Rainer G. Schaefer

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.

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).

Determination of gross kinetic parameters for petroleum formation from Jurassic source rocks of different maturity levels by means of laboratory experiments

Abstract Solvent-extracted core samples of Lower Toarcian carbonate-rich shales (“Posidonia Shale”, Lias ϵ) of the Hils syncline area, Lower Saxony Basin, northern Germany, covering a wide maturity range (vitrinite reflectance, Rr = 0.48, 0.88 and 1.45%) and a Kimmeridge mudstone (Rr = 0.45%) are pyrolyzed at three different heating rates (0.1, 0.7 and 5.0 K/min). By application of non-isothermal first-order reaction kinetics to the resulting hydrocarbon evolution curves activation energy distributions between 46 and 70 kcal/mol and pre-exponential Arrhenius factors are obtained by an iteration method. Hypothetical hydrocarbon generation curves are calculated for the Lias ϵ assuming three geological heating rates (0.5, 5 and 50 K/106a). Peak hydrocarbon generation temperatures range from 135 to 165°C.

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.

Generation and migration of light hydrocarbons (C2C7) in sedimentary basins

Sixty-five samples from selected source bed-type shale sequences from three exploration wells were analysed for yield and detailed composition of light hydrocarbons(C2C7) by a new hydrogen stripping/capillary gas chromatographic technique. In spite of low maturation levels (0.35–0.55% vitrinite reflectance), significant generation of ethane and propane was recognized in a Jurassic source bed sequence bearing hydrogen-poor kerogens. Light hydrocarbon generation in another and mature Jurassic source rock sequence is controlled by kerogen quality. Associated with a change from hydrogen-poor to hydrogen-rich kerogens, yields of total and most individual hydrocarbons exhibit orders-of-magnitude increases. At the same time, iso/n-alkane ratios for butanes, pentanes and heptanes decrease significantly. A study of an interbedded marine/nonmarine coal-bearing sequence of Upper Carboniferous age from the Ruhr area, West Germany, revealed that a marine shale unit in comparison to the adjacent coal seam is more prolific in generating n-alkanes of increasing molecular size. A case history for migration of light hydrocarbons by means of diffusion through shales is presented. In two shallow core holes in Campanian/Maastrichtian shales in West Greenland, upward diffusion of ethane to pentane range hydrocarbons is an active process within the near-surface 3 m interval. Diffusive losses within this interval amount to 99.8% for propane, 85.6% for n-butane and 38.9% for n-pentane.

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.

Kinetics of Petroleum Formation and Cracking

One of the most fundamental problems in basin modeling as related to petroleum exploration is assessing the temporal and spatial limits of petroleum generation in sedimentary basins. It is well known that petroleum is generated from macromolecular sedimentary organic matter as it thermally degrades upon burial. The multitude of chemical reactions involved are unknown in detail (Philippi 1965; Welte 1965) but are recognized to be quasi-irreversible Suck and Karweil 1955; Hanbaba and Juntgen 1969; Tissot 1969). The organic components of subsiding sedimentary rocks are generally far away from thermodynamic equilibrium (Dayhoff et al. 1967; Tackach et al.1987). Consequently, the formation of oil and gas in nature is controlled by chemical reaction kinetics, in particular by non-isothermal kinetics because temperature changes as a function of time under geological conditions (Tissot and Espitalie 1975).

Organic geochemistry of crude oils from the Vienna Basin and an assessment of their origin

Abstract A representative collection of oil samples and Neogene rock samples from the Vienna Basin has been analyzed by organic geochemical methods. The relatively high maturity of the crude oils and the characteristics of their hydrocarbon composition makes it unlikely that they are derived from the Neogene sediments of the Vienna Basin. Furthermore, investigation of the rock samples also showed that these sedimentary strata do not have the potential for generation of liquid hydrocarbons. However, there is some potential for gas generation. Even the deepest Neogene sediments (> 4000 m depth) just have reached the onset of the hydrocarbon generation zone. From these results it has to be concluded that the bulk of the oil found so far in the Vienna Basin originated from pre-Tertiary sediments below the Neogene basin.

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.