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The optical properties have been examined of three vitrinite-rich coals (carbon, daf: 82.5, 88.0 and 93.1%), carbonized at temperature intervals of 25 °C over the greater part of the range 20–950 °C using a heating rate of 2.45 °C/min. The small temperature interval employed has allowed a satisfactory correlation between alterations in the physical state of the carbonized products and fluctuations in the rates of change of their optical properties during carbonization. In general, as temperature rises the variation of all the optical properties reflects the growth and development of organized polycondensed aromatic systems, but in detail reflectivity does not prove as sensitive an indicator of structural changes as do other optical parameters, particularly refractive index. The behaviour of the refractive-index curves of the carbonized vitrinites is governed by changes in the heights of the crystallites forming the aromatic systems. The shape of these curves closely follows the shape of curves produced in independent X-ray diffraction studies giving the heights of crystallites. While all the refractive-index and all the absorptiveindex curves are similar in shape and trend to one another, each of these curves for carbonized coking-coal vitrinite is displaced from the equivalent curves for low-rank bituminous and anthracitic vitrinites. The displacement can be related to the formation of the highly-plastic stage during carbonization of the coking-coal vitrinite, which ultimately yields weakly cross-linked structures with good crystallite orientation, in contrast to the strongly cross-linked structures with lower ordering developed in carbonization of vitrinites of both lower and higher rank.

The US Bureau of Mines has developed an automated reflectance scanning microscope system, primarily for determination of volume proportions, size distribution, and mode of association of the physical (petrographic) components of coals. The process, which is potentially more rapid and objective and less tedious than a visual microscopic procedure, involves magnetic tape recording of several million reflectance readings on micrometer sized areas of polished coal surface, and computer analysis and interpretation of some characteristics of the coal components. Results of petrographic analysis by automated reflectance scanning microscopy and conventional visual microscopy have been compared for four coals on the basis of a partially arbitrary interpretation of reflectance distributions. Such comparisons indicate fairly good agreement of results by the two methods for at least three of the coals, particularly in showing the considerably higher proportions of exinite and lower amounts of vitrinite in dull coal samples contrasted with bright coals. Despite the interpretation being arbitrary, the interest of other laboratories in such research, and possible advantages of greater objectivity and rapidity of analysis and correlation with coal utilization, encourage further study along these lines.

The volatile content of coal can be calculated from the percentage of vitrinite, exinite, and inertinite if the volatile composition of the pure macerals can be determined. Results of determinations made in this way do not differ significantly from those of direct experimental determinations.

Over 1,000 representative coal samples from the Donets, Kuznetsk, Karaganda, Moscow, Kama, Minusinsk, Kizel, Chelyabinsk, Aldan (South Yakutian), Turgay, and other basins were studied petrographically. Coal microcomponents were divided into four broad groups (vitrinite, semivitrinite, fusinite; and leiptinite) that were used to classify coal types, to deduce conditions of accumulation and transformation, and to compute reserves. Vitrinite type coals constitute about 65 percent of Soviet coal reserves. High vitrinite contents (82-90%) typify Middle and Upper Carboniferous coals from the Donets and South Yakutian basins, and from the Kol'chugino series of the Kuznetsk basin. Coals from the Pechora and Minusinsk basins are also high (70-79%) in vitrinitous components. Fusinitous coals form about 32 percent of the U. S. S. R. coal reserves. Fusinite coal predominates in the Permian coals of the Tungus and Kuznetsk basins, in Lower Carboniferous seams of the Karaganda basin, and in Jurassic coals of the Angren...

We attempted the application of the petrogaphic method to the production of the boundry coke.Informations obtained are as follows.1) It is impossible to predict the foundry coke strength by the index and iso-coke strength line obtained by Schapiro.2) The reason why iso-coke strength line obtained by Schapiro cannot apply to the prediction of the foundry coke strength is based on the difference between the carbonization process of the foundry coke and that of the iron coke.Especially it was shown that a portion of vitrinite of high rank coal reacted as inerts by the quantitative analysis of anisotropic texture about the foundry coke.3) We made Optimum Reactives/Inarte Ratio and Rank Index for prediction of the foundry coke by micro coke oven's data, and drew up the iso-DI3015 lines for prediction of the foundry coke by the petrographic analysis and 250kg coke oven's data, about many kinds of individual coal and blend coal.4) It is possible to estimate the microchemical analysis values such as the volatile matter, fixed carbon and free swelling index, and the coking properties of coal by mean maximum reflectance of vitrinite.5) It is able to check the noncoking coal such as weathed coal and oxidized coal by using the corelation between mean maximum refletaace and free swelling index or maximum dilatation of dilatometer test.

