rocks of potentially variable maturity levels. At the centre of the mud volcano, the distinct kink observed in the pore water isotopic and dissolved species composition and its depth level change between subsequent cruises indicates a high frequency of fluid/gas expulsion. The seawater signature of the pore fluid in the upper part of the sediment column suggests that each expulsion is followed by a downward migration of overlying seawater. Such downward advection of bottom seawater may not only be related to the replacement of the ejected gas but also to the replacement of lower density pore fluid by bottom seawater. This mechanism is thought to be a common feature at mud volcanoes and gas seepage sites. In the peripheral zone, pore fluid is at seawater concentration and isotopic composition, down to a coring depth of 430 cmbsf. We suggest that the deeper downward advection of bottom seawater is related to the less frequent fluid/gas expulsion occurring at these sites compared to the centre sites. In this paper we show that a combination of geochemical tools used for hydrocarbons and pore water geochemistry is a powerful approach to detect sources of ascending fluid and to reveal post-eruptional fluidodynamic processes within mud volcanoes.

Source and significance of selected polycyclic aromatic hydrocarbons in sediments (Hovea-3 well, Perth Basin, Western Australia) spanning the Permian–Triassic boundary

Stable carbon isotopic data for polycyclic aromatic hydrocarbons (PAHs; chrysene and triphenylene, benzo( e)pyrene, 9-methylphenanthrene) in sediments of comparable thermal maturity from a petroleum drill core (Hovea-3 well, Perth Basin, Western Australia) spanning the largest mass extinction event in the past 500 million years (Permian–Triassic boundary) are consistent with an algal kerogen source in the basal Triassic as opposed to a combustion and/or higher-plant source in the underlying Permian sediments. The δ 13C values of extractable PAHs are less negative in the Permian inertinitic interval (−24.7‰ to −29.2‰) than the Triassic sapropelic interval (−30‰ to −33.3‰). A strong correlation is evident between δ 13C of individual PAHs, δ 13C of kerogen and Rock-Eval pyrolysis hydrogen index providing unequivocal evidence that PAH δ 13C is mainly controlled by organic matter type. Abundant algal-derived PAHs in basal Triassic sediments may be a global feature; the lack of coal globally in the basal Triassic sediments is also consistent with this observation. The Early Triassic coal gap appears to coincide with the global extinction of peat-forming plants at the end of the Permian, coal did not reappear until the Middle Triassic and Permian levels of plant diversity and thick peat deposits did not recover until the Late Triassic. The PAH isotopic data support the contention that a global anoxic event at the end of the Permian period contributed to the gradual worldwide asphyxiation and ultimate extinction of many marine and terrigenous organisms.

Experimental investigation of the compositional variation of petroleum during primary migration

Anhydrous non-isothermal heating experiments were conducted under controlled compressive stress on cylindrical plugs of six oil shales from Permian through Miocene age. The objectives of this study were to investigate molecular and compound class fractionation during petroleum generation and expulsion and to highlight causes of compositional variations due to primary migration from source rocks. Molecular compositions of n-alkanes, triterpanes, methylphenanthrenes and fatty acids of the original samples (initial bitumen), the samples after pyrolysis (residual bitumen) and the expelled products were compared. Acyclic alkanes were preferentially expelled with compositions depending on source kerogen and partial evaporative loss from the open experimental system expulsion. No substantial molecular fractionation within various compound classes or preferential primary migration of n-alkanes relative to acyclic isoprenoids occurred. Cyclic triterpanes and aromatic hydrocarbons were found only in small concentrations in the expelled bitumen indicating preferential retention relative to acyclic alkanes. Palmitic (C 16) and stearic (C 18) acids occurred as dominant compounds in the initial and residual bitumen. In the expelled bitumen they constitute virtually the only fatty acid species, indicating that expulsion efficiency is strongly dependent on concentration and generation intensity.

