Ratios Of Kerogen Research Articles

What is the meaning of hydrogen-to-carbon ratio determined in Archean organic matter?

The search for hydrocarbon molecular biomarkers in Archean metasediments is of prime importance for deciphering the early evolution of life. Suitable criteria are required to identify promising targets for further molecular biomarkers. Possible criteria include the Hydrogen-to-Carbon (H/C) atomic ratio used as a proxy of the aliphatic content of the kerogen matrix. However, H/C ratio values exhibit large variation in Archean kerogens and their significance remains poorly understood. In this study, we thus investigate the significance of the H/C ratios of Archean kerogens by combining elemental analyses, Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS), Rock-Eval pyrolysis and Raman spectroscopy. First, NanoSIMS investigations show the H/C ratio of kerogen can be compromised by residual minerals. In addition, Rock-Eval pyrolysis underlines the fact that thermal cracking of Archean kerogens does not just release hydrocarbon covalently linked to the macromolecular network but also a complex mixture of organic pools distinguished by their thermal maturity. Therefore, the H/C ratio alone cannot be used to probe the preservation of aliphatic compounds bound to kerogen since it can be biased by the presence of (i) residual bitumen, as well as (ii) refractory organic matter in secondary hydrothermal veins whose syngenecity is debatable. Rock-Eval pyrolysis then provides a useful and complementary method to check the significance of H/C atomic ratio as a proxy for hydrocarbon preservation in Archean kerogens.

Kerogen maturation data in the Uinta Basin, Utah, USA, constrain predictions of natural hydrocarbon seepage into the atmosphere

Natural seepage of methane from the lithosphere to the atmosphere occurs in regions with large natural gas deposits. According to some authors, it accounts for roughly 5% of the global methane budget. I explore a new approach to estimate methane fluxes based on the maturation of kerogen, which is the hydrocarbon polymer present in petroleum source rocks and whose decomposition leads to the formation of oil and natural gas. The temporal change in the atomic H/C ratio of kerogen lets us estimate the total carbon mass released by it in the form of oil and natural gas. Then the time interval of active kerogen decomposition lets us estimate the average annual formation rate of oil and natural gas in any given petroleum system, which I demonstrate here using the Uinta Basin of eastern Utah as an example. Obviously, this is an upper bound to the average annual rate at which natural gas seeps into the atmosphere. After adjusting for biooxidation of natural gas, I conclude that the average annual seepage rate in the Uinta Basin is not greater than (3100 ± 900) tonne yr−1. This is (0.5 ± 0.15)% of the total flux of methane into the atmosphere over the Basin, as measured during aircraft flights. I speculate about the difference between the regional 0.5% and the global 5% estimates.

The atomic H/C ratio of kerogen and its relation to organic geochemical parameters: implications for evaluating hydrocarbon generation of source rock

In this study, kerogen samples, including 19 from northwestern Taiwan, 15 from China, and 16 from Australia were analyzed, and integrated with 49 published data, to explore the significance of atomic H/C ratio as a parameter of source rock evaluation. Organic matters in the samples studied are of type II/III kerogen based on the relationship between HI and Tmax. The H/C ratio, as well as the HI, S1, and S2, generally decreases with the maturity increasing. The H/C ratio decreases slightly from 1.1 to 0.7 with the maturity increasing from Ro 0.55 to 0.85 %. Samples with H/C ratio in this range show significant change in certain other geochemical parameters (e.g., HI, S1, S2, S1 + S2, S1/(S1 + S2), S1/TOC, (S1 + S2)/TOC, Tmax). The atomic H/C value is in the range of 0.7–1.1, the value of the corresponding HI shows a stable increasing trend. This shows that organic matters in an oil window of Ro% = 0.55 and atomic H/C = 1.1 have reached a certain maturity and hydrocarbon potential. The QI, HI and H/C ratio shows a certain correlation, all increasing accordingly. It is inferred that hydrocarbon potential started from Ro 0.55 % and atomic H/C ratio 1.1 in this study.

