Hils Syncline Research Articles

3D basin modeling of the Hils Syncline, Germany: reconstruction of burial and thermal history and implications for petrophysical properties of potential Mesozoic shale host rocks for nuclear waste storage

AbstractJurassic sedimentary sequences suitable for nuclear waste storage in northern Germany consist of organic-lean claystone and were uplifted to < 100 m depth in the Hils Syncline area (southern Lower Saxony Basin). This Hils Syncline, showcasing a northwestward increase in thermal maturity, facilitates the study of shale petrophysical properties influenced by burial history. This study introduces a 3D-thermally calibrated numerical model of the Hils Syncline area to analyze its geodynamic evolution and maturity variations. It provides new vitrinite reflectance and sonic velocity data for modeling calibration and erosion estimation. The Hils Syncline area has undergone continuous subsidence, interrupted by a Cretaceous uplift documented by an erosional unconformity. During the latest Early Cretaceous, Jurassic rocks underwent maximum burial reaching up to several thousand meters depth and temperatures up to 160 °C in the northwest. The Late Cretaceous inversion caused stronger erosion towards the northwest removing up to 3300 m of sediment compared to about 1300 m in the south, according to vitrinite reflectance-based estimations. Numerical modeling results along the study area indicate decreasing porosity and permeability northwestward with increasing thermal maturity. Porosity and vertical permeability decreased to 5–14% and 2.8 × 10–23 to 1.5 × 10–19 m2 [1 mD = 10−15 m2], respectively, while vertical thermal conductivity increased to 1.30–2.12 (W/m/K). These trends of porosity/permeability and thermal conductivity with burial align with sonic velocity and published experimental porosity data, except for the thermally most mature region (Haddessen). This anomaly is tentatively attributed here to localized overpressure generation in the Posidonia Shale during maximum burial, affecting both the underlying Pliensbachian and overlying Doggerian units. Graphical abstract 3D numerical model of the Hils Syncline and surrounding area revealing that a northwestward increase in maximum burial resulted in higher temperatures and varying maturity levels. While most locations align well with calibration data (i.e. measured vitrinite reflectance and porosity), discrepancies arise in the Haddessen/Bensen area. The mismatch between porosity, vitrinite reflectance, and sonic velocity response indicates local overpressure in the northernmost region mainly during the Cretaceous. It was likely caused by gas generation in the Posidonia Shale affecting nearby Lower and Middle Jurassic units.

Lower Jurassic (Pliensbachian–Toarcian) marine paleoenvironment in Western Europe: sedimentology, geochemistry and organic petrology of the wells Mainzholzen and Wickensen, Hils Syncline, Lower Saxony Basin

AbstractOver the past few decades, Toarcian (Early Jurassic) black shale deposits of NW Europe have been extensively studied, and the possible global and regional mechanisms for their regional variation have been discussed. In this context, the black shales of the Northwest German Basin are still sparsely studied with regard to their palaeo-depositional history. This study aims to understand the connection between regional and global influences on the widespread Early Toarcian oceanic anoxic event by examining two wells covering the Upper Pliensbachian to Upper Toarcian sediments in the Northwestern German Basin. The core intervals were analysed using a multidisciplinary approach, including geochemistry, biostratigraphy and organic petrography. Marine palaeoenvironmental changes were reconstructed, and sediment sequences were stratigraphically classified to allow a supra-regional stratigraphic correlation. The results reveal complex interactions between sea level changes, climate warming, basin confinement, and Tethys–Arctic connectivity resulting in the Toarcian black shale deposition. Upper Pliensbachian sediments were deposited under terrigenous influence, shallow water depths, and predominantly oxic bottom water conditions. The deposition of black shale is characterized by algal organic material input and anoxic bottom water conditions. Strong correlations between water stratification, anoxia, and bioproductivity suggest that global warming and intensification of monsoonal rainfall, continental weathering, and increasing freshwater and nutrient inputs were the main factors controlling the formation of black shales. Prolonged deposition of OM-rich sediments in the NWGB may be related to intensified monsoonal precipitation in northern Europe and enhanced Tethys–Arctic connectivity at the serpentinum–bifrons transition. Graphical Abstract

