D. Leythaeuser

Aromatic components of coal: relation of distribution pattern to rank

Abstract Aromatic distribution patterns have been evaluated for a series of twenty-four German high volatile bituminous B to low volatile bituminous coals of Upper Carboniferous Westphalian C-, B- and A-ages using high performance liquid chromatography and glass capillary gas chromatography. The study concentrates on dicyclic and tricyclic aromatic hydrocarbons most of which have been identified by combined gas chromatography-mass spectrometry. The distribution patterns of the methylhomologs of naphthalene and phenanthrene are strongly controlled by rank. A sudden increase of individual isomer ratios at 0.9% Rm coincides with an abrupt shift of the sporinite fluorescence colour from yellow towards red. A recently developed aromatics-derived maturity parameter, the Methylphenanthrene Index (MPI), correlated well with the vitrinite reflectance data over the whole rank range. Deviations have been attributed to variations of maceral composition or migration phenomena. The influence of artificial thermal alteration (350 and 400dgC) on the aromatic distribution pattern of coal is described.

Composition of soluble organic matter in coals: relation to rank and liptinite fluorescence

Abstract Study of a series of twenty-six German high volatile bituminous B to low volatile bituminous coals of Upper Carboniferous age by recently refined analytical methods (‘flow-blending’ extraction, medium pressure liquid chromatography, HPLC, glass capillary gas chromatography and spectral fluorescence microscopy) reveals that yield and composition of soluble organic matter are strongly controlled by rank. In particular, the following points of inflection are noted in rank trends around 0.9% vitrinite reflectance: a maximum in yields of total soluble organic matter, aromatic hydrocarbons and n -alkanes; the most pronounced change in aromatic hydrocarbon composition; a trend reversal for pristane/ phytane ratios; a gradient change in the odd/even-predominance of long chain n -alkanes; appearance of a bimodal n -alkane distribution; and a sharp drop in concentration of individual n -, and isoprenoid alkanes. This discontinuity in rank trends around 0.9% Rm is interpreted to reflect a major change in reaction types, i.e. a shift from predominantly hydrocarbon generating to predominantly fragmentation reactions. Rank trends of maceral fluorescence exhibit the following pronounced changes over a similar but broader rank range: Different types of the maceral sporinite show a relatively abrupt shift of the fluorescence colour from yellow towards red between 0.8–0.9% Rm while up to about 1.0% Rm a sharp increase is recorded in the proportion of fluorescent vitrinite. This coincidence at a near-equal rank stage suggests a common cause for changes in yield and composition of the soluble organic matter and the maceral fluorescence of these coals.

Quantification of loss of calcite, pyrite, and organic matter due to weathering of Toarcian black shales and effects on kerogen and bitumen characteristics

Abstract Comparison of geochemical data on Posidonia Shale (Early Toarcian) from a shallow unweathered core and from adjacent weathered exposures of the same fades was performed. Results revealed that quantitatively the most affected petrographic constituent is pyrite and that organic matter and carbonate were also altered by weathering. Due to this process of weathering, the bulk composition of organic matter, especially the molecular composition of soluble organic matter and the fluorescence colour of organic particles, are changed. Estimates of weathering rates reveal that the annual release of sulphur and organic carbon from black shales may significantly add to anthropogenic pollution.

Relationship between rank and composition of aromatic hydrocarbons for coals of different origins

Distributions of di- and tricyclic aromatics were determined for Upper Carboniferous to Oligocene coals of various origins. In addition to Upper Carboniferous Ruhr coals, Wealden (Lower Creataceous) coals from North-west Germany, Lower Cretaceous coals from Western Canada, and Tertiary coals from Japan and Colombia were studied. The coals ranged from sub-bituminous to anthracite in rank and showed vitrinite reflectance from 0.4 to 4.9% Rm. The previously established relationship between the rank of coal in terms of vitrinite reflectance and the distribution patterns of individual aromatic hydrocarbon isomers has been verified and refined. It is demonstrated how the above relationships can be used to calculate vitrinite reflectance from the Methylphenanthrene Index (MPI 1). The “chemical” rank levels of coals can now be defined in terms of the calculated vitrinite reflectance (Rc).