Stable sulfur isotope partitioning during simulated petroleum formation as determined by hydrous pyrolysis of Ghareb Limestone, Israel

Hydrous pyrolysis experiments at 200 to 365°C were carried out on a thermally immature organic-rich limestone containing Type-IIS kerogen from the Ghareb Limestone in North Negev, Israel. This work focuses on the thermal behavior of both organic and inorganic sulfur species and the partitioning of their stable sulfur isotopes among organic and inorganic phases generated during hydrous pyrolyses. Most of the sulfur in the rock (85%) is organic sulfur. The most dominant sulfur transformation is cleavage of organic-bound sulfur to form H 2S (gas). Up to 70% of this organic sulfur is released as H 2S (gas) that is isotopically lighter than the sulfur in the kerogen. Organic sulfur is enriched by up to 2‰ in 34S during thermal maturation compared with the initial δ 34S values. The δ 34S values of the three main organic fractions (kerogen, bitumen and expelled oil) are within 1‰ of one another. No thermochemical sulfate reduction or sulfate formation was observed during the experiments. The early released sulfur reacted with available iron to form secondary pyrite and is the most 34S depleted phase, which is 21‰ lighter than the bulk organic sulfur. The large isotopic fractionation for the early formed H 2S is a result of the system not being in equilibrium. As partial pressure of H 2S (gas) increases, retro reactions with the organic sulfur in the closed system may cause isotope exchange and isotopic homogenization. Part of the δ 34S-enriched secondary pyrite decomposes above 300°C resulting in a corresponding decrease in the δ 34S of the remaining pyrite. These results are relevant to interpreting thermal maturation processes and their effect on kerogen-oil-H 2S-pyrite correlations. In particular, the use of pyrite-kerogen δ 34S relations in reconstructing diagenetic conditions of thermally mature rocks is questionable because formation of secondary pyrite during thermal maturation can mask the isotopic signature and quantity of the original diagenetic pyrite. The main transformations of kerogen to bitumen and bitumen to oil can be recorded by using both sulfur content and δ 34S of each phase including the H 2S (gas). H 2S generated in association with oil should be isotopically lighter or similar to oil. It is concluded that small isotopic differentiation obtained between organic and inorganic sulfur species suggests closed-system conditions. Conversely, open-system conditions may cause significant isotopic discrimination between the oil and its source kerogen. The magnitude of this discrimination is suggested to be highly dependent on the availability of iron in a source rock resulting in secondary formation of pyrite.

Modelling of the hydrocarbon generation history and volumetric considerations of the coal-sourced Lulita Field, Danish North Sea

The hydrocarbon generation history of the Middle Jurassic Lulita Field in the Søgne Basin in the northeastern part of the Danish Central Graben has been predicted by modelling the Lulita-1Xc well located on-structure and a pseudowell located in the kitchen area. The Lulita Field consists of east-dipping Middle–Upper Jurassic sediments in a structural trap. Waxy oil, sourced from Middle Jurassic coals and type III kerogen (and possibly type I kerogen) shales, occurs in Middle Jurassic sandstone reservoirs sealed by Upper Jurassic marine shales. Maturity modelling (vitrinite reflectance) was carried out by using the PresRo ® kinetic model and a heat flow history calibrated from both the tectonic-subsidence history and maturity data. An effective oil window (‘oil expulsion window’) between 0.85% and 1.8%R o for the Middle Jurassic coal and type III kerogen source rocks was applied. Timing for onset of oil generation differs between the Lulita-1Xc well and the pseudowell. The top of the source rocks becomes mature during the Late Cretaceous in the kitchen area and during the Oligocene in the Lulita-1Xc well. However, oil expulsion from the entire Middle Jurassic source rocks is modelled to have started from the Paleocene in the kitchen area and appears to have continued through to the present day. No, or little, expulsion is modelled in the Lulita-1Xc well, indicating that the Lulita Field was charged by updip migration of hydrocarbons from the Søgne Basin kitchen to the east. Hydrocarbon charging pre-dates the time of potential diagenetic modification of the reservoir, thus limiting reduction in reservoir porosity and permeability. Based on the known coal source rock volume and quality, calculated maximum generated and expelled hydrocarbons cannot account for the estimated in-place volumes of oil. The additional hydrocarbons were probably generated from coaly shales and shaly coals that form the lateral equivalent of the Middle Jurassic coals in the kitchen area, possibly with contributions from lacustrine to brackish type I kerogen shales.