Vitrinite maceral associations, Gondwana (Permian) coking coal deposits, Eocene-Miocene noncoking coal deposits, Gondwana basins stratigraphic correlation

Low-ash vitrinite concentrates of representative South African coals have been prepared and analysed. Various properties when plotted against carbon content show inflections at 1 1 2 to 2% lower carbon content than normal British coals. These differences are ascribed to the heating effects of dolerite intrusions.

Diagnostic analcime-, heulandite-, and laumontite-bearing mineral assemblages from four areas in the Upper Carboniferous and the Permian of New South Wales have been correlated with the rank of the associated coals, represented by the carbon content of vitrinite. The coals are high-volatile bituminous to semi-anthracitic; their vitrinites contain from about 82 per cent up to an estimated 91 per cent carbon. Analcime-quartz is found associated with the lower-rank, the laumontite-rich assemblages with the higher-rank coals. Comparison with a similar correlation in Northern Yakutia (U.S.S.R.) indicates that diagnostic zeolites are not necessarily associated with coals of the same range of rank in different areas. Unlike the occurrence and stability range of burial metamorphic zeolitic assemblages, degree of coalification is probably independent of partial pressures of H2O and CO2: it is controlled mainly by maximum depth of burial, its duration, and the geothermal gradient.

Coal macerals were exposed to blown air at a temperature of 175 degrees C for different periods of time up to 500 hours. In vitrinite and exinite, the drop in percentage of carbon and hydrogen increases up to 300 hours, but in fusinite the drop is negligible.

The sill is an analcine basanite with phenocrysts of labradorite, two clinopyroxenes, and olivine. The coal is from a bore drilled in the Dirty seam of the Baralaba Coal Measures in the Bowen Basin, Queensland. The intrusive rock was penetrated in the bore from 1514 to 1532 /sup 1///sub 2/ ft depth, directly below the Dirty seam. The phenocrysts in the narrow chilled margins indicate a temperature of intrusion of not more than 1160/sup 0/C. The coal was metamorphosed from semi-anthracite (92% C) to anthracite, meta-anthracite (97% C), and vesicular natural coke towards the contact. The vitrinite shows a progressive increase in mean maximum reflectance from 2 to 12%. Particularly in the meta-anthracites, two distinct types of vitrinite can be distinguished on the basis of reflectance. These are considered to express the different behaviour on carbonization of Vitrinite A and Vitrinite B.

1. A study of the physicochemical properties, petrographic composition and preparability of anthracites from the Shadrinsk, Elbashinsk, Gorlovsk, and newly prospected regions of the Listvyanskii deposits in the Gorlovsk Coalfield has shown that they are highly metamorphosed anthracites of high quality. 2. These anthracites resemble anthracites from the Listvyanskii deposits which were studied earlier; this indicates a certain constancy of composition of the organic substance in the anthracites, and the same recommendations can be made for their industrial exploitation as those for Listvyanskii anthracites. 3. The results were used to formulate a scheme for laboratory tests of core samples of Gorlovsk anthracites which will allow geologists to assess the reserves of minerals found by prospecting teams as potential technological raw material.

Investigation of bituminous-coal macerals, accomplished through the Institut fur Brennstoff Chemie (Fuel Chemistry Institute) of Aachen, is summarized here. Coal from five Ruhr coal beds was processed to obtain pure macerals in quantities sufficient to study their individual structure and chemical composition. Vitrinite, micrinite and exinite were recovered accordingly, and their chemical and physical properties studied; detailed, graphical interpretation of these data has been included. Chemical composition is derived for vitrinite, exinite, micrinite, and fusinite individually. Chemical structure of the macerals is discussed in relation to aromatically bound carbon content. It has been possible thus far, to determine maceral properties with respect to coalification rank as well as to indicate differentiation of these properties in macerals from the same coal bed. --D. D. Fisher.

