Niels H. Schovsbo

Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish

The evolution of Earth’s biota is intimately linked to the oxygenation of the oceans and atmosphere. We use the isotopic composition and concentration of molybdenum (Mo) in sedimentary rocks to explore this relationship. Our results indicate two episodes of global ocean oxygenation. The first coincides with the emergence of the Ediacaran fauna, including large, motile bilaterian animals, ca. 550–560 million year ago (Ma), reinforcing previous geochemical indications that Earth surface oxygenation facilitated this radiation. The second, perhaps larger, oxygenation took place around 400 Ma, well after the initial rise of animals and, therefore, suggesting that early metazoans evolved in a relatively low oxygen environment. This later oxygenation correlates with the diversification of vascular plants, which likely contributed to increased oxygenation through the enhanced burial of organic carbon in sediments. It also correlates with a pronounced radiation of large predatory fish, animals with high oxygen demand. We thereby couple the redox history of the atmosphere and oceans to major events in animal evolution.

From shale oil to biogenic shale gas: Retracing organic–inorganic interactions in the Alum Shale (Furongian–Lower Ordovician) in southern Sweden

Methane-rich gas occurs in the total organic carbon–rich Alum Shale (Furongian to Lower Ordovician) in southern Sweden. The lower part of the thermally immature Alum Shale was impregnated by bitumen locally generated by heating from magmatic intrusions from the Carboniferous to the Permian. Organic geochemical data indicate that the migrated bitumen is slightly degraded. In the upper Alum Shale, where methane is the main hydrocarbon in thermovaporization experiments, centimeter-size calcite crystals occur that contain fluid inclusions filled with oil, gas, or water. The Alum Shale is thus considered a mixed shale oil–biogenic shale gas play. The presented working hypothesis to explain the biogenic methane occurrence considers that water-soluble bitumen components of the Alum Shale were converted to methane. A hydrogeochemical modeling approach allows the quantitative retracing of inorganic reactions triggered by oil degradation. The modeling results reproduce the present-day gas and mineralogical composition. The conceptual model applied to explain the methane occurrence in the Alum Shale in southern Sweden resembles the formation of biogenic methane in the Antrim Shale (Michigan Basin, United States). In both models, melting water after the Pleistocene glaciation and modern meteoric water may have diluted the contents of total dissolved solids (TDS) in basinal brines. Such pore waters with low TDS contents create a subsurface aqueous environment favorable for microbes that have the potential to form biogenic methane. Today, biogenic methane production rates, with shale as the substrate using different hydrocarbon-degrading microbial enrichment cultures in incubation experiments, range from 10 to 620 nmol per gram and per day.

Oil–source-rock correlation of the Lower Paleozoic petroleum system in the Baltic Basin (northern Europe)

The correlation of lower Paleozoic marine source rocks with reservoired oils by biomarkers is complex because of the uniform early Phanerozoic biomass (bacteria and algae) and the lack of land plant and animal input. Accordingly, the main source rocks for the most prolific oil province in the Baltic Basin are still a matter of debate. The 10 source rocks and 15 oil samples from 5 northern European countries bordering the Baltic Sea Basin were analyzed by gas chromatography (GC) with flame ionization detector, GC–mass spectrometry (GCMS), and GCMS/MS to detect acyclic isoprenoids and aliphatic, aromatic, and nitrogen, sulfur, and oxygen biomarkers. Chemometric tools were applied to screen for meaningful source- and age-related biomarkers and to highlight genetics. Extended tricyclic terpane ratios, C24 tetracyclic terpane/C26 tricyclic terpane ratios, and relative C29 sterane concentrations are considered the most promising biomarkers in differentiating Llandovery shales from Cambrian to Ordovician Alum Shale and for correlation with expelled oil. The uranium irradiation–related C26–C28 triaromatic-steroid concentrations provide possible distinguishing criteria for the source potential of the different Alum Shale units. Enhanced oil maturation by volcanic intrusion is highlighted by sterane biomarkers and polycyclic aromatic hydrocarbons. The Alum Shale is here considered the main source rock for oil accumulations in lower Paleozoic reservoirs of the Baltic Basin. Oil seepage occurring in Ordovician limestone was mainly generated by the Middle Cambrian Alum Shale, and Middle Cambrian sandstone reservoirs were mainly sourced by Upper Cambrian and Lower Ordovician Alum Shale with higher maturity. Considerations about the assessment of migration distance are based on carbazole concentrations and C29 sterane isomerization. Advanced studies to unravel detailed lower Paleozoic oil–source-rock correlations are based on meaningful biomarkers, offer approaches to significantly reduce the exploration risk in this area, and could be applied to similar early Paleozoic petroleum systems in other basins.

Organic-inorganic Interactions During Reactive Fluid Flow in Carbonates

The talk will present results about organic-inorganic interactions leading to porosity formation due to migration of source-rock-derived corrosive fluids through carbonate reservoirs. Two aspects will be covered: (i) detectable processes at the nanometer scale by imaging techniques, and (ii) generic hydrogeochemical modelling results of porosity creation at a play scale (from source rock to reservoir).

Elasticity and Density of Paleozoic Shales from Bornholm

Elasticity and Density of Paleozoic Shales from Bornholm The Paleozoic shales exposed on the island of Bornholm contain intervals with more than 10% organic matter which is post mature with respect to the formation of hydrocarbons. Four shallow bore holes have been drilled through the 250 m thick shale interval and partly cored and logged with geophysical probes. We studied how the organic content and mineralogical composition influence mass density and elastic properties as measured from core analysis and logging data. The shales have porosity in the range of 1%-10% and modelled permeability (from porosity and BET) of generally less than 0.1 μDarcy. We found that solid density and elastic stiffness parameters only vary insignificantly with solid composition, when TOC is lower than 5%, but that mass density and TOC are correlated when TOC is higher than 5%. A similar correlation was not seen for TOC and Sonic logging data. A reason can be that the content of low-density kerogen and high-density pyrite is correlated. According to our data, the shales have weak to medium degree anisotropy of stiffness and compare well with other studied shales.