Organic Matter In Sedimentary Rocks Research Articles

Chemical and Petrographic Classification of Kerogen/Macerals

The use of the term «kerogen», originally described as the organic matter in oil shale, has been extended to include all nonsoluble solid organic matter in sedimentary rocks. Kerogen is divided into «types» based on elemental composition and potential maturation path. «Maceral» nomenclature, originally developed to describe the organic components of coal, has also been extended to source rocks and, later, oil shale. The extension and overlap of these two fields creates problems as attested to by the proliferation of vitrinite terminology and the application of the term vitrinite to organic matter that is not clearly derived from woody tissue. Other problems include the fundamental complex and wide-ranging nature of Type II kerogen which is now acknowledged to be a highly heterogeneous material.

Application of Thermal Analysis to the Study of Organic Matter in Sedimentary Rocks

Application of Thermal Analysis to the Study of Organic Matter in Sedimentary Rocks

TROUBLESHOOTING VITRINITE REFLECTANCE PROBLEMS USING FAMM: A GIPPSLAND AND OTWAY BASIN CASE STUDY

'Fluorescence alteration of multiple macerals' (FAMM) is a multi-parameter microprobe technique developed by CSIRO to provide the petroleum industry with an objective method of estimating thermal maturity of organic matter in sedimentary rocks. The technique is closely related to vitrinite reflectance methods, although the identification of vitrinite is not necessary; FAMM is based on the fluorescence alteration characteristics of vitrinite, inertinite and liptinite macerals in a sample. It is a powerful tool for the identification of casings, reworked material and hydrogen-rich (perhydrous) vitrinite, and has the capacity to correct for the effects of vitrinite reflectance suppression.The technique has been tested on Cretaceous coals from the Latrobe Group in Volador-1, offshore Gippsland Basin, where the identification of vitrinite is unequivocal, and on dispersed organic matter (DOM) in various clastic lithologies from the Cretaceous Waarre and Eumeralla Formations intersected by Flaxmans-1 and Port Campbell-4, onshore Otway Basin, where the identification of vitrinite is difficult and subjective.For the Volador-1 sequence, the FAMM-derived vitrinite reflectance (equivalent reflectance) and the measured vitrinite reflectance (VR) are similar, although the VR data are more scattered about the VR-depth regression line. The variability in the vitrinite reflectance data is largely due to compositional variation in the vitrinite. This variation however does not affect the equivalent vitrinite reflectance data to the same degree.In the sections intersected by Flaxmans-1 and Port Campbell-4, FAMM data indicate that vitrinite reflectance suppression is responsible for the large deviations of the measured values from the VR—depth regression line. In some of these samples the suppression effect may be as much as 0.20 per cent absolute.The FAMM technique offers a valuable supplement to VR determinations, or even a valid alternative in many practical situations.

Optical description of amorphous kerogen in both thin sections and isolated kerogen preparations of Precambrian to Eocene shale samples