Polyphase evolution of the East Gobi basin: sedimentary and structural records of Mesozoic–Cenozoic intraplate deformation in Mongolia

AbstractMapping and correlation of 2D seismic reflection data define the overall subsurface structure of the East Gobi basin (EGB), and reflect Jurassic–Cretaceous intracontinental rift evolution through deposition of at least five distinct stratigraphic sequences. Three major northeast–southwest‐trending fault zones divide the basin, including the North Zuunbayan (NZB) fault zone, a major strike‐slip fault separating the Unegt and Zuunbayan subbasins. The left‐lateral NZB fault cuts and deforms post‐rift strata, implying some post‐middle‐Cretaceous movement. This fault likely also had an earlier history, based on its apparent role as a basin‐bounding normal or transtensional fault controlling deposition of the Jurassic–Cretaceous synrift sequence, in addition to radiometric data suggesting a Late Triassic (206–209 Ma) age of deformation at the Tavan Har locality. Deposits of the Unegt subbasin record an early history of basin subsidence beginning ∼155 Ma, with deposition of the Upper Jurassic Sharilyn and Lower Cretaceous Tsagantsav Formations (synrift sequences 1–3). Continued Lower Cretaceous synrift deposition is best recorded by thick deposits of the Zuunbayan Formation in the Zuunbayan subbasin, including newly defined synrift sequences 4–5. Geohistory modelling supports an extensional origin for the EGB, and preliminary thermal maturation studies suggest that a history of variable, moderately high heat flow characterized the Jurassic–Cretaceous rift period. These models predict early to peak oil window conditions for Type 1 or Type 2 kerogen source units in the Upper Tsagantsav/Lower Zuunbayan Formations (Synrift Sequences 3–4). Higher levels of maturity could be generated from distal depocentres with greater overburden accumulation, and this could also account for the observed difference in maturity between oil samples from the Tsagan Els and Zuunbayan fields.

Source facies of the Paleozoic petroleum systems in the Tabei uplift, Tarim Basin, NW China: implications from aryl isoprenoids in crude oils

Aryl isoprenoids have been detected for the first time in crude oils from Paleozoic petroleum systems in the Tabei uplift, Tarim Basin, northwestern China. The principal compounds possess the 1-alkyl-2,3,6-trimethyl substitution pattern characteristic of diaromatic carotenoids found in the Chlorobiaceae family of photosynthetic sulfur bacteria, with a predominance of C13 – C23 homologues in these samples. Flash pyrolysates of the asphaltene fractions isolated from crude oils show an unusual abundance of 1,2,3,4-tetramethylbenzene, an indicator for a significant contribution of diaromatic carotenoids to the source kerogen of these oils. The wide distribution of these compounds in crude oils reveals that penetration of the photic zone by sulfidic waters during deposition enabled prolific growth of photosynthetic green sulphur bacteria (Chlorobiaceae). This suggests that the source rocks for the Paleozoic petroleum systems in the Tabei uplift were likely deposited under euxinic conditions with sulfate and sulfide-rich water bodies, which doesn't support previously published conclusions of a Middle-Upper Ordovician source that marks the slope facies at the margins of structural uplifts.