Organic matter maturation under the influence of a deep intrusive heat source: A natural experiment for quantitation of hydrocarbon generation and expulsion from a petroleum source rock (Toarcian shale, northern Germany)

Four shallow boreholes were drilled in the Hils syncline, northern Germany, in order to determine quantitatively the amount of hydrocarbons generated and expelled during maturation of a typical kerogen-type-II-bearing source rock. The holes penetrated the carbonceous Lias ϵ shales (Posidonia shale, Lower Toarcian) and part of the adjacent Dogger α and Lias δ mudstones. The maturity of the organic matter in the cores recovered ranges from immature (0.48% R0) to overmature 1.45% R0) due to location of the Hils syncline in the vicinity of the Vlotho Massif, which is deep-seated intrusive body. Facies variations of the Lias ϵ within the short geographical distances in the study area are negligible. Organic matter mass balance calculations were based on detailed organic geochemical analyses of residual material in the Lias ϵ shales (kerogen, bitumen etc.) and on the evidence of a uniform initial composition of these sediments in the study area. Dead carbon determinations supported this latter criterion but were not used as a parameter in the calculations. About 50% of the initial kerogen was transformed into oil, gas and inorganic compounds during the vitrinite reflectance increase from 0.48 to 0.88% Ro and only marginally more during the maturity increase from 0.88 to 1.45% Ro. Only a small portion of the generated material remained in the source rock even at a relatively early stage of generation (0.68% Ro). Expulsion efficiency of oil plus gas reached a value of 86% at the end of the main generation stage (0.88% Ro).

Temperature dependency of long-chain alkenone distributions in recent to fossil limnic sediments and in lake waters

Distribution patterns of C37 and C38 polyunsaturated long-chain alkenones (LCAs) serve as proxies for the determination of paleotemperatures for marine surface waters. We studied Recent/Subrecent and Late Glacial/Holocene sediments from Germany, Austria, Russia, and the U.S. to test for a correspondence between LCA distribution and surface water temperature in limnic systems. Previously, reports of LCA occurrence were restricted to sediments of 6 wide distributed freshwater and alkaline lakes. In this study 13 of 27 investigated lakes contained LCAs in surface sediments with concentrations varying between 12 to 205 μg/g TOC. Late Glacial to Holocene sediment sequences from Lake Steisslingen and Lake Wummsee, (Germany), Lake Pichozero (Russia), and Brush Lake (U.S.A.) contained abundant LCAs with averaged concentrations of 33 to 7536 μg/g TOC. For the first time we observed the occurrence of LCAs within in the water column of oligotrophic Lake Stechlin (NE-Germany). Alkenones were restricted to the zone of maximum chlorophyll concentration within the water column indicating that LCAs have a biosynthetic origin and can be attributed to phototrophic (micro)algae. Attempts to identify the producing organism, however, were not successful. Culture experiments allow various phytoplankton to be excluded as producers. Alkenone-producing algae are evidently of small size, hindering microscopical identification. LCAs commonly occur in high concentrations in Late Glacial sediments, mainly during the cold period of the Younger Dryas, whereas the Holocene usually is devoid of polyunsaturated alkenones. The episodic occurrence of LCAs restricts their utility as proxies for continuous geological records. Furthermore, lack of microscopical verification and the episodic distribution allow for different producers of unsaturated alkenones in Recent and Late Glacial sediments. An empirical relationship between LCA distribution and temperature was observed. In fossil sediments from Lake Steisslingen, there is a good correspondence between Uk37 and the temperature-controlled δ18O isotope ratio of lake chalk. Comparison of LCA patterns obtained from the uppermost centimetres of lake sediments with averaged summer surface water temperatures of the lakes studied, demonstrates a trend of covariance (r2: Uk′37 = 0.90, Uk37 = 0.67; n = 9). Hence, the same mechanism that causes temperature-dependence of LCA patterns in marine systems might be effective in limnic settings. Identification of alkenone producers and their culture under controlled temperature are still mandatory before LCAs can be routinely applied as paleotemperature proxy in limnic systems.

Microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in Toarcian source rocks of different maturities

Abstract This study is part of a comprehensive research program on the generation and migration of petroleum from the organic-rich Posidonia Shale (Lower Toarcian, Jurassic) of the Hils syncline, northern Germany. Four cores of the complete Posidonia Shale (with varying amounts of underlying Pliensbachian and overlying Aalenian mudstones) provided the sample base. Depending upon their distance from a presumed subjacent heat source, the Vlotho Massif, the cores display progressive levels of increasing thermal maturation with R 0 -values (0.48, 0.68, 0.88 and 1.45%) which span the limits of the oil-generation window. Primary stratigraphic, lithologic and petrographic attributes of the four sections are all very similar. Each includes a lower marlstone member (about 5 m thick) overlain by 11–31 m of calcareous clay-shale. Clay minerals, calcite, pyrite, kerogen, and quartz are the principal constituents. Thus, there is no evidence that depositional conditions of the Posidonia Shale varied significantly over the sampled area. The omnipresence of framboidal pyrite, clearly formed from the activity of sulphate-reducing bacteria at or near the depositional interface, is strong evidence that uniform reducing (anoxic) conditions were persistent at all localities. This conclusion supports the assumption that initially each section was identical in overall properties and that significant differences observed in geochemistry, organic macerals, and other properties are directly related to the variable levels of thermal maturation. Kerogen from the least mature section ( R o = 0.48%) is Type II and composed of alginite A, alginite B, liptodetrinite, bituminite and some terrestrial (sporinite, vitrinite and inertinite) macerals. With increasing maturation the alginite macerals exhibit a progressive change in morphology and fluorescence properties. Alginite of the least mature section displays a strong green fluorescence. At 0.68% R o , alginite macerals exhibit yellow-orange fluorescence and are structurally well preserved, although geochemical data indicate generation and loss of large amounts of oil. At 0.88% R o , hydrocarbon generation is almost complete and few primary alginites remain. They have been transformed into secondary liptinites, designated as metaalginite, which have similar fluorescence colours. At the highest maturation level (1.45% R o ), metaalginites are only weakly fluorescent with a shift toward red colours. Calcite- and/or bitumen-filled bedding-plane micro- and macrofractures occur exclusively in the organic-rich Posidonia Shale at maturity levels of 0.68% R o and above. Such features are totally absent in the immature Posidonia section and in the adjacent organic-poor mudstones. This observation indicates that the fractures developed in direct response to overpressuring (i.e. pore pressure exceeding lithologic load) related to oil and gas generation in the source rock. The fractures undoubtedly provided important expulsion conduits for generated products.

Keys to the depositional history of the Posidonia Shale (Toarcian) in the Hils Syncline, northern Germany

Abstract The Posidonia Shale from northwest Germany displays a threefold stratigraphic subdivision: lower marlstone, middle calcareous shale with bivalve shells, and upper calcareous shale. Deposition commenced after transgression over a disconformity on the Pliensbachian. Absence of current features indicates depositional conditions below wave base. Sediment infilling to water depths coincident with wave base probably caused cessation of Posidonia Shale deposition. Calcitic carbonate, organic matter and detrital clay are principal primary components. TOC values are 2–15%. Macerals are mostly from marine phytoplankton; terrigenous macerals are minor. HI values are high (500–800). Detailed sampling reveals a thin (4.5 m) low carbon zone within the middle shale; water column oxidation and bacterial alteration may account for this facies. Clay and organic matter relationships suggest that land-derived nutrients controlled phytoplankton productivity; carbonate constituents were a diluent to organic matter accumulation. Absence of burrowing, abundance of organic matter, and pervasive diagenetic pyrite demonstrate continual anoxia below the sediment-water interface. However, C/S relationships suggest that a euxinic water layer did not extend far above the depositional surface. The occurrence of benthic bivalves in the middle shale indicates that bottomwater tolerable to epibenthos sometimes prevailed. Sulphate-reducing bacteria modified the primary organic matter causing a 20–30% loss.