Precise maturity assessment over a broad dynamic range using polycyclic and heterocyclic aromatic compounds

The construction and reliable application of maturation indices is extremely important in deep Earth exploration, yet predicting levels of maturation on a molecular level, especially at overmature stages, is still a major challenge. Here, we report robust, broad ranging and precise maturity parameters that were developed using a continuous core of thermally overmature lacustrine deposits (Shahezi Fm.) from the deep Songke-2 Well (SK-2), Songliao Basin, China, augmented by shallow cores of lower maturity marine deposits (Posidonia Shale) from the Hils Syncline, Germany.The novel indices presented here are based on the general process of thermally-induced cyclization, aromatization and aliphatic chain cracking of hundreds to thousands of compounds during maturation. The newly developed parameters, with an extremely broad dynamic range extending from catagenesis and into metagenesis (vitrinite reflectances (Ro) range from 0.9% to 2.2%), are based on polycyclic and heterocyclic aromatic compounds measured using Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS). The high coefficient of correlations between the new maturity parameters with Ro and Tmax demonstrate their utility for assessing precisely the thermal maturity of overmature shales.

Changes of composition and content of tricyclic terpane, hopane, sterane, and aromatic biomarkers throughout the oil window: A detailed study on maturity parameters of Lower Toarcian Posidonia Shale of the Hils Syncline, NW Germany

A total of 30 core samples of organic matter-rich Lower Toarcian shale (Posidonia Shale) were geochemically analyzed to study systematic changes in the quantitative and qualitative composition of specific biomarkers at different thermal maturities ranging from 0.48 to 1.45% vitrinite reflectance (VRr). The samples are derived from 5 wells (Hils Syncline, NW Germany) each representing a different level of maturation (0.48, 0.68, 0.73, 0.88, 1.45 %VRr). In NW Germany, Posidonia Shale was deposited under marine, oxygen-deficient bottom water containing initially high amounts of TOC (above 10%) and marine type I-II kerogen. Upon maturation, organic carbon has been lost and hydrogen index values have decreased. Biomarker ratios reflecting this maturation very well include ratios of short over long-chain n-alkanes, ratios of pristane and phytane over nC17 and nC18, Ts/(Ts + Tm), C29Ts/(C29Ts + C29H), ratios based on DMDBTs, and TA(I)/TA(I + II) ratios (triaromatic steroids). The evolution of different steroid-, hopane-, naphthalene-, phenanthrene- and dibenzothiophene-related parameters is documented and compared to literature.Concentrations of individual hydrocarbons were measured in order to obtain quantitative data on biomarker abundance and their evolution upon maturation. Whereas there is a general trend of increasing biomarker content from immature (0.48 %VRr) to early mature (0.68 %VRr) conditions and of decreasing biomarker content with maturation from 0.68 to 0.88 %VRr, there are some exceptions from this trend, especially at low levels of maturity. Almost no polycyclic biomarkers were detected at the highest maturity level of 1.45 %VRr in well Haddessen, where n-alkanes and aromatic hydrocarbons are predominant. The overall pattern of polycyclic biomarker abundance is not only determined by isomerization and aromatization processes, but mainly by preferential degradation of thermally less stable biomarkers leading to the observed modifications of biomarker maturity parameters.Furthermore, outer and inner sections from the 10 cm wide cores were compared in order to find out, whether 40 years of dry storage in a core repository affected the biomarker parameters. No significant and systematic differences have been observed, with exception of the most mature Haddessen well, where low molecular weight n-alkanes (<nC21) are more abundant in the outer part.

