Hils Syncline Research Articles

Intraformational redistribution of selected trace elements in the Posidonia Shale (Hils Syncline, NW Germany) caused by the thermal influence of the Vlotho Massif

Selected samples of NW German Posidonia Shale (Hils Syncline area) were separated into five fractions (bitumen, kerogen, pyrite, carbonates, silicates) and chemically analyzed for trace element concentration using ICP-OES analyses. In immature Posidonia Shale (0.53% R o) Co, Cu, Mo, Ni and Zn occur mainly in the kerogen and pyrite fractions. In mature Posidonia Shale (1.45% R o) these trace elements are mainly fixed in the silicate fractions. We postulate a thermally induced, intraformational redistribution of Co, Cu, Mo, Ni and Zn, caused by heat from a deep-seated igneous intrusion, the “Vlotho massif”.

Hydrocarbon-bearing fluid inclusions in calcite-filled horizontal fractures from mature Posidonia Shale (Hils Syncline, NW Germany)

Hydrocarbon-bearing fluid inclusions in calcite-filled horizontal fractures from mature Posidonia Shale (0.88% R o) were investigated by microscopic, microthermometric and chemical methods. The fluid inclusions predominantly contain both a liquid hydrocarbon phase, which shows bright UV-fluorescence colours (yellow, brownish, light blue) and a non-fluorescent hydrocarbon gas phase. The different UV-fluorescence colours probably reflect the different gross chemical compositions of the trapped liquid hydrocarbons. The saturated hydrocarbon fraction of the fluid inclusions (mostly secondary inclusions) predominantly consists of low-molecular-weight n-alkanes (C 9C 12), while C 12C 22- n-alkanes are more abundant in the adjacent rock matrix. The molecular composition of the fluid inclusions appears to reflect a mobile hydrocarbon phase, which was entrapped within the calcitic fill of the horizontal fractures. Pressure-corrected trapping temperatures were estimated for primary fluid inclusions (123°–162°C). This temperature range is in agreement with the results from numerical simulation studies, which indicate a maximum temperature of 148°C coinciding with the period of fracture formation, calcite filling, and entrapment of primary fluid inclusions.

Extractable alkyldibenzothiophenes in Posidonia Shale (Toarcian) source rocks: Relationship of yields to petroleum formation and expulsion

Extractable C 0 to C 2 dibenzothiophenes were determined by capillary gas chromatography in 125 rock samples of the Posidonia Shale formation with vitrinite reflectance (R r) between 0.4 and 1.5% mostly derived from the Hils syncline area in northwestern Germany. Average yields of individual methyldibenzothiophenes in the range of 2–110 μg/g total organic carbon (TOC) are comparable to respective data for forty-seven rock samples of the Kimmeridge Clay formation (0.4–0.9% R r) from the Dorset coast and different regions of the North Sea including the Brae Oilfield area ( Leythaeuser et al., 1988a,b) Alkyldibenzothiophenes in samples of either formation display distinct yield profiles which agree with those of C 15+ soluble organic matter and hydrocarbon groups, i.e., an immature zone with low yields at 0.4–0.5% R r is followed by a zone of enhanced yields between 0.5 and 0.9% R r, which corresponds to the oil-generation window. Yields are low again in overmature Posidonia Shale samples beyond 1.4% R r. Yield profiles are disturbed towards their maximum around 0.7% R r due to highly variable depletion by erratic petroleum expulsion. Maturity parameters, such as the methyldibenzothiophene ratio (MDR) and the ethyldibenzothiophene ratio (EDR) are based on discrepancies in yield profiles among individual C 1and C 2 dibenzothiophenes that can be attributed to variations in kinetic stability. The parameters are virtually unaffected by depletion, as evidenced by variabilities, much lower for MDR and EDR than for the yields of the components employed. They are likely to be influenced by geothermal heating rates however. When compared to Kimmeridge Clay samples, MDR and EDR increase more gradually with R r beyond 0.7% in Posidonia Shale samples that have experienced enhanced geothermal heating rates owing to an intrusive body in the Hils syncline area.

