Equivalent Vitrinite Reflectance Values Research Articles

Permian carbonaceous rocks from the Bonito Coalfield, Santa Catarina, Brazil: Organic facies approaches

Permian sedimentary rocks of the Rio Bonito Formation, Paraná Basin, in the Santa Catarina state, Brazil, contain three important coal seams, in a descending order: Barro Branco, Bonito Superior and Pre-Bonito Superior. Artinskian carbonaceous shale samples corresponding to the so called “Bonito coal” were sampled at the mine face of the coal bed of the Bonito I underground mine. These samples were analyzed in order to discuss their petrographical, geochemical, palynological, and paleobotanical characteristics. Palynological analyses demonstrated abundant and well-preserved palynomorphs showing a dominance of bisaccate pollen grains related to gymnospermic plants. Wood fragments preserved as coalified compression show the presence of lycophyta. Palynofacies on samples demonstrated good results for the recognition of preserved organic matter and changes in paleoenvironmental conditions along the sedimentary sequence. Palynofacies and palynological analyses indicate changes in paleoecological conditions that have favored the development of different plant communities during the depositional time span from a forested gymnosperm at the base to a lycophyte, arborescent plant dominated scenario at the top suggesting a shift to coastal environment. The microscopic maceral-group analyses reveal a predominance of liptinite and vitrinite over inertinite, while among microlithotypes, carbargilite is abundant. The joint occurrence of inertinite as a maceral in the carbonaceous rock and charcoal in associated shales suggests that the mire and near surroundings were regularly subject to wildfires. Mean random vitrinite reflectance (Rr) varies from 0.88 to 1.04%. The liptinite spectral fluorescence parameters obtained for samples show the λmax values varying from 610 to 650 indicating equivalent vitrinite reflectance (Rreq) values from 0.83 to 1.10%. These results show that the equivalent vitrinite reflectance (Rreq) correlates well with the measured vitrinite reflectance (Rr). The “Bonito Coal” is a high-volatile bituminous coal according to the ISO, 11760: 2005 standard. Total organic carbon varies from 2.4 to 44.0wt.% and sulfur is present in moderate to high contents. GC-MS analyses evidenced a predominance of even saturated hydrocarbons and the presence of αβ-hopanes and αββ-steranes.

Enhanced late gas generation potential of petroleum source rocks via recombination reactions: Evidence from the Norwegian North Sea

Gas generation in the deep reaches of sedimentary basins is usually considered to take place via the primary cracking of short alkyl groups from overmature kerogen or the secondary cracking of petroleum. Here, we show that recombination reactions ultimately play the dominant role in controlling the timing of late gas generation in source rocks which contain mixtures of terrigeneous and marine organic matter. These reactions, taking place at low levels of maturation, result in the formation of a thermally stable bitumen, which is the major source of methane at very high maturities. The inferences come from pyrolysis experiments performed on samples of the Draupne Formation (liptinitic Type II kerogen) and Heather Formation (mixed marine–terrigeneous Type III kerogen), both Upper Jurassic source rocks stemming from the Norwegian northern North Sea Viking Graben system. Non-isothermal closed system micro scale sealed vessel (MSSV) pyrolysis, non-isothermal open system pyrolysis and Rock Eval type pyrolysis were performed on the solvent extracted, concentrated kerogens of the two immature samples. The decrease of C 6+ products in the closed system MSSV pyrolysis provided the basis for the calculation of secondary gas (C 1–5) formation. Subtraction of the calculated secondary gas from the total observed gas yields a “remaining” gas. In the case of the Draupne Formation this is equivalent to primary gas cracked directly from the kerogen, as detected by a comparison with multistep open pyrolysis data. For the Heather Formation the calculated remaining gas formation profile is initially attributable to primary gas but there is a second major gas pulse at very high temperature (>550 °C at 5.0 K min −1) that is not primary. This has been explained by a recondensation process where first formed high molecular weight compounds in the closed system yield a macromolecular material that undergoes secondary cracking at elevated temperatures. The experiments provided the input for determination of kinetic parameters of the different gas generation types, which were used for extrapolations to a linear geological heating rate of 10 −11 K min −1. Peak generation temperatures for the primary gas generation were found to be higher for Heather Formation ( T max = 190 °C, equivalent to R o appr. 1.7%) compared to Draupne Formation ( T max = 175 °C, equivalent to appr. R o 1.3%). Secondary gas peak generation temperatures were calculated to be 220 °C for the Heather Formation and 205 to 215 °C for the Draupne Formation, respectively, with equivalent vitrinite reflectance values ( R o) between 2.4% and 2.0%. The high temperature secondary gas formation from cracking of the recombination residue as detected for the Heather Formation is quantitatively important and is suggested to occur at very high temperatures ( T max approx. 250 °C) for geological heating rates. The prediction of a significant charge of dry gas from the Heather Formation at very high maturity levels has important implications for petroleum exploration in the region, especially to the north of the Viking Graben where Upper Jurassic sediments are sufficiently deep buried to have experienced such a process.