Geochemical composition of oils from the Gifhorn Trough and Lower Saxony Basin in comparison to Posidonia Shale source rocks from the Hils Syncline, Northern Germany

Geochemical composition of oils from the Gifhorn Trough and Lower Saxony Basin in comparison to Posidonia Shale source rocks from the Hils Syncline, Northern Germany

Evolution of porosity and pore types in organic-rich, calcareous, Lower Toarcian Posidonia Shale

Low and high resolution petrographic studies have been combined with mineralogical, TOC, RockEval and porosity data to investigate controls on the evolution of porosity in stratigraphically equivalent immature, oil-window and gas-window samples from the Lower Toarcian Posidonia Shale formation. A series of 26 samples from three boreholes (Wickensen, Harderode and Haddessen) in the Hils syncline was investigated. The main primary components of the shales are microfossiferous calcite (30–50%), clay minerals (20–30%) and Type II organic matter (TOC = 7–15%, HI = 630–720 mg/gC in immature samples). Characteristic sub-centimetric light and dark lamination reflects rapid changes in the relative supply of these components. Total porosities decrease from 10 to 14% at Ro = 0.5% to 3–5% at Ro = 0.9% and then increase to 9–12% at Ro = 1.45%. These maturity-related porosity changes can be explained by (a) the primary composition of the shales, (b) carbonate diagenesis, (c) compaction and (d) the maturation, micro-migration, local trapping and gasification of heterogeneous organic phases. Calcite undergoes dissolution and reprecipitation reactions throughout the maturation sequence. Pores quantifiable in SEM (>ca. 50 nm) account for 14–25% of total porosity. At Ro = 0.5%, SEM-visible macropores1 are associated mainly with biogenic calcite. At this maturity, clays and organic matter are not visibly porous but nevertheless hold most of the shale porosity. Porosity loss into the oil window reflects (a) compaction, (b) carbonate cementation and (c) perhaps the swelling of kerogen by retained oil. In addition, porosity is occluded by a range of bituminous phases, especially in microfossil macropores and microfractures. In the gas window, mineral-hosted porosity is still the primary form of macroporosity, most commonly observed at the organic-inorganic interface. Increasing porosity into the gas window also coincides with the formation of isolated, spongy and complex meso- and macropores within organic particles, related to thermal cracking and gas generation. This intraorganic porosity is highly heterogeneous: point-counted macroporosity of individual organic particles ranges from 0 to 40%, with 65% of organic particles containing no macropores. We suggest that this reflects the physicochemical heterogeneity of the organic phases plus the variable mechanical protection afforded by the mineral matrix to allow macroporosity to be retained. The development of organic macroporosity cannot alone account for the porosity increase observed from oil to gas window; major contributions also come from the increased volume of organic micro- and meso-porosity, and perhaps by kerogen shrinkage.

Flow-through extraction of oil and gas shales under controlled stress using organic solvents: Implications for organic matter-related porosity and permeability changes with thermal maturity

Four core plugs from the Lower Jurassic Posidonia Shale of the Hils Syncline in northern Germany have been subjected to flow-through extraction tests with dichloromethane (DCM) under controlled stress conditions in a tri-axial flow cell. The samples represent a maturity sequence from 0.53 to 1.45% VRr. The bitumen sequentially extracted from the natural pore space of the shale plugs was analyzed for its geochemical composition. Changes in rock matrix density, porosity and permeability resulting from the removal of soluble organic matter were determined. The relative porosity increase of the plugs after extraction ranged from 6 to 13% and correlated with the extract yield. Klinkenberg-corrected permeability coefficients measured with helium increased by a factor of 17.0 for the immature/early mature sample and 26.6 for the overmature sample. Petrographical investigations after extraction indicate that fluid flow occurred predominantly parallel to bedding as evidenced by open fractures and fractures bearing residues that apparently precipitated from the DCM solution. Compositional variations of the extracts over time are interpreted in terms of the organic geochemical inventory of bitumen associated with the natural pore system and its accessibility at different maturity levels. These patterns deviate strongly from the bulk rock extracts of the powdered samples.