Isotopengeochemische Untersuchungen an organischen Substanzen aus Sedimenten der Hilsmulde (Nordwestdeutschland)

Abstract Model coalification experiments have been carried out using organic matter in sediments from the Hils syncline. In the experiments we wanted to elucidate, to what extent different facies situations can be observed in the investigated region and which conclusions can be drawn regarding the type of originating substances for hydrocarbons adsorbed in drilling cores. As a result of chemical and isotope analyses the organic substances should be characterized not only as pure sapropelites but show in different degree the participation of humic organic matter. The obtained data of the several evaluation methods used for the characterization of organic matter are in good agreement for the particular samples.

Kerogen maturation in a reference kerogen Type II series: the Toarcian shales of the Hils syncline, NW Germany

Studies on the generation and migration of petroleum in the reference Type II source rocks of the Hils syncline were published some years ago by Rullkötter et al. (1988) and Leythaeuser et al. (1988). In the present study, kerogens of these reference samples ranging from 0.48 to 1.45% vitrinite reflectance ( R o ) were isolated and analysed. After having confirmed their similarity with the Toarcian shales of the Paris basin series, an immature kerogen of this latter series was added to the German natural series. This same kerogen was heated in sealed gold tubes at various temperatures for one day. These two series of kerogens allowed us to follow the thermal maturation both under natural and artificial conditions and to compare them to find common maturation parameters. Comparison of samples relies both on bulk analyses of kerogens and on molecular analyses of their thermal degradation products. Bulk analyses comprise elemental analysis, 13C NMR spectroscopy and Rock-Eval pyrolysis. Molecular analyses are based on mass balance of saturated and aromatic hydrocarbons in extracts and structural analyses of saturated hydrocarbons by gas chromatography and mass spectrometry. These analyses show that the evolution of the natural series is adequately reproduced by the heated samples, the latter having the advantage of recovering all the compounds formed during kerogen maturation from methane to coke, without losses during sampling or by migration phenomena as in natural samples.

Evolution of individual hydrocarbons (C 1C 4) by nonisothermal pyrolysis of petroleum source rocks

Nonisothermal pyrolysis of Toarcian oil shales from the Hils Syncline area, conducted at a low heating rate of 0.1°C/min, resulted in evolution profiles of individual alkanes (C 1C 4) and alkenes (C 2C 4) from which kinetic parameters and amounts of hydrocarbons were derived. The immature ( R m=0.48%) shale sample displayed at least four methane-generation reactions. The activation energies assessed by simple nonisothermal kinetics for the generation of alkanes (C 1C 4) and alkenes (C 2C 4) increased with increasing maturity of the shales, with the position of the maximum dependent on the individual hydrocarbon. Increasing shale maturity appeared to promote alkane and alkene formation.

A study of the Holzener Asphaltkalk, northern Germany: observations regarding the distribution, composition and origin of organic matter in an exhumed petroleum reservoir

The Holzener Asphalkalk deposit is located on the southwestern flank of the Hils Syncline, northern Germany. It consists of a series of Kimmeridgian-Portlandian bitumen-impregnated limestones (cumulative thickness of nearly 200 m) overlain unconformably by Neocomian claystones and underlain by older Jurassic and Triassic sediments. The most bitumen-rich limestone layers have been extensively mined with the result that there is three-dimensional access to what is probably an exhumed series of stacked petroleum reservoirs. Geochemical, petrological and petrophysical techniques were used to gain some insight into the distribution and the source of the organic matter in two of the 13 layers of the deposit. Most disseminated organic matter was found to be soluble in organic solvents and rich in non-hydrocarbons. Its distribution appears to be controlled by ooid versus carbonate matrix abundances, the occurrence of fractures and degree of impregnation. Small (meso- to micro-) pores have high bitumen saturations whereas large pores (meso- to macro-) have much lower saturations. Mature petroleum is thought to have once filled all pores, but alteration has probably removed as much as 85% of it, mainly from the large pores. Regional geochemical considerations, pyrolysis and stable carbon isotope data indicate that the original petroleum was most probably derived from the underlying Posidonia Shale (Toarcian) source rock. Asphaltene pyrolysis data indicate that an algal lacustrine co-source might have also contributed to the deposit.

