H. Wehner

Petrography, inorganic and organic geochemistry of Lower Permian carbonaceous fan sequences (“Brandschiefer Series”) — Federal Republic of Germany: Constraints to their paleogeography and assessment of their source rock potential

Abstract Lower Permian clastic and volcaniclastic sediments from the western edge of the Bohemian Massif (northern Bavaria, F.R.G.) were studied petrographically and chemically to decipher their complex history of deposition, their origin of elements (C specifically) and to assess their economic potential (hydrocarbons, Ba, U). The deposit is an alluvial fan sequence, among which pyroclastic sediments are intercalated. Chemical discrimination plots allow these acid to intermediate tuffs to be assigned to the calc-alkaline clan. These volcanites and the bimodal clastic sediments plead for the assumption of an active continental margin. Zr, Ba, Th, Ce and P are delivered by volcanic activity into these basins, while the organic matter originated from plant debris. Postsedimentary redeposition led to the removal of elements from the basinal fine-grained sediments and trapping of Ba, Na and hydrocarbons into the more porous mid-fan sections. The organic matter plays a minor role in U fixation, since it had already lost its sorption capacity at the time of basin evolution. S, C and Fe plots as well as the use of the DOP (=degree of pyritization) led to the conclusion that this pyrite in the more central parts has been syngenetically precipitated under iron-limited conditions. This environment may be considered as an area of U preconcentration and Ba mineralization. Moreover, it shows a good sourcerock potential, mainly for gas.

Thermogenic hydrocarbons from the offshore Calypso hydrothermal field, Bay of Plenty, New Zealand

Hydrothermal gases from shallow seafloor vents in the Bay of Plenty, New Zealand contain CO2, CH4, and the higher gaseous hydrocarbons up to i-, n-C4H10. The gases are similar to those discharged at fumaroles on the nearby White Island. Carbon isotope compositions for CO2 fall between −3.4‰ and −5.5‰ PDB and reflect a shallow magmatic carbon source. The δ13C values of CH4 range from −24.6‰ to −28.9‰ PDB and the δD values vary between −122‰ and −135‰ SMOW. The CH4 isotope values and the presence of the higher hydrocarbon compounds such as C2H6 and C3H8 with δ13C values near −20‰ PDB suggest hydrocarbon production by high-temperature maturation of sedimentary organic matter and mixing (∼1:1) of the thermogenic CH4 with abiogenic CH4. Long-chained hydrocarbons occur in dredged samples close to the active vents. Their n-alkane distribution has a high to moderate odd–even predominance and an extensive hopane series, indicative of higher land-plant waxes and prokaryotic membranes in the source. Substantial amounts of unsubstituted polynuclear aromatic hydrocarbons (PAH) mark the transition from aliphatic- to aromatic-dominated bitumens, consistent with extensive source maturation resulting from thermal stress. The bitumens are interpreted as pyrolysates derived from buried near-coastal vegetation and terrestrial detritus under various thermal regimes, mixed with immature seafloor organic matter.

Biodegradation of Crude Oils

Petroleum from well sites in the Gifhorn Trough (Lower Saxony, NW-Germany) and the Maracaibo Basin (Venezuela) contained various types of microorganisms capable of degrading crude oils. Genetically related oils were inoculated with the isolated microorganisms and the degradation of the oils was followed by chromatographic techniques. Parameters important for the reactions (pH, supply of oxygen, nitrogen and phosphorus, reaction medium) were monitored and optimized. The degradation of n-alkanes was followed closely. Microorganisms active in degradation (yeast, bacteria) easily survived a period of inactivity due to missing nutrients and were reactivated within hours to degrade newly added crude oil. Under substrate-limiting conditions selectivity of degradation was found, destroying medium-chain n-alkanes (C20, C21) at a faster rate than long-chain n-alkanes (C30, C31). During degradation the physical parameters of the crude oils (e.g. density, viscosity, average molecular weight) were altered and shifted into the direction of heavy oil. In vitro degraded oil is very similar to oil degraded in nature. Aromatic hydrocarbons and biomarker molecules (steranes and triterpanes) were not degraded under the conditions used. Pyrolysis-GC analysis of asphaltenes revealed no significant changes in the composition of pyrolyzates during biodegradation. There is sufficient evidence that heavy oils - besides some other effects - are generated by the in situ-biodegradation of conventional oils.