The electrical conductivity of Posidonia black shales — from magnetotelluric exploration to rock samples

ABSTRACTWe carried out a magnetotelluric field campaign in the South–East Lower Saxony Basin, Germany, with the main goal of testing this method for imaging regional Posidonia black shale sediments. Two‐dimensional inversion results of the magnetotelluric data show a series of conductive structures correlating with brine‐saturated sediments but also with deeper, anthracitic Westphalian/Namurian coals. None of these structures can be directly related with the Posidonia black shale, which appears to be generally resistive and therefore difficult to resolve with the magnetotelluric method. This assumption is supported by measurements of electrical resistivity on a set of Posidonia shale samples from the Hils syncline in the Lower Saxony basin. These rock samples were collected in shallow boreholes and show immature (0.53% Ro), oil (0.88% Ro), and gas (1.45% Ro) window thermal maturities. None of the black shale samples showed low electrical resistivity, particularly those with oil window maturity show resistivity exceeding 104 Ωm. Moreover, we could not observe a direct correlation between maturity and electrical resistivity; the Harderode samples showed the highest resistivity, whereas the Haddessen samples showed the lowest. A similar trend has been seen for coals in different states of thermal maturation. Saturation of the samples with distilled and saline water solutions led to decreasing electrical resistivity. Moreover, a positive correlation of electrical resistivity with porosity is observed for the Wickensen and Harderode samples, which suggests that the electrical resistivity of the Posidonia black shale is mainly controlled by porosity.

Mineralogical changes in organic-rich Posidonia Shale during natural and experimental maturation

The diagenesis of clay minerals and organic matter in Posidonia Shale has been typically studied independently of each other. Here, the results of an experimental investigation focused on the mineralogical changes of organic-rich shales are presented to understand the dissolution and precipitation processes of minerals as a function of thermal maturity. Three core samples of Posidonia Shale (Hils Syncline, Germany) with increasing natural thermal maturity (Wickensen (WIC) 0.53% R0, Dohnsen (DOH) 0.73% R0, Haddessen (HAD) 1.45% R0) were reacted in experimental aliquots of 0.3M KCl solution (rock/fluid ratio 1:5) sealed in gold capsules at 300°C and 120 bars for 27days. Following treatment, the calculated maturities (Easy-R0 values) of the two samples with the lowest vitrinite reflectance values indicates thermal enhancement (WIC 1.14% R0, DOH 1.15% R0), whereas the most mature sample remained unchanged (HAD 1.45% R0). The educts and products were analysed by transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). In all experiments kaolinite and carbonates were extensively dissolved whereas quartz underwent partial dissolution. Reaction products consisted of lath shaped Mg2+ rich 1Md illite together with K-feldspar in the form of sanidine. Trioctahedral saponite precipitated only in the oil producing sample (WIC), along with less sanidine and 1Md illite. The unexpected formation of metastable 1Md illite at 300°C instead of stable 2M1 illite is attributed to low K+ reactivity due to either depletion of this element by the early formation of sanidine and/or the chemical effects of organic acids that shift the stable phase into the K-poor stability field. The precipitation of saponite and the limited sanidine and 1Md illite formation in sample DOH is attributed to high dolomite content and hence Mg2+ availability, along with the more extensive release of hydrocarbon compounds and further reduction of K+ reactivity in this sample. It is concluded that the neoformation of metastable phases (1Md illite and trioctahedral smectite) observed in short-term, higher-temperature experiments may differ significantly from prolonged mineral transformations occurring under natural conditions.