Analysis and characterization of samples from sedimentary strata with correlations to indicate the potential for hydrocarbons

The Rock-Eval pyrolysis and TOC analysis have been widely used to evaluate the source rock quality. The atomic H/C ratio of kerogen, however, has been overlooked in source rock evaluation. In this study, coal and carbonaceous samples, including 26 from northwestern Taiwan, 12 from China, and 4 from the United States were analyzed, and integrated with 157 published data, to explore the significance of atomic H/C ratio as a parameter of source rock evaluation. Two different linear trends were observed in the cross-plot of S 1 versus S 2. Field outcropped shale or C-shale exhibits a steeper slope compared to that of coal samples which can be attributed to the compositional difference in their organic material. A rather strong positive correlation for H% versus S 2 illustrates the contribution of H-containing macerals, especially exinite. Organic matters in the samples studied are of type II/III kerogen based on the relationship between HI and T max. The H/C ratio, as well as the HI, S 1, and S 2, generally decreases with the maturity increasing. The H/C ratio decreases slightly from 1.1 to 0.7 with the maturity increasing from R o 0.55 to 0.85%. Samples with H/C ratio in this range show significant change in certain other geochemical parameters (e.g. HI, S 1, S 2, S 1 + S 2, S 1/(S 1 + S 2), S 1/TOC, (S 1 + S 2)/TOC, T max). The (S 1 + S 2)/TOC ratio (defined as QI) was used as an indicator of the hydrocarbon potential. The QI, HI, and H/C ratio show a certain correlation, all increasing accordingly. The QI of the samples analyzed in this study is approximately 100–380 (mgHC/gTOC), similar to that of most humic coals for oil and gas generation. Samples with R o value lower than 0.55% always show significant variation in their HI, ranging from 80 to 520 mgHC/gTOC. It is inferred that hydrocarbon potential started from R o 0.55% and atomic H/C ratio 1.1 in this study.

Dike intrusions into bituminous coal, Illinois Basin: H, C, N, O isotopic responses to rapid and brief heating

Unlike long-term heating in subsiding sedimentary basins, the near-instantaneous thermal maturation of sedimentary organic matter near magmatic intrusions is comparable to artificial thermal maturation in the laboratory in terms of short duration and limited extent. This study investigates chemical and H, C, N, O isotopic changes in high volatile bituminous coal near two Illinois dike contacts and compares observed patterns and trends with data from other published studies and from artificial maturation experiments. Our study pioneers in quantifying isotopically exchangeable hydrogen and measuring the D/H (i.e., 2H/ 1H) ratio of isotopically non-exchangeable organic hydrogen in kerogen near magmatic contacts. Thermal stress in coal caused a reduction of isotopically exchangeable hydrogen in kerogen from 5% to 6% in unaltered coal to 2–3% at contacts, mostly due to elimination of functional groups (e.g., OH, COOH, NH 2). In contrast to all previously published data on D/H in thermally matured organic matter, the more mature kerogen near the two dike contacts is D-depleted, which is attributed to (i) thermal elimination of D-enriched functional groups, and (ii) thermal drying of hydrologically isolated coal prior to the onset of cracking reactions, thereby precluding D-transfer from relatively D-enriched water into kerogen. Maxima in organic nitrogen concentration and in the atomic N/C ratio of kerogen at a distance of ∼2.5 to ∼3.5 m from the thicker dike indicate that reactive N-compounds had been pyrolytically liberated at high temperature closer to the contact, migrated through the coal seam, and recombined with coal kerogen in a zone of lower temperature. The same principle extends to organic carbon, because a strong δ 13C kerogen vs. δ 15N kerogen correlation across 5.5 m of coal adjacent to the thicker dike indicates that coal was functioning as a flow-through reactor along a dynamic thermal gradient facilitating back-reactions between mobile pyrolysis products from the hot zone as they encounter less hot kerogen. Vein and cell filling carbonate is most abundant in highest rank coals where carbonate δ 13C VPDB and δ 18O VSMOW values are consistent with thermal generation of 13C-depleted and 18O-enriched CO 2 from decarboxylation and pyrolysis of organic matter. Lower background concentrations of 13C-enriched carbonate in thermally unaffected coal may be linked to 13C-enrichment in residual CO 2 in the process of CO 2 reduction via microbial methanogenesis. Our compilation and comparison of available organic H, C, N isotopic findings on magmatic intrusions result in re-assessments of majors factors influencing isotopic shifts in kerogen during magmatic heating. (i) Thermally induced shifts in organic δD values of kerogen are primarily driven by the availability of water or steam. Hydrologic isolation (e.g., near Illinois dikes) results in organic D-depletion in kerogen, whereas more common hydrologic connectivity results in organic D-enrichment. (ii) Shifts in kerogen (or coal) δ 13C and δ 15N values are typically small and may follow sinusoidal patterns over short distances from magmatic contacts. Laterally limited sampling strategies may thus result in misleading and non-representative data. (iii) Fluid transport of chemically active, mobile carbon and nitrogen species and recombination reactions with kerogen result in isotopic changes in kerogen that are unrelated to the original, autochthonous part of kerogen.