Chemical reactions and stability of biomarkers and stable isotope ratios during in vitro biodegradation of petroleum

Abstract Petroleum from the Gifhorn Trough (Lower Saxony, F.R.G.) and the Maracaibo Basin (Venezuela) were analyzed for microorganisms capable of biodegradation. Genetically related oils were inoculated with the isolated microorganisms and the degradation of the oils was followed by chromatographic techniques. Parameters important for the reactions (pH, oxygen concentration, nitrogen, and phosphorus, reaction medium) were monitored and optimized. The degradation of n-alkanes was followed closely. It was shown that the microorganisms responsible for degradation (yeasts and bacteria) could easily survive a period of inactivity due to missing nutrients and could be reactivated within hours to degrade newly added petroleum. Under conditions in which oxygen or nitrogen was limited, selectivity was observed: medium-chain n-alkanes (C18, C20) were degraded faster than long-chain (C30) or short-chain (C14) n-alkanes. Branched-chain alkanes were also degraded. The physical parameters of the oils (e.g. density, viscosity, average molecular weight) were altered as expected during degradation. In vitro degraded oil is very similar to oil degraded in nature. It was found that biomarker molecules are resistant to degradation under the conditions used. Pyrolysis-GC analysis of asphaltenes revealed no significant changes in the composition of pyrolyzates during biodegradation. A small shift in the carbon isotope ratios in the aromatic hydrocarbon fraction was observed between biodegraded and unaltered samples.

Evaluation of the effects of native minerals on the organic matter of Aleksinac oil shale based on the composition of free and bound bitumens

Abstract In order to evaluate the adsorption and catalytic effects of indigenous carbonates and silicates on the organic matter of Aleksinac oil shale, the free and the bound bitumens were isolated and analyzed by organic geochemical methods. The differences in the gross compositions indicated an adsorption effect of both the carbonates and silicates in the examined oil shale. The maturation parameters, based on n-alkanes and isoprenoid alkanes pristane and phytane, indicated that bitumen 3 was of the highest, and bitumen 1 of the lowest, apparent maturity. The silicates were shown to have had a thermocatalytic effect on the organic matter of this sediment. Clay minerals had a catalytic effect primarily on sterane aromatization, rearrangement of steranes into diasteranes and their (20R) → (20S) isomerization. All evidence obtained by analysis of steranes and aromatic steroids suggested that the differences in bitumen compositions should be explained by the effect of clay minerals rather than by variations in the origin of the precursor biomass.

Investigation of the catalytic effects of indigenous minerals in the pyrolysis of Aleksinac oil shale substrates: Steranes, triterpanes and triaromatic steroids in the pyrolysates

Abstract Evidence for catalytic effects of indigenous minerals in the pyrolysis of Aleksinac oil shale kerogen was found by investigating the liquid products obtained in the Micro Activity Test (MAT) pyrolysis of raw shale and its demineralized substrates. By comparing the corresponding fragmentograms of a demineralized sample with those of a substrate containing only silicate minerals, differences in the side-chain isomerisation of the steranes and triterpanes, and in the side-chain cleavage of the triaromatic steroids were observed. These findings corroborated the previous conclusion, based on MAT parameters and TG weight losses, that the catalytic effect of the indigenous mineral components during the pyrolysis of Aleksinac oil shale kerogen is very low.

Geochemistry and lithofacies of Permo-Carboniferous carbonaceous rocks from the southwestern edge of the Bohemian Massif (Germany). A contribution to facies analysis of continental anoxic environments