Maturity-Driven Generation and Transformation of Acidic Compounds in the Organic-Rich Posidonia Shale as Revealed by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

The Posidonia Shale in the Hils syncline in North-West Germany represents a natural maturity sequence from 0.48 to 1.45% vitrinite reflectance (R0). In this study, the molecular composition of the acidic compounds of six Posidonia Shale samples with a different maturation level has been determined using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with electrospray ionization (ESI) in the negative ion mode. The changes in the distribution of elemental and compound classes as well as of double-bond equivalents (DBE) and carbon numbers at the different maturity levels are described in detail. Aromatic N1, N1O1, and N1S1 compounds are preferably formed during ongoing maturation, while the amount of O2 compounds decreases. With increasing thermal stress, condensation and aromatization of the acidic NSO compounds increases. In the sample of highest maturity (R0 1.45%), which has been shown to have a relevant shale gas potential, a significant enrichment of nonalkylated and methylated highly aromatic N1, N1O1, and N1S1 compounds has been observed, which can be the result of thermally induced side chain cracking of compounds with a higher alkylation degree. The other cracking products would be short chain hydrocarbons contributing to a significant amount to the overall gas potential of the Posidonia Shale.

Alteration of organic material during maturation: A pyrolytic and infrared spectroscopic study of isolated bisaccate pollen and total organic matter (Lower Jurassic, Hils Syncline, Germany)

Six samples of Lower Jurassic mudstones (uppermost Pliensbachian, uppermost Lias δ) from boreholes drilled at the southwestern rim of the Hils Syncline (Lower Saxony Basin, NW-Germany), covering a thermal maturity series ranging from immature to overmature, were analyzed to gain insight into the chemical structure and alteration of bisaccate pollen during maturation as well as the differences in results from various analytical methods. Laser-pyrolysis–gas chromatography–mass spectrometry (La-Py–GC–MS) and micro-Fourier transform infrared (μ-FTIR) spectroscopy applied to handpicked bisaccate pollen, and Curie Point-Py (CP-Py–GC–MS) and FTIR spectroscopy applied to sieve-fractionated kerogen concentrate from the same samples indicated considerable differences in composition. Ratios of the main compound groups, e.g. O containing components, aliphatic hydrocarbons and aromatic hydrocarbons, plotted vs. maturity showed systematic changes in composition with maturation. The study also focussed on the comparability of micro- and macroscale spectroscopic and Py methods. The results from FTIR analysis and μ-FTIR analysis, as well as those from La-Py–GC–MS, showed similar trends, e.g. defunctionalization and increasing aromatization, which mostly agree with common knowledge about the changes in the structure of organic matter, whereas the results from CP-Py–GC–MS showed wide differences, especially at higher stages of maturity, i.e. a strong increase in aliphatic vs. aromatic hydrocarbons. Together, the results indicate that both IR spectroscopy and Py–GC–MS are suitable for studying the alteration of palynomorphs during maturation and their complementary use is recommended for comprehensive characterization of alteration.

BIB-SEM study of the pore space morphology in early mature Posidonia Shale from the Hils area, Germany

Abstract This contribution reports on the study of the pore space morphology in two early mature (VR r = 0.59 and 0.61) samples of Posidonia Shale from the Hils Syncline in Germany, using Broad Ion Beam (BIB) polishing and high resolution Scanning Electron Microscopy (SEM). This allows imaging pores with resolution down to 10 nm in equivalent diameter (d eq ), and quantitative estimation of porosity. Using a combination of BSE and SE detectors and semi-automatic segmentation of the gigapixel images, the representative elementary area of the samples, on the scale of a few mm, is inferred to be about 140 × 140 μm 2 . Pore morphologies and pore sizes are clearly related to the mineral fabric, with large differences: very large (typically several microns) pores with internal faceted crystal morphology in recrystallized calcite clasts, and smaller pores (d eq cf. Schizosphaerella nanofossils (typically 200 nm). Pores are less common in organic material and in pyrite aggregates. Pore characteristics are very similar for both samples, and porosity resolvable by BIB-SEM is 2.75 and 2.74%. Pore size distribution can be described by a power-law, with an exponent about 2.0 and 2.2, respectively, for the pore population excluding the fossils. Pores in the carbonate fossils show dual-power-law distribution with power-law exponents of about 1 and 3. By extrapolating the power-law distribution for each sample, total porosity is estimated to be 4.8% (− 0.9, + 1.7%), and 6.5% (− 2.7, + 7.2%), respectively. This estimate can be compared with 3.4–3.7 and 3.3–3.6% as measured by mercury injection porosimetry. We interpret this difference to reflect the unconnected (for mercury) part of the porosity. Comparison of imaged pores and mercury injection porosimetry suggest a very high pore body to pore throat ratio. This results in a pore model where large pores, represented mainly by pores in fossils and calcite grains, are connected via a low-porous (and low-permeable) clay-rich matrix with pore throats below 10 nm in both samples.