A kinetic model for thermally induced hydrogen and carbon isotope fractionation of individual n-alkanes in crude oil

A quantitative kinetic model has been proposed to simulate the large D and 13C isotope enrichments observed in individual n-alkanes (C 13–C 21) during artificial thermal maturation of a North Sea crude oil under anhydrous, closed-system conditions. Under our experimental conditions, average n-alkane δ 13C values increase by ∼4‰ and δD values increase by ∼50‰ at an equivalent vitrinite reflectance value of 1.5%. While the observed 13C-enrichment shows no significant dependence on hydrocarbon chain length, thermally induced D-enrichment increases with increasing n-alkane carbon number. This differential fractionation effect is speculated to be due to the combined effect of the greater extent of thermal cracking of higher molecular weight, n-alkanes compared to lower molecular weight homologues, and the generation of isotopically lighter, lower molecular weight compounds. This carbon-number-linked hydrogen isotopic fractionation behavior could form the basis of a new maturity indicator to quantitatively assess the extent of oil cracking in petroleum reservoirs. Quantum mechanical calculations of the average change in enthalpy (ΔΔH ‡) and entropy (ΔΔS ‡) as a result of isotopic substitution in n-alkanes undergoing homolytic cleavage of C-C bonds lead to predictions of isotopic fractionation that agree quite well with our experimental results. For n-C 20 ( n-icosane), the changes in enthalpy are calculated to be ∼1340 J mol -1 (320 cal mol -1) and 230 J mol -1 (55 cal mol -1) for D-H and 13C- 12C, respectively. Because the enthalpy term associated with hydrogen isotope fractionation is approximately six times greater than that for carbon, variations in δD values for individual long-chain hydrocarbons provide a highly sensitive measure of the extent of thermal alteration experienced by the oil. Extrapolation of the kinetic model to typical geological heating conditions predicts significant enrichment in 13C and D for n-icosane at equivalent vitrinite reflectance values corresponding to the onset of thermal cracking of normal alkanes. The experimental and theoretical results of this study have significant implications for the use of compound-specific hydrogen isotope data in petroleum geochemical and paleoclimatological studies. However, there are many other geochemical processes that will significantly affect observed hydrogen isotopic compositions (e.g., biodegradation, water washing, isotopic exchange with water and minerals) that must also be taken into consideration.

Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China. Part 2: Maturity assessment