Four macerals from an hvab coal have been dehydrogenated. As expected, different yields of hydrogen were obtained. Vitrinite and exinite, which evolve 25 and 31 atoms of hydrogen per 100 carbon atoms, respectively, are evidently rich in hydroaromatic structures. Fusinite, which yields only 5 atoms of hydrogen, is thought to be mainly aromatic. Micrinite, which is similar to vitrinite in both ultimate and proximate analyses, gives only 18 atoms of hydrogen. Micrinite must therefore contain some type of hydrogen which is not hydroaromatic but which does not greatly alter the ultimate or proximate composition from that of the vitrinite. Vitrinite and fusinite from an hvcb and an lvb coal show differences similar to that found with the hvab coal. The data also confirm that whereas all three fusinites (from coals of different ranks) are highly aromatic, the vitrinites become more aromatic as coalification proceeds and rank increases.

Chromatographic, infrared, and other analytic data for vitrinite, fusinite, and exinite concentrates from samples of abnormally high-sulfur Tertiary coals of Assam (India) are presented and compared with similar data for normal (coking) coals from the Jharia field.

The relation between the reflectance of vitrinite in oil and the carbon content (or content of volatile matter) is the same for Mesozoic and Cenozoic coals of New Zealand as for Paleozoic coals from other parts of the world.

The optical properties at 546 nm of three vitrinites (daf carbon contents 82.5, 88.0 and 93.1%) have been examined after pre-oxidation for 14 days at 105 °C and carbonization at intervals of 50 °C within the temperature range 300–800 °C. In general, pre-oxidation prior to carbonization produces only relatively small changes in the trends of optical parameters with temperature that were observed for the same vitrinites when carbonized fresh; the properties of the anthracitic vitrinite displayed little modification. Reflectivities of carbonized pre-oxidized vitrinites of bituminous rank are higher than those for the equivalent fresh vitrinites up to 500 °C, supporting the suggestion that oxidation raises aromaticity and consequently the reflectivity. The most noticeable optical difference occurs with the bituminous-rank vitrinites, in the variation with carbonization temperature of their refractive-index curves, which parallel the behaviour of the L c curves of X-ray diffraction studies. In particular, the refractive-index track for the carbonized pre-oxidized vitrinite of coking-coal rank no longer shows the same marked contrast to the refractive-index tracks of vitrinites of higher and lower rank, as it does at temperatures above 600 °C when carbonized fresh. The absorptive indices follow similar trends to the reflectivities for all the chars.

The changes of anisotropic texture of foreign coals were examined.It was showed that anisotropic texture from Vitrinite is equal to Japanese coals.In medium volatile coals, first star structure appeared, then it was changed to fibrous texture.In low volatile coals, the changes from droplet to completely fibrous texture was shown.Anisotropic texture from inert maceral are classified into Fusitelike and Fragmentary texture.Quantitative analysis of anisotropic texture was examined about coke, which produced in 250kg standard test coke oven.Good correlation was established between anisotropic texture and coke qualities.We insisted that the study for inprovement of coke quality had to pursue the change of anisotropic texture during plastic state.

Summary The pattern of variation of reflectivities and bireflectances in vitrinites of low bituminous rank in coal scares from the Middle Coal Measures (Westphalian) of Northumberland has been examined in relation to the angle which the scares and the vitrinite banding make with the sedimentary bedding. The normal uniaxial negative optical character of the vitrinites is maintained when the scare margins and the edges of the vitrinite bands are parallel to the stratification, with the isotropic section in the plane of the bands. However, when the scare and the banding are in a vertical position, the optical properties of the vitrinites appear similar to those of biaxial materials. This pattern of variation can be attributed primarily to overburden pressure which, when the vitrinite bands depart from parallelism with the stratification, distorts both the form anisotropy and the specific anisotropy of the aromatic lamellae that constitute the greater part of the molecular structures of the vitrinites at this level of coalification.

An ESR investigation was made of coals and of the corresponding tars produced by rapid pyrolysis and trapped at 100°K in the resonant cavity of a spectrometer. The coals give rise to an ESR signal which can be resolved into a broad and a narrow line. New information was obtained from the saturation curves: the paramagnetic centres responsible for the narrow line are located in inertinite; the width of the broad line increases from vitrinite to exinite. The tars have a larger proportion of aliphatic compounds as compared with the original coal. This ‘renovation’ seems to be related to the complex depolymerisation and repolymerisation reactions during rapid pyrolysis. By reheating the tars from 100° to 400°K, one observes a gradual disappearance of the free radicals.