Origin and deposition of organic matter in continental chert of the Middle Permian Gufeng Formation in the northeastern Yangtze platform

The Middle Permian Gufeng Formation in the northeastern Yangtze platform, China, contains radiolarian chert that is characterized by black color, high content of Fe, Mo, Ni, Cu, and Zn. Based on the inorganic geochemical features, previous work concluded that the cherts were deposited under sulfate-reducing conditions. In this study, organic carbon (C org), N and S contents of the cherts and H/C and O/C ratios of kerogen were examined to explore the origin of the organic matter. The concentration of C org in the black chert is extremely high relative to other cherts in accretionary complexes and continental platforms. The average C/N ratio of organic matter is 35, indicating that the organic matter originates from land plants. The kerogen from the chert is classed as type IV based on the H/C and O/C ratios, suggesting that oxidized or reworked organic matter was supplied to the Gufeng basin. Therefore, the organic matter is considered to originate from terrestrial plants or reworked material from land. Due to the depositional setting on a continental platform, nutrient supplied with organic matter from land probably played an important role in abundant biogenic productivity and sedimentation of the radiolarian chert on the continental platform.

Experimental controls on D/H and 13C/12C ratios of kerogen, bitumen and oil during hydrous pyrolysis

Large isotopic transfers between water-derived hydrogen and organic hydrogen occurred during hydrous pyrolysis experiments of immature source rocks, in spite of only small changes in organic 13C/12C. Experiments at 330 °C over 72 h using chips or powder containing kerogen types I and III identify the rock/water ratio as a main factor affecting ΔδD for water and organic hydrogen. Our data suggest that larger rock permeability and smaller rock grain size increase the H-isotopic transfer between water-derived hydrogen and thermally maturing organic matter. Increasing hydrostatic pressure may have a similar effect, but the evidence remains inconclusive.

Carbon, nitrogen, and sulfur geochemistry of Archean and Proterozoic shales from the Kaapvaal Craton, South Africa