Mineral Preferred Orientation and Microstructure in the Posidonia Shale in Relation to Different Degrees of Thermal Maturity

AbstractThe thermal maturity of samples of the Posidonia Shale collected from the Hils Syncline, northern Germany, varies significantly as a function of location indicating variations in local history. Synchrotron X-ray diffraction was used to document the composition and the preferred orientation of four samples of the Posidonia Shale with different degrees of maturity (0.68–1.45%, Ro) to determine possible effects on diagenesis and preferred orientation. Overall, the degree of preferred orientation of all clay minerals (illite-smectite, illite-mica, and kaolinite) and in all samples is similar, with (001) pole figure maxima ranging from 3.7 to 6.3 multiples of a random distribution (m.r.d.). Calcite displays weak preferred orientation, with c axes perpendicular to the bedding plane (1.1–1.3 m.r.d.). Other constituent phases such as quartz, feldspars, and pyrite have a random orientation distribution. The difference in thermal history, which causes significant changes in the maturity of organic matter, influenced the preferred orientation of clay minerals only marginally as most of the alignment seems to have evolved early in their history. Synchrotron X-ray microtomography was used to characterize the three-dimensional microstructure of a high-maturity sample. Low-density features, including porosity, fractures, and kerogen, were observed to be elongated and aligned roughly parallel to the bedding plane. The volume of low-density features was estimated to be ~7 vol.%, consistent with previous petrophysical measurements of porosity of 8–10 vol.%. Transmission electron microscopy analysis of samples with different degrees of maturity (0.74%Ro and 1.45%Ro) was used to document microstructures at the nanoscale as well as the presence of kerogen. In the high-maturity sample, pores were less abundant while minerals were more deformed as shown by fractured calcite and by kinked and folded illite. Some of the porosity was aligned with clay platelets.

Methane‐related microbial gypsum calcitization in stromatolites of a marine evaporative setting (Münder Formation, Upper Jurassic, Hils Syncline, north Germany)

AbstractFossil stromatolites may reveal information about their hydrochemical palaeoenvironment, provided that assignment to a specific microbial community and a corresponding biogeochemical mechanism of formation can be made. Tithonian stromatolites of the Münder Formation at Thüste, north Germany, have traditionally been considered as formed by intertidal cyanobacterial communities. However, thin sections of the stromatolites show elongated angular traces of former gypsum crystals in a dense arrangement, but no algal or cyanobacterial filament traces. Moreover, high Fe2+ and Mn2+ contents, oxygen‐isotope and sulphur‐isotope ratios of carbonate‐bound sulphates, and sulphurized hydrocarbon biomarkers of the stromatolitic carbonate indicate that CaCO3 precipitation occurred near the oxic–anoxic interface as a result of intensive bacterial sulphur cycling rather than photosynthetic activity. Furthermore, anaerobic oxidation of methane by Archaea may have driven CaCO3 precipitation in deeper parts of the biofilm community, as reflected by high concentrations of squalane with a strongly negative δ13C in conjunction with evaporite pseudomorphs showing extremely low δ13CCarb ratios. Consequently, the Thüste stromatolites are now interpreted as having initially formed by gypsum impregnation of biofilms. Subsequently, early Mg‐calcitic calcitization within the biofilms occurred because of combined bacterial iron, manganese and sulphate reduction, with an increasing contribution of anaerobic oxidation of methane with depth. This model plausibly explains the prominent preservation of signals derived from oxygen‐independent metabolic pathways, whereas virtually no geochemical record exists for an aerobic community that may, nevertheless, have prevailed at the stromatolite surface. Photic‐zone stromatolites with a prominent signal of anaerobic oxidation of methane may be common in, and indicative of, oxygen‐depleted sulphate‐bearing environments with high rates of methane production, conditions that possibly were fulfilled at the Archaean to Proterozoic transition.

