Matthias Radke

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.

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).

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.

Organic geochemistry of freshwater and alkaline lacustrine sediments in the Green River Formation of the Washakie Basin, Wyoming, U.S.A.

Abstract Two members of the Green River Formation in the Washakie Basin have been analysed by organic geochemical and organic petrographic techniques and the results placed in a sequence stratigraphic framework. One of these members, the Laney Shale, was deposited in a hydrologically closed, alkaline lake under an arid climate and was characterized by high concentrations of alginite-rich organic matter whereas the other, the Luman Tongue, was deposited in a hydrologically open, freshwater lake under a humid climate and consisted of organic-poor profundal lake mudstones and coaly lake margin sediments. Potential source rocks in both lake types have the potential for generating high-wax oil at high subsurface temperatures (150–175°C at a heating rate of 5 K/Ma) but the alkaline system is clearly more prolific. The most abundant biological marker hydrocarbons were the 4-methylsteranes in both lake types, though dinosteranes were found only in the Laney Shale. Oleanane was absent from the Luman Tongue despite an abundance of angiosperm palynomorphs. Intraformational heterogeneities were best documented for the Laney Shale where molecular differences in kerogen type occur at the parasequence level, with early transgression-, maximum transgression- and rejuvenation stages of lake history having its own kerogen type and distinctive high-wax oil fingerprint. Thermal lability was shown to be linked to kerogen structure, in particular the presence of alicyclic and oxygen-containing moieties, and these in turn are correlated with total organic carbon content and thence productivity/degree of preservation. Some kerogen components appear to have originated via selective preservation whereas others may have been formed by diagenetic condensation reactions.

Generation and distribution of aromatic hydrocarbons in coals of low rank

Abstract The generation of C11+-aromatic hydrocarbons has been investigated in a total of thirteen coal samples with mean random vitrinite reflectance (Rr) between 0.45 and 1.00% mainly derived from the Lower Cretaceous German Wealden formation in the Lower Saxony Basin. Yields of total aromatics and selected dicyclic and tricyclic aromatic molecules which were extractable before and after heat treatment of the samples at 350 and 400°C have been determined by medium and high pressure liquid chromatography followed by capillary gas chromatography. Regular variations with increasing Rr in the carbon-normalized yields demonstrated that aromatic hydrocarbon generation was strongly controlled by rank. In contrast, variations in maceral composition appeared to be of minor importance. Based on the conceptual model of “open” and “closed” poers, discrepancies in the quantitative distributions of extractable aromatics among unheated and heated coals were attributed to a restricted mobility of molecules contained in “closed” pores, which were accessible to solvent extraction only after the rigid physical coal structure had partially been disintegrated by heat treatment at 350°C.

Geochemical effects of petroleum migration and expulsion from Toarcian source rocks in the Hils syncline area, NW-Germany

For mature, oil-prone source rocks of Toarcian-age in the Hils syncline area, NW-Germany, the geochemical effects of primary migration of petroleum are shown. Expulsion has occurred at high efficiencies primarily via a network of macro- and micro-fractures. The effects of this fracture-controlled expulsion with respect to gross and molecular composition of C15+-soluble organic matter are documented. Samples from fractured intervals have expelled total C15+-soluble organic matter more efficiently than adjacent unfractured intervals. A systematic comparison of samples taken at cm-intervals away from macro-fractures reveals compositional gradients which are interpreted to reflect preferential depletion of saturated and aromatic hydrocarbons from the source rock shale into the fractures. Extracts of shale samples directly at fracture walls are enriched in NSO-compounds and highly polar asphaltene-like fractions. The composition of residual petroleum fluids extracted from the fracture-fill materials is variable. In some cases there is a close match with that of the shale at some distance away from the fractures. In other cases there is geochemical evidence to suggest that petroleum fluids were chemically altered during lateral flow along the fractures. Based on available geological and geochemical evidence a conceptual model is proposed to explain fracture development and fluid flow processes. Rapid heating by a nearby intrusive body at shallow depths when the Toarcian sediments still contained about 15–20% pore water has played an important role: overpressuring of aqueous pore fluids with abundant dissolved carbonate caused intense fracturing of the rich intervals in the mature Toarcian sections. Finally, the closely interdependent relationships between petroleum generation and petroleum migration processes are documented. Especially, the evolution of these relationships with maturity progress from immature (0.48% Rm) to mature (0.68 and 0.88% Rm) and overmature (1.45% Rm) stages is shown. For example, in the maturity interval from 0.68 to 0.88% Rm most of the measured shale compaction and porosity decrease is achieved by expulsion of petroleum. For some samples there is an inverse relationship between kerogen quality and extractable hydrocarbon yields suggesting that expulsion efficiencies were higher from the better-quality intervals because the petroleum achieved higher saturations and hence higher relative permeabilities.

Quantitative gas chromatography — mass spectrometry analysis of geological samples

Abstract The absolute concentration of individual biological markers in the hydrocarbon fractions of crude oils and sediment extracts can be determined by the addition of a known amount of an alkane standard before gas chromatography-mass spectrometry analysis. This method adds a new dimension to biological marker analysis in petroleum exploration. It has shown that the concentrations of steroid and triterpenoid hydrocarbons in sediment extracts decrease considerably between the onset of petroleum generation and its peak. The concentration of these compounds in more than forty crude oils from the Williston Basin vary by more than a factor of one hundred. This variation, in addition to distribution patterns, in this sedimentary basin helps in the division of oils into families. Furthermore, absolute concentrations of biological markers in crude oils allow to detect mixing of oils of different maturities and uptake of biological markers from the organic matter indigenous to reservoir rocks. This is illustrated with a study of oils from the North Slope of Alaska and from North-west Germany.

Retene in pyrolysates of algal and bacterial organic matter

Abstract Retene, dehydroabietin, and 1,2,3,4-tetrahydroretene have been identified by GC–MS in extractable aromatic hydrocarbon fractions produced by pyrolysis at 200 and 300°C of pure strains of the green alga Chlorella protothecoides and the cyanobacterium Synechocystis sp. The results suggest phytoplankton as a possible source of retene in Recent and ancient sediments.