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.

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

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, R r = 0.48, 0.88 and 1.45%) and a Kimmeridge mudstone ( R r = 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/10 6a). Peak hydrocarbon generation temperatures range from 135 to 165°C.

Evolution of Network of Clastic Petroleum Source Rock During Catagenesis (Lower Toarcian Posidonia Shale, Hils Syncline, Northwestern Germany): ABSTRACT

The Posidonia shale represents the most prolific petroleum source rock of central Europe. Completely cored sections of this lower Toarcian oil shale interval from six shallow boreholes were studied in order to investigate the influences of lithofacies, diagenesis, hydrocarbon content, and compaction on the pore network at various catagenetic stages. Based on geochemistry, petrophysics, and basin modeling, the following conclusions can be drawn. (1) Of the two lithofacies penetrated, the pore network of the marlstone is more permeable than that of the claystone. Therefore, according to this criterion, the marlstones ought to have expelled hydrocarbons earlier than did the claystones. (2) Early diagenetic carbonate cementation has reduced the size and volume of the pores at specific stratigraphic levels. However, around the contact of the two lithofacies, updip-streaming formation waters created secondary porosity which can be considered a migration avenue for generated hydrocarbons from the source rock. (3) Before hydrocarbon expulsion, soluble organic matter, which is evenly distributed within the pore network, occupied up to 60% of the available pore space. (4) With progressive maturation of the organic matter, analyzed pore size and porosity values became continuously smaller. Using the method of computer-aided integrated basin modeling, they can explain the observedmore » porosity reduction trend only partly by mechanical compaction. Mechanical compaction is computed considering the changes in effective stress by taking into account several parameters controlling water flow and pressure buildup. As a consequence, the diminution of the pore network during catagenesis seems to be tied to the collapse of the alginite structures.« less

Maturity-related compositional changes in the low-molecular-weight hydrocarbon fraction of Toarcian shales

Compositional changes in the light hydrocarbon content of sedimentary rocks and petroleum are known to reflect their maturation history. The present contribution comprises the study of the low-molecular-weight hydrocarbon fraction in core samples of Lower-Toarcian age (“Posidonia shale”, Lias ϵ). The samples series, collected by shallow-borehole drilling in the Hils syncline, Lower Saxony Basin, northern Germany, represents an excellent petroleum source rock (type-II kerogen) of a wide maturity range (mean vitrinite reflectance, R m, between about 0.5 and 1.5%). About 30 compounds in the C 2−C 7 molecular range were determined quantitatively using a combined thermovapourization/capillary gas chromatography method. Both quantity and composition (particularly, of the C 7 hydrocarbon fraction) are discussed in relation to vitrinite reflectance values measured at five locations ( R m = 0.48, 0.66, 0.68, 0.88, and 1.45%). Correlations between R m and paraffin/naphthene concentration ratios are described. Also, the isomer concentration ratios of 2-methylhexane/3-methylhexane and of trans- to cis-1,2-dimethylcyclopentane are clearly related to the maturity of the organic matter.

Natural and artificial maturation of biological markers in a Toarcian shale from northern Germany