Ostracod assemblages, palaeoenvironment and cyclicity of Purbeck-type sediments of the Münder Formation (Lower Cretaceous, Hils Syncline, N-Germany)

Berriasian marlstones in the upper part of the Münder Formation (“Purbeck”) of the Hils Syncline have been regarded as freshwater deposits with minor brackish influence. However, similar to some marlstones of the English Purbeck Limestone Group, it remained unclear whether these deposits rich in non-marine ostracods are lacustrine or marine-lagoonal in origin. Facies analysis of the sediments revealed strikingly symmetric cycles composed of (1) red-brown massive marlstones of distal alluvial plains, (2) greenish-grey bedded marlstones with microbial lamination of the supralittoral zone, (3) dark-grey charophyte marlstones and (4) light-grey charophyte limestones with scattered porostromate oncoids of the eulittoral zone, (5) poorly bedded light-grey micritic limestones and (6) clayey marlstones of the sublittoral zone. Ostracod associations revealed by cluster analysis and associated microfossils indicate cyclic salinity changes coincident with lithofacies alternations. A Theriosynoecum-association is restricted to eulittoral and shallow sublittoral lithofacies types and reflects freshwater to oligohaline conditions. This association is accompanied by the freshwater gastropods Gyraulus and Provalvata as well as charophytes of the genera Atopochara, Clavator and Globator. Supralittoral marlstones are characterized by a mixed Cypridea-association, composed of autochthonous Cypridea of ephemeral freshwater pools and transported faunal elements. Increased salinities of miohaline to mesohaline conditions are indicated for sublittoral lithofacies types by a Mantelliana-association. Maximum flooding is indicated by the appearance of small foraminifera. Variations of monospecific ostracod stable isotope ratios coincide with the cyclic salinity fluctuations interpreted from ostracod assemblages along the section. This correlation of lithofacies types and ostracod faunas implies that salinities increased with increasing water level and vice versa. Consequently, a coupling of the water level of the Hils Basin to short-term eustatic sea-level changes must be assumed. Therefore, the Hils Basin is interpreted as a marine embayment during the Berriasian with freshwater spreading out on top of marine-brackish water, but not a lake. A similar approach combining micropalaeontological and sedimentological evidence may help to distinguish between marginal-marine sediments affected by cyclic sea-level changes and lacustrine sediments displaying climatically-driven cyclicity in the Purbeck-type area and elsewhere in Europe.

99/00168 Isomeric identification and quantification of polycyclic aromatic hydrocarbons in environmental samples by liquid chromatography tandem mass spectrometry using a high pressure quadrupole collision cell

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 R0-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 (Ro = 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% Ro, 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% Ro, 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% Ro), 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% Ro 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.

Fluoren-9-ones and carbazoles in the Posidonia Shale, Hils Syncline, northwest Germany