Abstract This paper presents a limited study of Precambrian kerogen and compares it to Phanerozoic kerogen. In both cases, most of the kerogen is amorphous. While the specific biological precursors of kerogen may be significantly different between the Precambrian and Phanerozoic, the mode of degradation and preservation in shales may be similar, and that is what is found in the description of the amorphous kerogen. It is suggested that similar methods of sample prepagation, description and classification can be applied to all sedimentary organic matter, regardless of age. Kerogen (disseminated organic matter of sedimentary rocks that is insoluble in nonoxidizing acids, bases and organic solvents, Hunt. 1979) occurs in many forms when observed with a microscope. Structured kerogens include spores pollen, unicellular algae, cuticle and cellular tissue of plants and coal fragments. The majority of kerogens have no structures relatable to precursor organisms and are called “amorphous”. This paper describes optical comparisons of various preparations of organic-rich sedimentary rocks and the information that can be derived from them. Optical descriptions of organic matter, in particular the amorphous fraction, in sedimentary rocks help us to understand the bulk chemical or petroleum-generating properties, as well as paleodepositional environments. There is no universally accepted method of preparing samples for optical study, nor is there a unified set of terms to describe the observations. Ideally, more information about the origin or preservation of organic matter can be obtained when it is studied in situ in the rock matrix in thin section, revealing relationships to depositional fabric and clues from mineralogy and microfossils. However, it is often necessary to physically concentrate the organic matter for study because it is too dispersed throughout the rock matrix to be seen in thin section. The concentrated, or isolated, organic matter is prepared either as polished mounts or strewn slides for optical study. More confident kerogen identifications can be made if the different preparations are examined with more than one microscopic lighting condition. Doubly polished thin sections of whole rock can be viewed with three common microscope lighting conditions (transmitted, reflected white and reflected blue light, or fluorescence). Visual kerogen strewn slides (which can also be viewed with the three lighting conditions) contain the isolated organic matter following acid removal of most of the mineral matrix and provide detailed morphological information about the kerogen itself. Polished mounts of isolated kerogen or whole rock (similar to polished coal plugs) can only be viewed with reflected white or blue light and are somewhat limited in the information they convey, despite their relative ease to prepare. Examples of amorphous kerogens from a variety of samples are shown including: thin section documentation of the occurrence of oil-prone amorphous kerogen in laminae of the Eocene Green River Formation: in stylolites of Cretaceous carbonates from Dubai; thin section illustration of solid bitumen rather than amorphous kerogen in paleoreservoir rocks from the Permian of Svalbard (Norway); detailed unicellar algal morphologies in strewn slides of Devonian samples from Russia; strewn slide details of gas-prone amorphous kerogen textures from Silurian Bolivian samples; and the occurrence of oil-prone amorphous kerogen in the Precambrian Nonesuch Formation. More Precambrian organic-rich rock need to be studied optically by the methods suggested here to verify the hypothesis that Precambrian kerogens are mostly amorphous. The best way to completely describe amorphous kerogens is to use both isolated kerogen strewn slides and whole rock polished thin sections.

Fractional composition of neutral asphaltenes from disseminated organic matter in sedimentary rocks

Chloroform extracted asphaltene fractions (chloroform bitumoid- “CB”) from cores of Mesozoic rocks from Central North Bulgaria were studied. It is expected that the asphaltene content hardly depends on the genetic and sedimentological factors influencing the overall composition of the disseminated organic matter (DOM). Indeed, the core samples were sedimentologically, and with respect to genetical oxidation of DOM similar, but of different degree of catagenesis. The neutral asphaltenes (10–22 wt% of CB, petroleum ether insoluble part of DOM) were studied. They were fractionated by GPC on Sephadex LH-20/CHCl 3–CH 3OH (95:5, v/v) system. A molecular sieve separation was carried out. Fractions in the MM n range 500–1250 were obtained. Sub-fractions from preparative TLC separation were examined by 1H NMR and by GC-MS. An explanation is proposed for the structural changes that had taken place during the maturation. Mass spectral data provided evidence for the presence of normal, cyclic alkanes (di-, tetra-) and 1,2,4-trimethyl-alkylbenzene. It could be assumed that the asphaltene from DOM of sedimentary rocks might be considered as potential precursors of hydrocarbons after maturation under proper thermobaric environments of lythogenesis.

History of the Earth's Atmosphere

This book is a synthesis of a series of outstanding research articles by Professor Budyko on climatology and by Professor Ronov on geochemistry and an expansion of their previous collaborative work on the evolution of atmospheric composition. Although other topics are briefly mentioned, the book is mainly devoted to the calculation of the levels of atmospheric CO2 and O2 over Phanerozoic time. Their basic assumption is that the worldwide abundance of carbonate minerals and organic matter in sedimentary rocks as a function of age, based on the exhaustive collection of data by A. B. Ronov and coworkers, can be used to calculate atmospheric composition. The reasoning is quite simple. It is assumed that CO2 given off to the atmosphere by volcanic and metamorphic degassing is removed mainly in the form of carbonate minerals in sediments (organic matter being much less abundant). The total mass of carbonate for each age period, divided by the length of the period, gives the rate at which CO2 is removed from the atmosphere via burial in sediments. Since so little CO2 is in the atmosphere, compared to fluxes to and from it on a geological time scale, rates of removal must equal rates of addition and so carbonate burial rates also give rates of addition to the atmosphere via volcanism. Since removal rates are also assumed to be directly proportional to the mass of CO2 present in the atmosphere (linear feedback), by determining the proportionality constant, one can directly calculate CO2 levels from rock abundances. For atmospheric O2 , production rates are calculated from the burial rates for organic carbon, and removal rates (via the oxidation of sulfides, ferrous minerals, and volcanic gases) are again assumed to be directly proportional to the mass of O2 in the atmosphere which, on equating production and removal rates, enables calculation of O2 levels over Phanerozoic time.