The utility of reflectance of the vitrinite component of coals as a criterion of degree of coalification has been investigated. Present standard systems for classification of coals by rank are intended mainly as guides for utilization behaviour, and there is a need for more widespread acceptance and usage of an objective rank criterion based more firmly on physical/chemical characteristics of a coal. The relations of reflectance of vitrinite to other commonly used or proposed rank parameters have been studied, using voluminous data supplied by United States and European laboratories. These relations were found to be excellent for high-rank bituminous coals, good for anthracitic coals, fair for low-rank bituminous coals, and rather poor for subbituminous coals, lignites, and brown coals. The data for the last category are least numerous, and work is continuing on their reflectance—rank characteristics, especially for the International Commission on Coal Petrology. The reflectance ranges established for present rank categories are listed and it is proposed that they be included in standard classification systems as supplementary criteria of rank.

Most previously reported work suggests that the optimum content of the reactive macerals (vitrinite and exinite) in a coal or coal blend is in excess of 70%. Studies of blends with New Zealand coals and coals from the Southern Coalfield of New South Wales show that above 50 % vitrinite there is little improvement in coke hardness and a decline in coke stability; below 50% vitrinite both indices of coke strength decrease markedly.

The variation of the electron spin resonance signal of a pure vitrinite (Barnsley seam, Dinnington Colliery, South Yorkshire) carbonized at various temperatures and showing only a simple line in the natural state was investigated. Above a treatment temperature of 600°C near the center of the wide line a hyperfine structure is observed.

Concentrates of the petrographic components of a high volatile A bituminous coal were obtained by centrifugal separation in salt solutions of adjusted specific gravity. Samples containing 94% vitrinite, 86% exinite, 87% micrinite, and 96% fusinite were obtained. Various physical and chemical properties of the concentrates were determined, e.g., chemical composition, helium density, surface area, indices of refraction, and X-ray scattering intensities. The components were found to differ considerably in chemical composition and in most of the physical properties. Some of the physical properties could be related to chemical composition.

Alterations of coal organic matter in the course of metamorphism are considered in the paper. Under the influence of temperature and pressure the organic matter contained in sedimentary rocks is liable to easier and quicker alteration than the inorganic substance. As a result of a study of oil- and coal-bearing sediments it is suggested that the transformation of the organic matter contained in coal disseminations should be used as an indicator of general sedimentary rock alterations, taking place during their submergence into higher temperatures and pressure zones. The reflectivity of vitrinite, determined by means of photocells under the microscope serves as a quantitative expression of different sedimentary rock transformation stages. Thus, 22 stages of sedimentary rock alteration have been isolated. These stages have been subdivided into 6 subgroups and 3 groups. Paragenetic relations between coal alterations and oil distribution in sedimentary rocks of different stages have been established.—Auth. En...

The behaviour of the dispersion curves of refractive and absorptive index of a vitrinite of high-volatile rank oxidized for different periods of time up to 128 days at 105 °C supports the suggestion that the aromaticity and/or condensation of the vitrinite rise when it is oxidized. Equivocal optical data previously obtained for oxidized vitrinites can probably be explained by the extensive use in earlier microscopical studies of wavelengths in the green region of the visible spectrum. Reflectivity-dispersion data indicate that there is greater possibility of distinguishing oxidized and weathered coals from fresh samples with the microscope by comparing their optical properties in the blue region of the visible spectrum or in the ultra-violet.

Heating value of borehole samples of lignite ranges from 500 to 1,000 Btu/lb within short distances. Much of this variation is caused by (1) weathering at shallow depth, resulting in a decrease in heat value, and (2) loss of moisture because of improper handling of the sample, resulting in an increase in heating value. Aside from these factors, the petrographic composition of lignite is the most important parameter that ultimately controls its heating values (assuming geologic history is the same after the peat stage). Chemical analyses of hand-picked pure maceral material exhibit considerable variation in elemental composition and heating values. On an as-received basis, the heating values of resinite and huminite (vitrinite) are 16,000 Btu/lb and 7,500 Btu/lb, respectiv ly. The hydrogen and carbon contents are 10 % and 80 %, respectively, for resinite and 5 % and 70 % for huminite, calculated on a dry, ash-free basis. Lignite mined near Larson, Burke County, North Dakota, contains resinite particles with a diameter up to 0.5 in. Published data (212 samples) indicate that the average heating value of this lignite is 460 Btu/lb more than the average value for all lignite in North Dakota (7,280 versus 6,820 Btu/lb). A presence of 5 % resinite could account for this difference in heating values. End_of_Article - Last_Page 963

Summary The mobility of silicogenic dust samples (cristobalite, erosil, tridymite, silicon dioxide and vitrinite) was studied by microelectrophoresis, which showed that they differed in surface potential. A relationship was found between surface potential and severity of tissue damage.