The thermal maturity of forty three crude oil samples collected from the cratonic portion of the Tarim Basin, NW China has been assessed using a variety of parameters. While compound ratios of diamondoids, alkylated naphthalenes, methyl phenanthrenes and methyl dibenzothiophenes showed consistent trends with increasing maturity, a large discrepancy was observed for the maturity levels of the oils determined from these parameters. Equivalent vitrinite reflectance values calculated from methylphenanthrene index 1 (MPI1), methyldibenzothiophene ratio (MDR) and methyldiamontane index (MDI) were in the range of 0.66–0.97%, 1.11–1.64% and 0.89–1.74% Ro, respectively. While we caution that some of the discrepancy may be related to calibration problems, the difference observed from different compound classes could be real, reflecting the multiple charge to petroleum accumulations, as the diamondoid hydrocarbons were possibly generated at later maturation stages than most aromatic hydrocarbons. As diamondoids with methyl substitution on the bridgehead positions are more stable than those with substitution on other positions, several compound ratios based on ethyl admantanes, dimethyl admantanes, trimethyl admantanes and dimethyl diamantanes also correlated with the established maturity indicators such as MDR and MDI. While condensates with high maturity levels represent the late charge hydrocarbons, some of them with relatively low maturity levels may be affected by evaporative fractionation processes. The normal gravity oils cover a wide maturity range, consistent with multiple charges and in-reservoir mixing. While the waxy oils with high apparent maturity levels may be the products of wax precipitation due to a later gas injection into an oil column, heavy oils with low apparent maturity levels could be the residue of oil that has lost its light end.

Effect of maturation on the indigenous δD signatures of individual hydrocarbons in sediments and crude oils from the Perth Basin (Western Australia)

We measured the stable hydrogen isotopic composition ( δD) of selected aliphatic hydrocarbons in nine sediments of various maturity and two crude oils, all from the northern Perth Basin (on-shore, Western Australia). The sediments and crude oils are from the Lower Triassic Hovea Member (Sapropelic Interval) of the Kockatea Shale. The n-alkanes, pristane and phytane from two immature sediments have δD values that represent the expected isotopic compositions of their precursors. Phytane is enriched in deuterium (D) relative to pristane in all sediment extracts and crude oils; this is attributed to either different sources, or to different isotopic effects during their derivation from phytol. With increasing maturity, pristane and phytane become enriched in D while the n-alkanes generally remain at a constant isotopic composition. A more rapid enrichment of D in isoprenoids relative to n-alkanes with increasing maturity suggests that hydrogen isotopic exchange is occurring via a carbocation mechanism. The phytane diastereomer ratio for each of the sediments correlates linearly with the progressive enrichment of D in phytane and therefore maturity, suggesting that significant hydrogen exchange occurs at the chiral carbons of phytane. The δD values of pristane and phytane from the crude oils are similar or more positive relative to the δD values of the n-alkanes. The average δD values of pristane and phytane for the sediments correlate well with equivalent vitrinite reflectance values, as does the conventional biomarker maturity parameter T s/ T m, indicating that δD values may be useful for establishing maturity over a wider range than conventional biomarker parameters. The results provide further evidence that care must be taken not to over-interpret δD values of sedimentary hydrocarbons in samples of high thermal maturity.

THE APPLICATION OF ELECTRON SPIN RESONANCE AS A GUIDE TO THE MATURATION AND TYPING OF ORGANIC MATTER IN THE NORTH SEA

In early electron spin resonance (ESR) analysis of North Sea wells, maturation of organic matter (OM) was expressed in terms of maximum palaeotemperature (MPT) based on North American calibrations that did not consider the influences of kerogen composition or overpressure. In the North Sea, the MPTs were anomalous in overpressured sequences and relative to other indices of OM maturation such as vitrinite reflectance, so the ESR method was abandoned there in geochemical studies. However, early empirical study of North Sea ESR data indicated that, in relation to functions that linked temperature and pore pressure, some ESR parameters were predictable without reference to MPTs.In order to re‐evaluate ESR parameters as indices of OM maturation, the physical factors (temperature and pressure) which affect OM maturation are related in the present paper to the ESR parameters “g” (spectral position) and Ng (spin density) at six well locations in the northern North Sea. A third ESR parameter, W (line width), is not an effective guide to maturation levels due to its complex relationship to the physical factors and kerogen types. However, cross‐plots of W versus “g” and Ng appear to be as effective as pyrolysis for kerogen typing. Levels of maturation investigated in the North Sea wells range through the equivalent vitrinite reflectance values of about 0.50–1.50%. The values of “g” and Ng have been differentiated for kerogen type, but undifferentiated values of “g” have also been studied.Regression analysis has shown that there are linear relationships between the ESR parameters “g” and Ng, and the physical factors present‐day temperature (To), “effective” temperature (Te), and differential pressure (Pd). Correlation coefficients for both “g” (undifferentiated and differentiated) and Ng (differentiated) relative to the physical factors are high; the highest values are for “g” and Ng relative to Te and Pd (r =−0.950 for “g” differentiated or undifferentiated, r = 0.944–0.976 for Ng differentiated, respectively). However, correlation coefficients were lower for “g” and Ng relative to To.More frequent high correlation coefficients and larger sample populations suggest that “g” (undifferentiated) is a more reliable index of OM maturation than Ng(differentiated). However, the estimation of levels of OM maturation is improved if both indices are used together.The ESR method appears to be effective both for estimating levels of OM maturation and for kerogen typing. It has a number of potential advantages over other geochemical methods: firstly, it is more sensitive for estimating OM maturation than most other methods; secondly, it can be used to analyze organic matter which is as old as Proterozoic; thirdly, it does not destroy the samples analyzed.