The lacustrine Condor oil shale sequence

Summary Organic matter in sedimentary rocks is derived from numerous types of organisms that lived in many different environments. Australian Tertiary oil shales, for example the Condor deposit, are termed lamosites and were formed in a lacustrine environment with the bulk of the organic matter derived from freshwater planktonic algae such as Pediastrum . The Condor oil shale deposit, located near Proserpine, Queensland, contains four oil shale units and a brown coal unit. The oil shales contain abundant lamalginite (or lamellar alginite), derived from planktonic freshwater algae, with a small quantity of vitrinite derived from terrestrial plants and other minor constituents. The lower oil shale unit contains a dark brown oil shale whereas the upper three oil shale units contain predominantly brown oil shale. The coal unit contains liptinite-rich cannel coals. Reflectance data for vitrinite indicate that the lower oil shale unit and the underlying coal unit are within the oil generation window.

Sedimentological subjects of organic matter in sedimentary rocks

Sedimentological subjects of organic matter in sedimentary rocks

THE MECHANISM OF PETROLEUM FORMATION

The totality of geochemical indicators of the formation of petroleum from the disseminated organic matter of sedimentary rocks is the fundamental evidence proving the biogenic origin of oil and gas. Geochemical modeling of the processes of transformation of organic matter into petroleum hydrocarbons indicate that oil and gas are predominantly products of thermocatalytic transformation of the original organic material. A substantial role is also played by biochemical processes of formation of petroleum hydrocarbons during the early stages of lithogenesis. A large amount of data on the composition and distribution of organic matter in different petroleum-source rocks, formation water, etc., have been gathered during the present phase of investigation. It has been established that the formation of petroleum hydrocarbons in insoluble organic matter and its quantitative rate are clearly associated with the genetic type and grade of metamorphism of the original organic matter and with the depth of subsidence of the enclosing sediments. The results of these studies are of great theoretical and practical significance. It is sufficient to state that on the basis of these data, it is possible to determine more objectively the mechanism of formation and primary migration of the hydrocarbons from the disseminated organic matter in themore » sedimentary rocks. 10 references.« less

New Developments in Microphotometry of Kerogen and Bitumen at Various Stages of Thermal Maturity and Applications to Hydrocarbon Exploration: ABSTRACT

Microphotometry is the computerized microscopic measurement of reflectance, fluorescence, and transmittance of organic matter in sedimentary rocks. Microphotometric data are an indispensable tool in exploration of hydrocarbons. The chromaticity of kerogen particles in fluorescence and transmitted light is derived from spectral analysis and provides new scales for thermal maturity: the Fluorescence Color Index and Transmittance Color Index. The TCI could substitute for the inaccurate TAI. Generation of crude within thermally mature kerogen particles shows characteristic fluorescence phenomena when microscopically observed and measured. Spectral data and chromaticity values provide an improved method for determining the thermal maturity of petroleum source rocks, in particular when vitrinite reflectance data are unreliable or unavailable. The oil floor in a basin is marked by the disappearance of the fluorescence of fossil alginite, exinite, amorphous organic matter, and most bitumen. This method provides a better mapping of thermally mature petroleum source rocks. End_Page 313------------------------------ Other applications of microphotometry are: (1) calculation of the effective organic carbon content from the amount of fluorescent oil-prone kerogen and its fluorescence characteristics, a technique providing a better estimation of the volume of hydrocarbons that may have been generated by a source rock, and (2) fingerprinting of various types of kerogen, bitumen, drilling contaminants, and caved rock material, and identification of kerogen and bitumen in the mineral matrix of rock samples. End_of_Article - Last_Page 314------------