The gem-grade jet found in Upper Cretaceous rocks of the Jet Basin, Wayne County, Utah, has been shown to be a vitrinitic, high-volatile B bituminous coal with aberrant chemical constitution. The residual structure is entirely that of taxodiaceous conifer wood. The abnormally high volatile content (62 percent) and low reflectance (0.25 percent) of Utah jet compared with other vitrinites of similar rank is produced by the unusual derivatives of cellulose and lignin of which the woody structure is composed.

The term "paper coal," previously applied to a coal in the U.S.S.R., describes a coal found in Parke County, Indiana. In contrast to the Russian coal, which consists of cuticles of arborescent lycopods, the cuticles of the Indiana paper coal resemble Sphenopteris bradfordii Arnold, the foliage of a seed fern. Exinite (cuticles and spore exines) constitutes 23.5 percent of the coal seam where it is unweathered. The papery texture occurs only where weathering has removed the vitrinite and has left a mat of resistant cuticles. Spore analyses indicate that the seam is one of the upper Pottsville coals. Many sporangia from the paper coal are composed of small, round non-sutured spores and an outside layer of spores resembling Torispora Balme. Cutinization suggests that these plants could store water. Fresh water periodically feeding into a saline swamp could have been adequate to sustain these xerophytic plants. Trace element analyses substantiate the belief that water conditions were predominantly marine. Cutinite isolated from the coal has a calorific value of 16,700 B.T.U. per lb and a volatile matter content of 85 percent on a dry, ash-free basis. Because cutinite has such a high volatile matter content similar cuticular coals possibly can be utilized most efficiently as hydrocarbon sources.

Sporinite is a term normally used to describe an organic constituent (maceral) that is widespread in coals, particularly those of Palaeozoic age. The maceral is formed from spore exines, which, over a range of coal rank, but primarily in the bituminous coals, display a substantial and continuous variation of chemical and physical properties with increasing coalification. The properties of sporinite eventually become similar in the low-volatile bituminous coals to those of the dominant coal maceral, vitrinite, which is derived primarily from lignin-cellulose-humic complexes in the original coal-forming peat. These property changes with rank can be mainly attributed to the influence of relatively small increases of temperature (

A vitrinite of 84% carbon content and its pyridine and benzene extraction products have been studied by small- and wide-angle X-ray diffraction on the solid samples and by small-angle X-ray diffraction in solution. Fourier transform methods have been used to determine interatomic distances in the structure and an attempt was made to derive values for the aromaticity from the shortest interatomic distances. Diamond's least-squares method has been applied to the wide-angle scattering of the samples. The significance of the different structural parameters obtained by this method is discussed. The scattering due to the intermolecular order has been studied under different aspects. The importance of the effect of statistical disorder on both the shape and the position of the interference maxima is discussed. The small-angle scattering of extracts in solution has been used to determine size and shape of the dissolved particles.

Leonardite is a naturally oxidized lignite characterized by high percentages of humic acids. Visual inspection is usually insufficient to distinguish leonardite from lignite. The low-angle X-ray scatterings from 8-2.5A° of lignite, leonardite and humic acids were analyzed. The primary subpeak shifts from 3.37A° for leonardite to 3.44-3.58A° for lignite and vitrinite. Leonardite and lignite can also be distinguished by examining the relative intensities of the 3.1-3.7A° sub-peak.