COAL AS A SOURCE OF OIL AND GAS: A CASE STUDY FROM THE BASS BASIN, AUSTRALIA

Only a few published geochemical studies have demonstrated that coals have sourced significant volumes of oil, while none have clearly implicated coals in the Australian context. As part of a broader collaborative project with Mineral Resources Tasmania on the petroleum prospectivity of the Bass Basin, this geochemical study has yielded strong evidence that Paleocene–Eocene coals have sourced the oil and gas in the Yolla, Pelican and Cormorant accumulations in the Bass Basin.Potential oil-prone source rocks in the Bass Basin have Hydrogen Indices (HIs) greater than 300 mg HC/g TOC. The coals within the Early–Middle Eocene succession commonly have HIs up to 500 mg HC/g TOC, and are associated with disseminated organic matter in claystones that are more gas-prone with HIs generally less than 300 mg HC/g TOC. Maturity of the coals is sufficient for oil and gas generation, with vitrinite reflectance (VR) up to 1.8 % at the base of Pelican–5. Igneous intrusions, mainly within Paleocene, Oligocene and Miocene sediments, produced locally elevated maturity levels with VR up to 5%.The key events in the process of petroleum generation and migration from the effective coaly source rocks in the Bass Basin are:the onset of oil generation at a VR of 0.65% (e.g. 2,450 m in Pelican–5);the onset of oil expulsion (primary migration) at a VR of 0.75% (e.g. 2,700–3,200 m in the Bass Basin; 2,850 m in Pelican–5);the main oil window between VR of 0.75 and 0.95% (e.g. 2,850–3,300 m in Pelican–5); and;the main gas window at VR >1.2% (e.g. >3,650 m in Pelican–5).Oils in the Bass Basin form a single oil population, although biodegradation of the Cormorant oil has resulted in its statistical placement in a separate oil family from that of the Pelican and Yolla crudes. Oil-to-source correlations show that the Paleocene–Early Eocene coals are effective source rocks in the Bass Basin, in contrast to previous work, which favoured disseminated organic matter in claystone as the sole potential source kerogen. This result represents the first demonstrated case of significant oil from coal in the Australian context. Natural gases at White Ibis–1 and Yolla–2 are associated with the liquid hydrocarbons in their respective fields, although the former gas is generated from a more mature source rock.The application of the methodologies used in this study to other Australian sedimentary basins where commercial oil is thought to be sourced from coaly kerogens (e.g. Bowen, Cooper and Gippsland basins) may further implicate coal as an effective source rock for oil.

Vanadium of petroleum asphaltenes and source kerogens (La Luna Formation, Venezuela): isotopic study and origin

Abstract High-resolution mass spectrometry indicates that the isotopic abundance of 50 vanadium (V) of the Late Cretaceous La Luna petroleum asphaltenes and related source kerogens of marine origin (both highly enriched with V>2000 ppm) is higher by about 3.5% than that of inorganic source (VOSO 4 ·5H 2 O, Merck). Similar results are obtained with the isotopic analysis of the asphaltenes (containing high V) extracted from the floating asphalts (Dead Sea, Israel). We propose that the difference in the 50 V/ 51 V values between the La Luna petroleum asphaltenes/source kerogens and inorganic source can be best ascribed to the biological processing of the seawater V. The fact that the isotopic composition of V of the vary over a very narrow range (2.46–2.52) suggests an essentially same (or similar) and fixed (micro)biological source of V. Isotopic analysis was also extended to the methanol-soluble fractions of the La Luna asphaltic petroleums (DM-119/-120/-124) highly enriched with extractable (alkyl) vanadyl-porphyrins (VO 2+ -P). This analysis shows that the isotopic abundance of 50 V for the methanol-soluble fractions agrees (within the limits of experimental error) with those of the asphaltenes/kerogens.

Hydrogen isotope values in lacustrine kerogen

Kerogen derived from plankton and sedge/cattail that accumulated in sediment during two distinct phases of Swan Lake's development was extracted from segments of radiocarbon dated cores. Carbon (C), hydrogen (H), and nitrogen (N) amounts, and the δ D values of kerogen extracts were determined. Kerogen extracts younger than 3900 YBP contained higher average H/C and N/C atom ratios than older kerogen (5000 to 3900 YBP). The more recent kerogen extracts were derived primarily from plankton in open water sediment and are characterized by significantly higher lipid and protein fractions than those from the sedge/cattail that formed the marsh layer. Plankton-derived kerogen was more depleted in δ D, averaging ∼20 per mil (‰) lighter than that from older extracts (∼5000 to 3900 YBP) which, according to pollen analysis, were dominated by sedge/cattail-derived kerogen. In the sediment profiles, differences in the δ D values appear more related to kerogen source than to postdepositional hydrogen loss or temperature.