The C, N, and S contents and VC and δ 13Cδ 34S values were analyzed for 100 shale samples from ten formations, 3.0 to 2.1 Ga in age, in the central and eastern regions of the Kaapvaal Craton, South Africa. The Kaapvaal shales are characterized by generally low contents of organic C (range 0.06–2.79 wt%, average 0.47 wt%), N (range <0.01–0.09 wt%, average 0.1 wt%), and S (range <0.01–1.63 wt%, average 0.1 wt%). The low N/C (<0.005) and H/C (mostly ∼0.2) atomic ratios in kerogens from the shales indicated that the Kaapvaal shales lost considerable amounts of N, C, S, and H during diagenesis and regional metamorphism (up to the greenschist facies). From the theoretical relationships between the H/C ratios of kerogen and organic C contents of shales, the original C contents of the Archean and Proterozoic shales from the Kaapvaal Craton are estimated to be on average ∼2 wt%. These values are similar to the average organic C content of modern marine sediments. This suggests that the primary organic productivity and the preservation of organic matter in the ocean during the period of 3.0 to 2.1 Ga were similar to those in the Phanerozoic era, provided the flux of clastic sediments to the ocean was similar. This would also imply that the rate of O 2 accumulation in the atmosphere-ocean system, which has equaled the burial rate of organic matter in sediments, has been the same since ∼3.0 Ga. The δ 34S values of bulk-rock sulfides (mostly pyrite) range from +2.7 to +7.4%‰ for seven sulfide-rich samples of ∼2.9 Ga to ∼2.6 Ga. These values are consistent with a suggestion by Ohmoto (1992) and Ohmoto et al. (1993) that most pyrite crystals in Archean shales were formed by bacterial reduction of seawater sulfate with δ 34S values between +2 and +10‰, and that the Archean seawater was sulfate rich. Changes in the δ 13C org values during maturation of kerogen were evaluated with theoretical calculations from the experimental data of Peters et al. (1981) and Lewan (1983), and from the observations by Simoneit et al. (1981) on natural samples. These evaluations suggest that the magnitudes of δ 13C org increase are much less than those estimated by Hayes et al. (1983) and Des Marais et al. (1992), and only about 2 to 3%‰ for the kerogens that decreased their H/C ratios from 1.5 to less than 0.3. Based on the relationships among sulfide-S contents, organic-C contents, and δ 13C org values, four different types of depositional environments are identified for the Archean and early Proterozoic shales in the Kaapvaal Craton: (I) euxinic marine basins, characterized by normal marine organisms with δ 13C org= −33 ± 3%‰ (II) near-shore, oxic marine environment, characterized by normal marine organisms with δ 13C org = −31 ± 3%‰; (III) hypersaline, low-sulfate lakes, characterized by organisms with δ 13C org= −2 ± 3%‰; and (IV) euxinic, marine basins which supported the activity of methanogenic and methanotrophic bacteria and accumulated organic matter with δ 13C org= −43 ± 3%‰. In contrast to the currently popular model positing a global anoxic ocean prior to ∼2.2 Ga (e.g., Des Marais et al, 1992; Hayes, 1994; Logan et al., 1995), this study suggests that the development of anoxic basins, which accumulated Group II and IV sediments, occurred only regionally and episodically during the period between 3.0 Ga and 2.1 Ga. This further suggests that the normal ocean has been oxic since at least ∼3.0 Ga. Diversifications of environments, as well as of biological species, had already occurred ∼3.0 Ga. The carbon isotope mass balance calculation suggests that the removal rates of organic C and carbonate C from the ocean and the weathering rates of organic C and carbonate C on the continents during the 3.0–2.1 Ga period were basically the same as those in the Phanerozoic era. This would have been possible only if the atmospheric P O 2 level had been basically constant since at least 3.0 Ga. The results of this study, therefore, add to a growing list of evidence that the atmosphere has been oxic (i.e., P O 2 > 1%PAL) since at least 3.0 Ga. The list of evidence includes the sulfur isotope data on Archean sedimentary rocks ( Ohmoto and Felder, 1987; Ohmoto et al., 1993), the Fe 3+Ti ratios of paleosols ( Ohmoto, 1996), and the paragenesis of minerals in the “detrital” gold-uranium ores in pre-2.0 Ga quartz pebble beds that suggests nondetrital origins for uraninite and pyrite in these deposits ( Barnicoat et al., 1997).

Atomic H/C Ratio of Kerogen as an Estimate of Thermal Maturity and Organic Matter Conversion

In recent years, the atomic H/C (hydrogen to carbon) ratio of kerogen as a way to assess the quality of organic matter in source rocks has been overlooked in favor of the more easily determined Rock-Eval hydrogen index. Rock-Eval pyrolysis provides fast, inexpensive, quantitative (mg HC/g rock) data without requiring kerogen isolation from the rock. Because of the general scatter in the data, many source rock interpreters consider Rock-Eval pyrolysis to be a screening analysis. In this paper I describe the benefits of using H/C ratios in source rock evaluations and present new correlations between atomic H/C ratios and thermal maturity, organic matter conversion, and expulsion volumetrics. Atomic H/C ratios of pyrolyzed kerogens have been correlated to the extent of thermal conversion of organic matter for both type I and type II kerogens. The excellent agreement between stoichiometric calculated hydrogen and carbon loss to observed losses from hydrous pyrolysis maturation experiments suggests that kerogen H/C ratios are excellent indicators of thermal maturity for end-member kerogen types. These data also offer a method to estimate percent organic matter conversion, provided that both the initial and present H/C ratios of the kerogen are known. Present H/C ratios can be measured, and initial H/C ratios can be reasonably estimated, from microscopic organic analysis of kerogen. For oil-prone source rocks, typical immature type I kerogens have H/C ratios of 1.35-1.50, whereas type II kerogens have H/C ratios of 1.20-1.35. Correlations of the amount of expelled oil in hydrous pyrolysis experiments to atomic H/C ratio of the spent kerogen offer exploration geologists a quick estimate of oil expulsion volumes. Based on hydrous pyrolysis experiments, measured H/C ratios, and calculated original TOC (total organic carbon) values, first-order volumetric approximations were made on three basins containing mature source rocks. Results compared favorably with published approximate-oil-in-place estimates for the Williston basin (Bakken shale), Los Angeles basin (Nodular shale), and the Illinois basin (New Albany Shale).