Generation of hydrocabon gases by thermal decomposition of organic matter in sedimentary rocks of the Tungusska syneclise, based on experimental data

Generation of hydrocabon gases by thermal decomposition of organic matter in sedimentary rocks of the Tungusska syneclise, based on experimental data

Uranium and thorium enrichment in rocks from the base of DSDP Hole 465A, Hess Rise, central North Pacific

Uranium and thorium are concentrated in Cretaceous limestone, chert, ash, basalt, and other rock types at Deep Sea Drilling Project Site 465 located on the southern Hess Rise in the central North Pacific. U concentrations, up to 194 ppm on a carbonate-free basis, are among the highest recorded for any deep-sea deposits. U was initially derived from seawater and concentrated by absorption on terrigenous (humic) organic matter in limestone in a shallow marine environment. U and Th were probably concentrated further by low-temperature hydrothermal fluids emanating from the basaltic basement. Mainly montmorillonite, an alteration product of basalt and ash, and organic matter in sedimentary rocks acted as hosts for U and Th. The unique combination of sediments rich in humic organic matter, abundant smectite in altered ash and basalt, and warm hydrothermal solutions provided the necessary conditions for migration and concentration of U and Th. To better understand the conditions limiting the migrating and concentration of U and Th, other rocks deposited during the ocean-wide Cretaceous anoxic events should be analyzed for these elements.

Quantitative evaluation of hydrocarbons produced by organic matter in sedimentary rocks

A method is given for estimating the forecast reserves of hydrocarbon gases on the basis of catagenesis conditions for dispersed organic matter. The method employs the historical approach to estimation of petroleum reserves in sedimentation basins and gives the most reliable estimate currently available for the generation potential. A formula presented can be used to estimate the forecast reserves of hydrocarbon gases for the basin as a whole and for individual zones, which is of considerable practical importance. (JMT)

Thermal properties and coal rank in rocks and coal seams of the southern Sydney Basin, new South Wales: A palaeogeothermal explanation of coalification

Measurements of thermal conductivity on 106 disc specimens of rocks from 275 m of the Permian to Triassic section of the Southern Coalfield of the southern Sydney Basin have been carried out in an effort to explain a high rank gradient in the Permian coals. The samples came from six diamond drill holes north and west of Wollongong, although one hole only provided specimens of a syenitic sill (n = 17, mean thermal conductivity = 2.36 W/m°K, s.d. 0.03). When combined with previously published data on chip specimens, with which there is good agreement, from a further four drill holes the mean thermal conductivity for the late Permian and early Triassic sandstones and shales is 3.20 W/m°K. Heat generation by Permian volcanic rocks below the coal measures (from about 1 to 3 μW/m 3), and by basement granitic rocks, appears to be consistent with previously reported heat flow for the southern Sydney Basin (about 80 mW/m 2). This heat flow is a relatively high value for the east coast of Australia. Younger (Mesozoic and Tertiary) intrusive and extrusive igneous rocks produce local coal-rank anomalies, but do not appear to have any regional effect. The rank of coals above the Permian volcanic rocks appears to be little affected by the presence of the igneous rocks and the coal-rank decreases towards the major area of vulcanicity. Organic matter in sedimentary rocks interbedded with the volcanic rocks is of relatively high rank but it appears that these thermal effects do not extend more than about 100 m above the base of the coal measures. The area of high rank north and northwest of Wollongong seems likely to be a regional effect associated with a combination of high heat generation in basement and the Permian rocks, and high heat flow from the basement. Greater cover on the coal measures, together with an increase in the proportion of rocks of relatively low thermal conductivity in the cover, may also influence the rank in the Permian coal.