Carbon isotopic composition of organic acids in oil field waters, San Joaquin Basin, California, USA

The carbon isotopic compositions of short-chain organic acids (C 2-C 5) in oilfield waters of the San Joaquin Basin exhibit a wide range of values, from +0.57 to −27.33‰ (PDB). Most, however, are in the range of −15 to −25‰. The bulk isotopic compositions of individual acids generally become isotopically depleted (more negative) with increasing carbon number (chain length) but are consistently more enriched than the isotopic composition of coproduced oils. Relationships between carbon number and isotopic composition of the acids suggest intramolecular isotopic fractionation. Calculated carboxyl carbons (COOH) are isotopically more enriched, typically by 10 to 38‰, than aliphatic carbons (CH). Calculated aliphatic carbon isotopic compositions cluster around two values, −23 and −28‰, consistent with the isotopic composition of coproduced oils derived from two different sources, Miocene Monterey–sourced oils (−22 to −25‰) and Eocene Kreyenhagen–sourced oils (−28 to −30‰). We suggest that the isotopic composition of the aliphatic carbons in the organic acids reflects the carbon isotopic composition of the coproduced oils (or their source kerogen). Isotopic compositions of carboxyl carbons fall within a range of −12.8 to +8.0, consistently heavier than aliphatic carbons. We interpret the enrichment of carboxyl carbons to be inherited from either biological precursors in the source rock kerogen or due to exchange with carbon in dissolved HCO 3. The carbon isotopic enrichment of carboxyl carbons is at odds with theories that attribute organic acids to the inorganic oxidation of hydrocarbons in the subsurface.

Compositional variability of crude oils and source kerogen in the Silurian carbonate-evaporite sequences of the eastern Michigan Basin, Ontario, Canada

Compositional variability of crude oils and source kerogen in the Silurian carbonate-evaporite sequences of the eastern Michigan Basin, Ontario, Canada

<i>Pediastrum</i> in Lake Sediment and Its Mineralization

A lot of algae fossils were found in Cenozoic lake sediments, especially Pediastrum fossil sedimentary deposits in some places of China and also in some nearby seacoasts. For imitating Pediastrum mine ralization in lake sediments, several species of Pediastrum are cultured using BBM medium and sea water with 18% salt. By optical and electronic microscope observation and photosyntheic pigment analysis, we found that Pediastrum boryanum, P.duplex var clathratum and P.tetras , etc. cannot grow well in sea water. If cultured in saline medium, the specimen will die in a short period of time. We consider that probably the Pediastrum fossil that appeared in marine sediments were transported to the sea by river. In order to prove the mineralization condition of in living organism and fossils of Pediastrum, the following analysis is made. The fat soluble substances are high and they contain a small amount of alkane, alkene fatty acid and a great deal of plant pigments. Pediastrum has high H/C and low O/C (atom number ratio) value. Therefore, it is a good resource of oil formation. The products of oxidative degradation of the insoluble cell wall of Pediastrum are similar to the production obtained by kerogen, which is known to be of algal origin. So, the insolable materials of cell wall are probably a source of kerogen during the deposition.

The vanadium isotopic constitution of petroleum asphaltenes: La Luna Formation (Venezuela)

High resolution mass spectrometry indicates that the isotopic abundance of 50Vanadium (V) of the Late Cretaceous La Luna petroleum asphaltenes and related source kerogen of marine origin (both highly enriched with V > 2000 ppm) is higher by about 3.5% than that of an inorganic vanadium source (VOS04?5H20, Merek). It is proposed that the difference in the 50V/51V values between the La Luna source kerogen/the associated petroleum asphaltenes and the inorganic source can be best ascribed to the biological processing of seawater V. The fact that the isotopic compositions of V vary over a very narrow range (2.46-2.50) suggest an essentially same (or similar) and fixed biological source of V.