Geochemical Indicators of Depositional Environment and Source-Rock Potential for the Upper Ordovician Maquoketa Group, Illinois Basin

Two depositional cycles are recognized within the Upper Ordovician Maquoketa Group of the Illinois basin in a core from the New Jersey Zinc 1 Parrish well of Fulton County, Illinois. Organic carbon (Corg), total sulfur, 13C content of the organic carbon (^dgr13Corg ), hydrogen and oxygen indices (HI and OI) from Rock-Eval pyrolysis, and yields of extractable organic matter (EOM) vary through the cycles. Dark-brown to black, laminated shales are present in the lower portion of each cycle and have high values of Corg (1.0-3.0%), HI (500-1000 mg hydrocarbon [HC]/g total organic carbon[TOC]), and EOM (500-2500 ppm), and more negative ^dgr13Corg values (^dgr13Corg = -30 to -30.5^ mil). Gray to greenish-gray, bioturbated shales are present in the upper portion of each cycle and have low values of Corg (<1.0%), HI (<500 mg HC/g TOC), and EOM (<500 ppm), and more positive ^dgr13Corg values (-28.5 to -29.5^pmil) compared to the laminated shales. Although thermally immature to marginally mature in this core, the laminated shales represent potential source rocks for petroleum because they contain good to excellent quantities of oil-prone organic matter and are more deeply buried in other areas of the basin. Kerogen elemental data and Rock-Eval data suggest that the source of organic matter in the Maquoketa was uniform, with the notable exception of graptolite-rich layers. Distributions of saturated hydrocarbons for Maquoketa samples resemble those derived from amorphous organic matter. The organic-walled microfossil Gloeocapsomorpha prisca was not identified in kerogens of the Maquoketa shale, and its absence is most likely due to an unfavorable depositional environment and/or climate. Variations in bulk geochemical data and carbon isotopic compositions within the Maquoketa Group indicate substantial reworking and degradation of organic matter associated with bioturbation and oxygenated depositional conditions. Organic-rich shale was deposited in an environment where circulation of the water column was sluggish and dissolved-oxygen concentrations were low, resulting in the preservation of organic matter. Organic-poor shale was deposited in an environment where circulation of the water column was strong, dissolved-oxygen concentrations were high, and burrowing organisms thoroughly reworked the sediment, resulting in the degradation of organic matter. A maximum difference in carbon isotopic ratios of 2^pmil between organic-rich and organic-poor units of the Maquoketa roup suggests that heterotrophic reworking of organic matter was important in the depositional environment. The carbon isotopic composition and H/C ratio of kerogen for graptolite-rich shale suggests that organic productivity may have played an intermittent role in the accumulation of organic matter in the Maquoketa shale.

Vertical variations of organic matter content in Guayuta Group (upper Cretaceous), Interior Mountain Belt, Eastern Venezuela

A geochemical study of the vertical variations of organic matter in the Guayuta Group (Querecual and San Antonio Formations) has been conducted on a series of samples taken from a sedimentary sequence outcropping in the Interior Mountain Belt. The results indicate that despite very high thermal maturity, the organic matter content in the samples is still very high and shows geochemical characteristics related to anoxic paleoenvironmental conditions of sedimentation. Based on micropaleontological data, mineralogical data, and trace metal (V/Ni) ratio of kerogen, the analysis of the sequence shows stronger anoxic depositional conditions toward the base of the section and an increase of clastic sediments toward the top. The total organic carbon of the sediments increases toward the base of the sequence. In addition, there are pseudocyclic patterns and heterogeneities of the organic carbon content distribution at the meter-scale. The results indicate that the lower part of this sedimentary sequence represents a major anoxic event, that can be correlated with other sequences of middle and upper Cretaceous age.