A fluorescence alteration of multiple macerals (FAMM) study of Netherlands coals with “normal” and “deviating” vitrinite reflectance

Suites of well-characterised Westphalian coals from The Netherlands with “normal” and “deviating” vitrinite reflectance have been examined by the FAMM (fluorescence alteration of multiple macerals) technique. The fluorescence alteration characteristics of the “normal” Netherlands coals change with rank in a similar manner to “normal” Australian Permian coals, except that the telovitrinites of the Netherlands coals appear to be relatively more perhydrous. This difference is also apparent in a comparison of Rock-Eval hydrogen index values. A fluorescence alteration diagram has been calibrated for the Netherlands Carboniferous from which accurate equivalent vitrinite reflectance values can be obtained, as well as suppression corrections for measured vitrinite reflectances. However, the errors in using the equivalent Australian Permian diagram for the determination of the suppression correction are small (≤0.05% absolute). The well Hengevelde-1 is used as an example to illustrate how corrections may be made to perhydrous samples with “deviating” vitrinite reflectance for maturation assessment in petroleum exploration.

COMPARISON OF PALAEOTEMPERATURE INDICATOR TECHNIQUES: AFTA, VR, FAMM AND ICA- CONSIDERATIONS FOR DESIGNING SAMPLING PROGRAMS

A comparison of thermal maturity techniques (AFTA, VR, FAMM and ICA) in a variety of geologic settings allows an assessment of the reliability of each technique and insight into basin thermal history. Five representative sample sets are presented. In the first set, well samples from southeast Asia yield a FAMM equivalent vitrinite reflectance (VRe) value of 0.53 per cent, significantly higher than the measured VR mean of 0.43 per cent. AFTA-derived VRe indicates expected maturity levels between 0.37 and 0.45 per cent. Petrographic evaluation identified a second vitrinite population composed of reworked material which may influence the FAMM result. In the second and third cases, outcrop and well samples of Woodford Formation from Oklahoma, USA, were analysed. Facies variability in these samples may have a greater influence on FAMM VRe results than on measured VR values and AFTA VRe. A sample of jet from England gave a FAMM VRe value of 1.0 per cent with comparable AFTA VRe greater than 0.70 per cent. Measured VR values occupy a narrow range and give a well-defined mean value of 0.26 per cent with strong petrographic evidence of geochemical suppression. Based on this evidence of vitrinite reflectance suppression, the FAMM VRe value appears to represent the actual maturity level. In the final suite, samples from the Otway Basin, Australia were analysed. Measured VR (1.19 per cent) and FAMM VRe (0.82 per cent) values are consistent with AFTA VRe which constrains the maturity to greater than 0.60 per cent. The liptinite-poor nature of this sample may cause an overcorrection calculation for the FAMM VRe value. In designing a thermal history study, collection of high-quality VR data including detailed maceral descriptions and qualitative fluorescence assessments should be completed first and then combined with other maturity techniques to define the timing and magnitude of thermal events.