The distribution of alkylated fluoren-9-ones and carbazoles, including their benzo- and dibenzo- and/or naphtho-annellated derivatives, have been investigated in rock extracts from a maturity sequence of Posidonia Shales from the Hils Syncline of northwest Germany. The fluoren-9-one and carbazole derivatives are quantitatively the most important compound types in the low polar heterocompound fraction obtained from the Posidonia Shale bitumens by liquid chromatography. This fraction comprises 9.3–27.5% of the total bitumen depending on maturity. The concentration of the fluoren-9-ones in the bitumens systematically increases with increasing maturity. Generally, a trend of decreasing degree of alkylation is observed with increasing maturity. However, at the molecular level, less systematic maturity-dependent variations are observed for the fluoren-9-one distribution than for the carbazole distributions. A remarkable exception is the 1-ethylfluoren-9-one (1-ethylfluoren-9-one + 1,8-dimethylfluoren-9-one) ratio which linearly decreases over the entire maturity range (0.48–1.45% R r). Among the carbazoles, dibenzo-and/or naphtho-annellated derivatives become more abundant at higher maturity. An oxidation of fluorenes may be ruled out as the major process for the formation of fluoren-9-ones since Posidonia Shale was deposited under anoxic conditions and fluorenes are not present in high amounts even in samples of low maturity. The fluoren-9-ones may rather be produced by cyclisation reactions of suitable precursors in the organic matter. Particularly, 2-carboxybiphenyls could be transformed to fluoren-9-ones either catalysed by Lewis acids, e.g. clay minerals, or solely under the influence of higher temperatures. The constant ratio of fluoren-9-ones and benzofluoren-9-ones suggests that selective formation/degradation and interconversion during maturation plays a minor role. this seems to be different for the carbazoles where a significant influence of maturity on the distribution of derivatives with different numbers of aromatic rings was observed.

Effect of maturity on carbazole distributions in petroleum systems: new insights from the Sonda de Campeche, Mexico, and Hils Syncline, Germany

Effect of maturity on carbazole distributions in petroleum systems: new insights from the Sonda de Campeche, Mexico, and Hils Syncline, Germany

Aromatic aldehydes and ketones in the Posidonia Shale, Hils Syncline, Germany

The low-polarity NSO compound fraction of Posidonia Shale bitumens has previously been shown to contain fluoren-9-ones and benzofluoren-9-ones as the quantitatively most abundant compound class. The present study, based on detailed gas chromatography-mass spectrometry describes the distribution of naphthaldehydes, alkylnaphthylketones, benzaldehydes, alkylphenylketones, 1-indanones and 1-tetralones as additional types of aromatic carbonyl compounds in these samples. Benzene-derived carbonyl compounds are less abundant than the other identified compound classes. The observed distributions show some remarkable maturity related variations. Particularly, the ratios of α- and β-isomers of naphthalene-derived carbonyl compounds behave in a similar manner to the corresponding hydrocarbons, i.e. they show a relative increase of the β-isomers with increasing maturity. The 1-indanone/(1-indanone + 1-tetralone) ratio (ITR) is suggested as a new heterocompound based maturity parameter. The carbonyl compounds described may be derived either from inter- and/or intramolecular acylation and formylation reactions or from oxidation of benzylic methylene groups of alkylated aromatic hydrocarbons.

Palaeomagnetism of the Jurassic Asphaltkalk deposits, Holzen, northern Germany

SUMMARY Palaeomagnetic and rock magnetic studies on the Kimmeridgian-Portlandian Asphaltkalk deposits of the Hils Syncline near Holzen in northern Germany suggest a connection between bitumen impregnation and magnetization in the limestones. A stable remanent magnetization is only found in the Asphaltkalk deposits, not in non-bitumen-impregnated limestones of the same age. The stable remanence was acquired after folding in this region, as revealed by a palaeomagnetic fold test. The timing of remanence acquisition is Late Cretaceous-early Tertiary, as derived from a comparison of the mean pole of the magnetic component with the European apparent polar wander path (APWP). The decay pattern of the natural remanent magnetization (NRM) during alternating field (AF) and thermal demagnetizaton indicates that the magnetization is carried by magnetite. Isothermal remanent magnetization (IRM) acquisition and thermal decay measurements, as well as hysteresis parameters, are consistent with this interpretation. This study suggests that the creation of authigenic magnetite during the lower Tertiary in the Asphaltkalk deposits might coincide with the timing of hydrocarbon migration into the unit.