A Novel Approach for Recognition and Quantification of Hydrocarbon Migration Effects in Shale-Sandstone Sequences

A detailed organic geochemical study of over 150 samples from two cores with a total combined length of 320 m (1,050 ft) through sequences of interbedded source rock-type shales (0.84% Rm maturity) and reservoir sandstones allowed recognition and quantitation of a number of migration effects. Detailed gas chromatography-mass spectrometry of steranes and triterpanes was used to insure that samples being compared to investigate migration effects contain organic matter of a similar type. Thin shales interbedded in sands and the edges of thick shale units are depleted in petroleum-range hydrocarbons to a much higher degree than the centers of thick shale units. For the alkanes, expulsion occurs with pronounced compositional fractionation effects: shorter chain length n-alkanes are expelled preferentially, and isoprenoid alkanes are expelled to a lesser degree than their straight-chain isomers. Based on material balance calculations, expulsion efficiencies were determined and found to be very high in certain instances. For thin interbedded shales, they decrease from about 80% around C15 to near zero in the C25+ region. There is no evidence for significant redistribution of steranes and triterpanes in the two sequences. Compared to C15 to C25 n-alkanes, they appear relatively immobile. The composition of the hydrocarbons impregnating parts of the reservoir sandstones is in agreement with expulsion occurring with pronounced fractionation based on molecular chain length. Hence, consideration of bulk expulsion efficiencies gives an unrealistic picture. Furthermore, the impregnation of a siltstone cap rock from an underlying hydrocarbon accumulation seems to have occurred by bulk-oil migration and without significant fractionation. The degree of hydrocarbon depletion of some of the shales of both sequences appears to be controlled by compaction, and the primary migration process appears to have occurred with chromatographic separation. The migration phenomena observed in both sequences lead us to propose that the main phase of expulsion can be preceded by an earlier sta e, during which the edges of thick shales and thin interbedded shales appear to be slowly and continuously depleted by the chromatographic processes. The composition of the hydrocarbon product accumulating in the reservoirs at this stage appears to be controlled primarily by physical processes rather than by the type and maturity of the organic matter in the generating source rock. By this mechanism, the origin of accumulations of light oils and gas-condensates in low mature sequences bearing predominantly terrestrial-derived organic matter can be explained. Finally, the migration effects documented in this study have some consequences for interpretation of geochemical data (e.g., the pristane/n-C17 ratio, a commonly accepted maturity parameter, has been shown to be also controlled by the degree of hydrocarbon expulsion).

Geochemical effects of primary migration of petroleum in Kimmeridge source rocks from Brae field area, North Sea. I: Gross composition of C15+-soluble organic matter and molecular composition of C15+-saturated hydrocarbons

In mature, oil-prone source rocks of the Upper Jurassic Kimmeridge Clay in the Brae field area. North Sea, the yield and the gross composition of solvent-extractabte pctroleum-likc compounds, as well as the molecular proportions of C15+-saturated hydrocarbons, are primarily controlled by the effects of hydrocarbon expulsion during primary migration. Within sample series extending from shale centres towards adjacent reservoir sandstones, regular and pronounced yield decreases have been documented, as well as marked compositional changes for C15+-soluble organic matter and its compound class fractions (saturated hydrocarbons, aromatic hydrocarbons, NSO-compounds) and for normal and isoprenoid alkanes. Relative migration losses indicating relative expulsion efficiencies were determined for the C15+-alkanes. Throughout the outer five meters of thick shales relative expulsion efficiencies increase regularly and drastically for all compounds considered, e.g. exceeding 90% for the C15+-n-alkanes in the outermost shale samples. Samples from thin (5–50 cm) shale layers interbedded between reservoir sandstones reveal similarly high expulsion efficiencies. For n-alkanes, the degree of depletion remains uniform throughout the molecular range C15 to C30i.e. no compositional fractionation occurred during primary migration, which is interpreted to reflect expulsion of oil as a single phase fluid. A simple conceptual model is proposed to explain the mechanisms and relationships of generation and expulsion processes in time and space. A geochemically-relevant conclusion is that the C15+-soluble organic matter recovered by solvent extraction of mature source rocks represents only a portion of the total non-volatile petroleum-like material originally generated.