Coal macerals can be separated using flotation process by taking advantage of differences in their surface properties. In this work, the effect of pH, air velocity (Jg) and frother concentration (FC) on the vitrinite separation of two coal samples from Colombia, Cerrejón and Jagua was studied. The experimental runs were carried out by flotation column at pH ranging from 4 to 10, FC from 2 to 6 ml of frother/kg of coal and Jg from 0.7 to 2.1 cm/s Jg. Results showed that both coals exhibited vitrinite recoveries above 60% at pH 5, Jg 1 cm/s and FC of 3 ml/kg of coal. For Cerrejón coal, the highest vitrinite recovery (63.55%) was obtained at pH 7, Jg 0.7 cm/s and FC of 4 ml de frother/kg of coal, whereas for Jagua coal, the highest vitrinite recovery (69.80%) was obtained at pH 4, Jg 1.4 cm/s, and 4 ml of frother/kg of coal respectively. It was also noted that both coals exhibited the highest value of vitrinite recovery at acid conditions compared with those obtained using basic conditions Los macerales del carbón se pueden separar usando procesos de flotación aprovechando las diferencias de sus propiedades superficiales. En este trabajo, se estudió el efecto del pH, la velocidad del aire (Jg) y la concentración de espumante (FC) sobre la separación de vitrinita de dos minas de carbones del Cerrejón y Jagua (Colombia). Las corridas experimentales se realizaron en una columna de flotación usando pH en el rango de 4 a 10; FC en el rango 2 a 6 ml de espumante/kg de carbón y Jg en el rango de 0.7 a 2.1 cm/s. Los resultados mostraron que ambos carbones presentaron recuperación de vitrinita superiores al 60% a pH 5, Jg 1 cm/s y FC 3 ml/kg de carbón. Para el carbón Cerrejón, la mayor recuperación de vitrinita (63.55%) se obtuvo a pH 7, Jg 0.7 cm/s and 4 ml de espumante/kg de carbón, mientras que para el carbón Jagua, la mayor recuperación de vitrinita (69.80%) se obtuvo a pH 4, Jg 1.4 cm/s and 4 ml de espumante/kg de carbón respectivamente. Se observó también que ambos carbones presentaron los más altos valores de recuperación del grupo vitrinita a condiciones acidas en comparación con las obtenidas a condiciones básicas

IT is a commonly held view that coals contain aromatic structures and that they become more graphite-like with increase in rank. X-ray studies have tended to confirm this concept ; however, recent work1 has shown that small (about 100 atoms or less) aromatic and tetrahedral structures that have comparable numbers of atoms give rise to overlapping X-ray diffraction patterns in the angular regions of the (10), (11) and (20) reflexions of graphite-like layers. The question of distinguishing between the two types of structures will have to be resolved eventually by rigorous and more detailed X-ray or electron diffraction studies. Knowledge of other physical and chemical properties would be of great help in such structural analyses. It has been reported2 that the extinction coefficients of a high-volatile A bituminous vitrinite in ultra-violet and visible light are too low to admit of an appreciable content of condensed aromatic structures. A detailed study of absorption became possible when we succeeded in cutting ultra-thin sections of coals of various ranks and in accurately measuring their thickness and refractive index by interferometry3.

Dehydrogenation of a Pittsburgh seam coking vitrinite is relatively insensitive to mild oxidation, but the yield of hydrogen is considerably reduced after oxidation at 150 °C for 66 h.

It is important that petroleum exploration geologists know the critical depth limits where oil is generated from original organic matter in sediments and where the oil changes to natural gas. Organic matter is very sensitive to temperature. The maximum temperature experienced is related to its depth of burial.CSIRO has used the composition and physical properties of various types of organic matter in shaly rocks as indicators for the degree of diagenesis caused by this heat alteration. The reflectance of vitrinite in associated coals is used as the primary standard, and carbon content of such coals as the secondary parameter to distinguish various stages of oil and gas generation. Depth-reflectance curves are useful 1., for estimating palaeogeothermal gradients, 2., for determining the degree of diagenesis at a particular depth and also 3., for estimating the approximate thickness of sediments subsequently lost after deposition.The petroleum potential of some Australian sedimentary basins is reviewed in the light of this knowledge. In the Northwest Shelf area and in the Capricorn and Otway Basins, the oil generation zone is deeper than in the Cooper, Galilee and Surat Basins. In the Bowen and Sydney Basins and several other small basins along the eastern coast of Australia, this zone is very shallow, and in some areas the oil generation zone has been completely lost by erosion. The areas most promising for oil fields are those where little erosion of sediments has taken place subsequent to deposition and diagenesis, provided that all other geologic factors for hydrocarbon accumulation are present.