Abstract The Permo-Carboniferous Schmidgaden, Weiden, Erbendorf and Stockheim basins from southern Germany host discontinuous seams of high ash coal and carbargillites which were mined for hard coal, explored for uranium and only recently have proved their source rock potential for gaseous and fluid hydrocarbons. Alteration and facies of these carbonaceous beds were geologically and chemically investigated. During silicification which is exclusively confined to the carbonaceous interbeds of pyroclastic depositions, element depletion prograded except for Zn, Pb, U. The anoxic environments under consideration may be subdivided into fluvial/swamp (Upper Westphalian beds from Schmidgaden, Weiden, Erbendorf and Stockheim) and lacustrine depositions (Upper Stephanian and Upper Autunian carbargillites from Erbendorf, Weiden-Bechtsrieth). SiO 2 , MgO, CaO, Mo and Zr have proved to be most suitable for recognition of these environments, whereas, U, K 2 O and notably Cu/Zn are less appropriate. In the ternary plots displaying the organic chemistry of extracts of these carbonaceous beds fluvial swamps have low and lacustrine beds high contents of saturated hydrocarbons. All samples show a pronounced OEP (=odd even predominance) of n -alkanes which partly are environment-controlled (terrestrial origin), partly maturity-controlled. The high iso- and cyclo-alkanes of Schmidgaden and bimodal n -alkyl distribution point to a contribution of algal material. Finally four basin types bearing carbonaceous rocks are discussed with respect to their economic potential and compared with basins elsewhere (Cerilly, Lodeve, St. Hippolyte, Uinta): type I: intramontane fault-bounded basin (swamp), type II: volcanic-depression with steep relief (swamps), type III: halfgraben (lake), type IV: volcanic-depression with smooth relief (lake). Types I and II are of interest for hard coal, organic-hosted U-deposits and mainly gasprone; types III and IV, however, show good source-rock potential for crude oil.

Organic geochemistry and organic petrology of organic rich sediments within the “Hauptdolomit” formation (Triassic, Norian) of the Northern Calcareous Alps

Organic rich intercalations (TOC up to 50%) occur within the “Hauptdolomit”. The principal components of the organic matter are lamellar alginite, prebitumen or solid bitumen (epi-impsonite). Organic geochemistry shows the OM to be marine and to have a high bacterial contribution. A mesohaline environment, low terrigenous supply, high production of HC-rich OM, high microbial activity and anoxic conditions are seen as factors favouring the formation of these rocks. Pyrolysis gas chromatography is used to characterize the OM based on the abundance of alkene/alkane doublets, alkyl-thiophenes, alkyl-benzenes and/or C15+-alkanes. Biomarker data shows that the samples cover a range from immature to overmature. Restricted lagoons and special basins within the HD carbonate platform are suggested as the two different paleoenvironmental settings for the deposition of these rocks.

Correlation studies on crude oils and source-rocks in the German Molasse Basin

Abstract Three types of oils can be distinguished in the German Molasse Basin according to their distributions of alkanes and aromatic hydrocarbons. GC-MS investigations, which till now have been restricted mainly to the analysis of steranes, have revealed the presence of only 2 oil families for which 2 different source-rocks must be assumed. We have tentatively concluded that the Tertiary oils have originated from Jurassic source-rocks of the “Germanic” facies, whereas the Triassic oils show some resemblance to Jurassic rocks of the “Helvetian” facies. The Tertiary foreland sediments have not yet been heated sufficiently to have generated oil.

The origin of sulfide mineralization in arenaceous rocks beneath carbonaceous horizons in fluvial depositions of late Paleozoic through Cenozoic age (SE Germany)

Abstract Sulfide mineralization in clastic rocks from SE Germany were mineralogically and chemically investigated in relation to host-rock lithology and their environment of deposition. The S isotopes and trace-element contents of pyrite were determined to decipher the origin of sulfur concentration in arenaceous rocks beneath carbonaceous horizons. Two principal types of CS distribution may be defined in these fluvial depositions. Type I may be encountered in meandering stream depositions and is of syn(dia)genetic origin. Type II was found in sediments laid down by braided streams and is likely to have been overprinted by an epigenetic pyritization. The sulfur of sulfides probably derived from evaporites. There is no evidence for these swamp environments to have been flooded by marine ingressions. The organic matter originated from higher-plant material and to a varying amount from algae and bacteria. Sedimentation did not occur in a strongly reducing environment. Biogenic reduction controlled by the humic matter preferentially removed 32 S from the sulfate of intrastratal solutions and conduced to negative δ 34 S-values at the top and increasingly more positive values with depth. Sulfate reduction commenced in an open system and later converted into a more or less closed system. Concluding from the data of organic chemistry the organic matter of type-I sediments may be categorized as “gas prone”. However, the clastic rocks of type II show small-scale hydrocarbon migration along with sulfidization. Their C isotopes attest to a slight change in the composition of kerogenous matter, owing to the shift of δ 13 C-values plotted as a function of host-rock stratigraphy.