The degree of metamorphism of organic matter in sedimentary rocks as a paleo-geothermometer, applied to the Upper Rhine Graben

The degree of metamorphism of organic matter in sedimentary rocks as a paleo-geothermometer, applied to the Upper Rhine Graben

Individual components of low-boiling (up fo 125° C) hydrocarbons in organic matter of sedimentary rocks

Individual components of low-boiling (up fo 125° C) hydrocarbons in organic matter of sedimentary rocks

Kerogen studies and geological interpretations

Abstract Kerogen is defined as the portion of organic matter in sedimentary rocks that is insoluble in organic solvents. It is formed from formerly living organic materials by diagenetic processes in the first few hundred meters of burial and is partially converted, under the influence of temperature and time, into petroleum by a series of complex, first-order reactions, termed catagenesis. The quantity and quality of petroleum formed is controlled by the concentration, type, and thermal maturity of the kerogen present in source beds and these parameters comprise the basis of all source bed studies. Catagenesis of hydrogen-rich algal and liptinitic kerogen accounts for most of the oil and gas generated in sedimentary rocks. Hydrogen-deficient, humic kerogen can yield only limited quantities of dry gas and some components of kerogen have little or no capacity to generate petroleum. The convertibility of any specific kerogen to petroleum can be estimated from its elemental composition and thermal maturity as defined by the reflectance of vitrinite. Kerogen studies play an important role in identifying effective source beds in order to predict the amount, type, and distribution of petroleum that can be expected in a basin. Maturity data is also useful in identifying certain geological features in wells, defining the provenance of sediments, unravelling the sedimentary and tectonic history of basins, and determining the boundaries between diagenesis, catagenesis, and metamorphism. Incorporation of kerogen studies and other geochemical data into conventional exploration programs will become essential as the search for petroleum becomes more difficult and expensive.

Distribution of Carbon in Crust of Earth: GEOLOGICAL NOTES

The total carbon in crust of the earth is about 9 × 1022 g. Continental and oceanic sediments contain 1.2 × 1022 g of organic carbon and 6.4 × 1022 g of carbonate carbon. There is about three times as much organic carbon in claystones and shales as in carbonate rocks and sandstones. The carbon in the oil and gas of petroleum reservoirs totals 1 × 1018 g or about 0.01 percent of the organic carbon in sedimentary rocks. The hydrogen in the organic matter of sedimentary rocks totals 9 × 1020 g or about 32 times as much as is needed to form all the petroleum in both reservoir and nonreservoir rocks. This inventory shows not only that there is adequate hydrogen to form oil in the sediments, but als that the entire process of the origin, migration, and accumulation of petroleum is extremely inefficient.

Free Hydrogen in Genesis of Petroleum: GEOLOGICAL NOTES

A model is proposed that postulates (1) a free-hydrogen source in the deep-seated environment, resulting from dissociation of water in the presence of minerals containing ferrous iron; (2) a continuous upward percolation of elemental hydrogen into the surface environment as a consequence of its high rate of diffusion relative to other constituents; and (3) a hydrogen sink near the surface formed by biologically catalyzed chemical reactions of free hydrogen with organic matter, ferric iron, sulfates, and atmospheric oxygen. It is suggested that the hydrogenation of biogenic organic matter in sedimentary rocks by free hydrogen of deep-seated origin should be investigated as a factor in the genesis of petroleum.

Effects of Incipient Metamorphism on Organic Matter in Mudrock

Comparison of organic matter in unmetamorphosed rocks with organic matter in metamorphosed rocks shows three consistent differences: (1) the absolute amounts of hydrocarbon material present are less in metamorphosed rocks than in unmetamorphosed rocks, (2) the relative amount of the total organic matter present as hydrocarbon material is less for metamorphosed than unmetamorphosed rocks, and (3) the extractable material and the isolated hydrocarbons from metamorphosed rocks have a higher proportion of saturated (compared to aromatic) species, and have lighter colors, lower melting points, and lower average molecular weights than analogous material from unmetamorphosed rocks. In addition, anomalously high carbon isotope ratios (^dgrC13) were observed in two samp es believed to have undergone higher degrees of incipient metamorphism. These results suggest that the nature and characteristics of organic matter in sedimentary rocks may provide a useful new basis for recognizing and classifying degrees of incipient metamorphism.