Steroid and terpenoid biological marker evolution was studied in a maturity sequence of Lias ϵ (Toarcian) shales and adjacent strata from three shallow boreholes in the Hils syncline, northern Germany. Excellent facies control within the study area ensures the comparability of the organic matter type among the stratigraphically equivalent samples of the natural maturation series. In the early hydrocarbon generation stage (0.68% R 0 ), a strong facies dependence of some sterane and hopane isomerisation values was found. Within the same borehole at only a few metres depth difference, the extent of sterane isomerisation at C-14 and C-17 was close to 80% in the Toarcian shale and between about 55 and 65% in the adjacent Dogger α and Lias δ sediments. Similar differences were observed for the 17α(H)-hopane/moretane and 22,29,30-trisnor-18α(H)-neohopane/22,29,30-trisnor-17α(H)-hopane ratios, whereas the 20S/(20S + 20R) sterane isomerisation ratio did not show this difference. The observed differences mostly disappear by the end of the main oil generation phase (0.88% R o ), where several reactions appear to have reached completion. Hydrous pyrolysis experiments, by variations of time and temperature, were used to simulate the natural evolution of the biological markers. Results for sterane isomerisation in the high-temperature range were compared to those published by Mackenzie and McKenzie ( Geol. Mag. 120, 417–470, 1983) for natural sediments. Kinetic data derived for this reaction from the hydrous pyrolysis results alone or from the combination of hydrous pyrolysis and natural data sets differ markedly from those determined by Mackenzie and McKenzie (1983). The hydrous pyrolysis data alone yield an activation energy for sterane isomerisation at C-20 of 131 kJ/mol, the combined data of 170 kJ/mol; the frequency factors are in the 10 5–10 8 s −1 range. The significance of the data is discussed in detail.

Source rock evaluation by well log analysis (Lower Toarcian, Hils syncline)

The feasibility of conducting petroleum source rock evaluations from well-log data was examined using three continuously cored sections (immature, mature and postmature stages) of the Lower Toarcian Posidonia Shale, northern Germany. Logs depicting natural gamma emission (K, U, Th), bulk density, sonic travel time and resistivity were utilised, as was the borehole televiewer for fracture control. Selected data from these logs were correlated with parameters of source richness, hydrocarbon content and maturity. The resulting three-dimensional cross plot shows at a glance well-by-well comparison of facies, maturity and hydrocarbon content. A simple correlation between organic-geochemical and petrophysical parameters is, of course, not perfect since log response is complex and affected by mineralogical and pore fluid properties of the rock as well as by organics. However, source rock characterization by wireline logging is still a fast and therefore economical method. Relative “quick-look” comparisons from well to well are possible without calibration to core data and can easily be performed at once at the drill site. The present application is limited to clastic source rocks in basins with simple geologic histories.

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% R̄ 0) to overmature 1.45% R̄ 0) 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% R̄ o and only marginally more during the maturity increase from 0.88 to 1.45% R̄ o. Only a small portion of the generated material remained in the source rock even at a relatively early stage of generation (0.68% R̄ o). Expulsion efficiency of oil plus gas reached a value of 86% at the end of the main generation stage (0.88% R̄ o).

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 C 15+-soluble organic matter are documented. Samples from fractured intervals have expelled total C 15+-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% R m) to mature (0.68 and 0.88% R m) and overmature (1.45% R m) stages is shown. For example, in the maturity interval from 0.68 to 0.88% R m 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.

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

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.

Geochemical Effects of Primary Migration of Petroleum and Their Relevance with Respect to Mechanisms and Efficiencies of Expulsion: ABSTRACT

Geochemical analysis of the petroleum residing in the pore system of source rocks after primary migration has occurred holds the key to better understanding the mechanisms and efficiencies of expulsion processes. Indeed, the detailed recognition of migration phenomena and quantification of their effects have been made by a combined approach which (1) examines the composition variation of residual oil within individual bodies of source rocks and its relationship to, inter alia, petroleum in juxtaposed reservoir rocks and (2) makes a material balance of petroleum-related chemical species. This approach has been applied in three separate case studies, two of which address the economically most important source rock units of central and northwest Europe: the Lower Jurassic Toarcian shales from the Hils syncline area (Federal Republic of Germany) and the Upper Jurassic Kimmeridge Clay Formation from the Brae field area, North Sea (U.K. Sector). The third study is of a series of coals and interbedded sediments of Late Carboniferous age from the Ruhr area (Federal Republic of Germany). A synthesis of results has revealed some fundamental differences in migration mechanisms and efficiencies which are strongly linked to organic richness, kerogen quality, and source rock fabrics. These differences concern especially the degree ofmore » microfracturing and the role of gas as transport agents.« less