Temperature effects on kerogen and on molecular and isotopic composition of organic matter in Pierre Shale near an igneous dike

A suite of siltstone samples from the Upper Cretaceous Pierre Shale from the contact zone of a 130-cm thick igneous dike near Wolcott, Colorado, U.S.A., was taken from the contact to 170 cm from the dike to study the effects of temperature on the organic matter. The sampled bedding interval was about 10 cm thick, so variation in lithology and type of organic matter is minimal. Vitrinite reflectance values ( R 0) increase from 0.4 far from the dike, to 3.3% near the dike contact. Geochemical measurements show systematic thermal effects analogous to those often observed for catagenesis and metagenesis in the depth range of 1–4 km within a sedimentary basin. The H/C ratio of kerogen and the hydrogen index (Rock-Eval) decrease most rapidly in the 0.6–1.7% R 0 range, in which the transformation ratio (Rock-Eval) increases from 0.1 to 0.3. Based on extraction of C 15+ compounds, the main increase of hydrocarbons and total extractable organic matter occurs between 0.6 and 1.0% reflectance. The saturated/aromatic hydrocarbon ratio increases almost twofold in this range of maturity. However, the pristane/phytane ratio is essentially constant through the hydrocarbon generation zone but decreases slightly at high levels of thermal alteration ( R 0 > 1.2%). The δ 13 C values for aromatic and saturated hydrocarbons are about −27 and −29‰, respectively, and are constant to about 1.0% R 0, then both become heavier by about 2‰ at higher R 0 values.

An approach to estimate the evolution of organic matter and its application to resource evaluation

On the basis of atomic H/C and O/C ratios of kerogen and by using a simple mathematical treatment, it is possible to determine the post-depositional evolution of organic matter. This technique further allows us to estimate the content of organic starting material, the amount of petroleum that can be released and that of pyrolytic natural gas generated. It is suggested that the metagenetic products of kerogen are mainly CO 2 and H 2. The latter controls the hydrogenation of previously produced hydrocarbons and the generation of methane at depth.

THE ORGANIC GEOCHEMISTRY OF OIL AND GAS GENERATION AND ITS APPLICATION TO BASS STRAIT AND THE NORTHWEST SHELF

A study of the chemical reactions occurring within the geological environment can throw new light on the occurrence of petroleum and natural gas deposits. The principles of chemical kinetics enable extrapolations to be made so that the depths and temperatures of typical hydrocarbon-forming reactions can be deduced when intervals of geological time are available. In general, temperature is the crucial parameter with corrections being possible for shorter or longer heating times.Living organic matter available for incorporation in sediments is of four basic types: carbohydrates, proteins, lignin and lipids. Only the last two are significant precursors of organic matter (kerogen) in consolidated sediments. Lipid-derived material is the principal source of oil, while lignin-related kerogen gives methane as its main gaseous hydrocarbon product. Generation occurs in zones where the rate of cracking of the source material into volatile hydrocarbons due to increasing temperature becomes significant on the geological time-scale. During and after migration of oil from its source to reservoirs (usually at shallower depths and lower temperatures), other processes can alter the initial light oil. In particular, biodegradation by microorganisms can result in a heavier naphthenic or aromatic crude by the loss of normal alkanes.Application of organic geochemistry to specific basins involves the measurement on uncontaminated core samples of certain parameters (organic carbon, extractable hydrocarbons, normal alkane distributions, reflectance of vitrinite, H/C ratio of kerogen and spore coloration). These data, together with activation energies for specific chemical reactions, geothermal gradients, rates of subsidence and knowledge of the geological history of the basin enable a general view of generation, migration and preservation of oil and gas to be obtained. Such an overview is given for the Gippsland Basin, where generation of both oil and gas in the lower Latrobe Group is indicated, and for the Rankin Platform of the Northwest Shelf where a deep source at greater than 4000 m is suggested for the gas and condensate.