Thermal History of the 1.1-Ga Nonesuch Formation, North American Mid-Continent Rift, White Pine, Michigan

The Nonesuch Formation, a black siltstone, was deposited in the Lake Superior portion of the mid-continent rift system during middle Proterozoic rifting. Despite its age of nearly 1.1 Ga, the Nonesuch Formation contains liquid petroleum and solid pyrobitumen. Numerical modeling techniques were used in this study to constrain the thermal history of the Nonesuch Formation at White Pine, Michigan. Thermal modeling addresses two issues: (1) the utility of illite-smectite expandability as a limiting parameter for describing the thermal history of ancient (Proterozoic) sedimentary rocks, and (2) the potential for hydrocarbon maturation within the Nonesuch Formation at White Pine. A range of potential burial histories for the Nonesuch Formation was constructed based on geologica evidence. Time-temperature histories were calculated for each burial history model assuming one-dimensional, transient, conductive heat flow. Results of numerical time-temperature models were then evaluated on the basis of organic and inorganic thermal maturity indicators including Rock-Eval® pyrolysis data and biomarker indices (correlated to equivalent vitrinite reflectance values), in addition to illite-smectite expandability. Illite-smectite expandability values appear generally consistent with measured organic thermal maturity values. The preferred set of cases also produces calculated illite-smectite expandability and vitrinite reflectance values consistent with thermal maturity indicators (that do not include vitrinite) collected in the field. Modeling results demonstrate tha a combination of elevated basal heat flow and rapid burial during the Proterozoic is required to produce the observed thermal maturities at White Pine, Michigan. More specifically, modeling indicates that maximum temperatures for the Nonesuch Formation, 110 to 125°C, were reached at about 1075 Ma, coincident with a maximum burial depth of about 6 km, although 4 km represents a more plausible value. The results support a thermal history consistent with in-situ oil generation at White Pine; however, thermal modeling alone cannot rule out the possibility that oil was generated and expelled elsewhere and migrated into the area with fluids expelled during an episode of postrift compression.

Microspectrofluorescence measurements of coals and petroleum source rocks

Good, reproducible microspectrofluorescence data, similar to published fluorescence trends, are obtainable from samples of coals and petroleum source rocks (shales). These data are reproducible when measured by different operators, different instruments, or at different times (for example, the remeasurement of samples several months apart). These fluorescence data are used to predict maturity or rank to within about 0.10% R 0 (mean vitrinite reflectance, oil immersion). Commercially available computerized fluorescence equipment, with customized software and specific hardware mod modifications, makes the acquisition of fluorescence data feasible. Fluorescence can be used for measuring low-maturity samples, in which the measurement of vitrinite reflectance is difficult. Source rocks from oil-rich basins frequently do not contain enough particles of vitrinite for a reflectance determination, but they may contain fluorescing particles which can be measured. Although the results from only two different kinds of fluorescence equipment (Zeiss Zonax and Vickers Gamma-Scientific) are discussed, any instrument with equivalent stability and sensitivity can be used to attain reproducible and accurate fluorescence data. We propose the use of the term “ R F” to report fluorescence data in equivalent vitrinite reflectance values. Although the principles and techniques of measuring fluorescence were introduced several years ago, much work remains to establish standardized methods of maesuring as well as data evaluation. The current method of reporting fluorescence data in terms of spectral characteristics (such as the wavelength of maximum intensity or wavelength ratios) is awkward to use and difficult to compare results from various laboratories. R F values convey rank or maturation meaning and can be easily compared among laboratories. Data from fluorescing grains in both coal and petroleum source rock samples are given in this study. Polished samples of coal, this sections, and kerogen strewn slides of source rocks are measured. Spectral data are evaluated by four parameters: the wavelength of maximum intensity (Peak), the red/green ratio ( Q), the coordinates of a metric color system (CIE), and the relative intensity at 490 nm. The measurement of intensity, reported in units of radiance, is possible with the Vickers Gamma-Scientific instrument. Measuring both fluorescent colors ( R F) and intensities may help explain some of the relationships among organic particles, maturation, and hydrocarbon generation.