B. Horsfield

The conversion of oil into gas in petroleum reservoirs. Part 1: Comparative kinetic investigation of gas generation from crude oils of lacustrine, marine and fluviodeltaic origin by programmed-temperature closed-system pyrolysis

The thermal alteration of reservoired petroleum upon burial was simulated comparatively by closed-system programmed-temperature pyrolysis of produced crude oils of lacustrine, fluviodeltaic, marine clastic and marine carbonate origin using the microscale sealed vessel (MSSV) technique. Bulk kinetics of oil-to-gas cracking and accompanying compositional changes were studied at heating rates of 0.1, 0.7 and 5.0 K/min. The oil type related variations of experimental cracking temperatures were small compared to those related to heating rate, but the high-temperature shift of gas evolution curves with increasing rate of heating turned out to be more pronounced for the marine than for the non-marine oils. Accordingly the kinetic frequency factors were derived to be higher for gas generation from the lacustrine and fluviodeltaic oils (A ≈ 4·1019 min−1) than from the marine oils (A ≈ 2·1018 min−1) and the gas potential vs. activation energy distributions were calculated to be centered around 71–72 kcal/mol for the former and around 67 kcal/mol for the latter. These kinetic parameters and compositional observations give some evidence that gas generation is accompanied by the formation of aromatic compounds in the case of the marine oils whereas alkene intermediates seem to be involved in the case of the non-marine high was oils. Under geological heating conditions (e.g. 5 K/My), the onset of gas generation and peak gas generation are extrapolated to occur at about 180°C and 225°C for the high wax oils. The marine oils turn out to be slightly less stable with peak gas generation at 215°C and the onset of decomposition reactions predicted at about 170°C. In the absence of reservoir bitumen and minerals severe oil-to-gas cracking is very unlikely to take place at temperatures less than 160°C, whatever the crude oil type or the geological heating rate.

Severity and timing of Cenozoic exhumation in the southwestern Barents Sea

The juxtaposition of the North Atlantic rift system with the borderlands of the Eurasian Arctic shelf has resulted in an unusual basin evolution in the southwestern Barents Sea. Exploration has concentrated on the Mesozoic sandstone plays of the Hammerfest Basin. These have yielded abundant gas and very little oil. Such findings are typical of peripheral North Atlantic margin basins that have undergone Cenozoic exhumation. This is generally thought to have occurred during the Palaeocene, Oligocene–Miocene or Pleistocene. Hammerfest Basin erosion is widely estimated to be in the range of 500–1500 m. However, the relative severity of individual episodes and impact on fluid dynamics within the basin are poorly constrained. We apply basin modelling tools to establish the sensitivity of the petroleum system to three cited scenarios for Cenozoic exhumation. Vitrinite reflectance and temperature data constrain burial and thermal histories. Calibration is not possible using heat flow variations alone; erosion timing and magnitude are required to match available data. The calibrated model indicates a significant thermal disequilibrium in the Hammerfest Basin at the present day resulting from Late Cenozoic exhumation. Forward modelling of Pleistocene ice sheets allows for an estimation of pressure and temperature fluctuations in response to glacial–interglacial cycles.

Prokaryotic Populations and Activities in an Interbedded Coal Deposit, Including a Previously Deeply Buried Section (1.6–2.3 km) Above ∼ 150 Ma Basement Rock

A largely terrestrial, lignite/coal-bearing, 148 m core from the Waikato Basin, New Zealand, was studied, with a multidisciplinary approach, for subsurface microbiology. The top ≈76 m was Latest Miocene-Late Pleistocene (≈0.4–5.5 Ma) sediments, which overlay an unconformity and a previously deeply buried (1,600–2,300 m, × 55–75°C) ≈69 m section of moderately indurated, Late Eocene-Early Oligocene (≈32–35 Ma) deposits. Below this is weathered, Late Jurassic metasedimentary basement rock (145.5–157.0 Ma). Similar cell numbers (mean 1.2 × 106 cm −3 ), high viability (4–32%), intact phospholipids (biomarkers for living Bacteria) and activity (sulphate reduction, DNA replication) occurred heterogeneously throughout the core, including the weathered basement rock. Substantial numbers of viable anaerobic heterotrophic and lignite-utilizing bacteria (means 3.4 × 10 4 , 3.0 × 10 3 cm −3 ) were present throughout the core. This is similar to some deep terrestrial formations but contrasts with the generally exponentially decreasing prokaryotic populations in sub-seafloor sediments. For Bacteria, ≈76% of the 16S rRNA gene phylotypes were similar above (31.98 m) and below (133.55 m) the 76 m unconformity, which together with similar cell numbers indicates limited deep burial impact/palaeosterilization, or effective re-colonisation. Archaeal populations were not dominant being only detected with general primers at 31.98 m and those detected with methanogen functional primers were different above and below the ≈76 m unconformity. Both dominant bacterial (Proteobacteria, Actinobacteria, Firmicutes, Chloroflexi) and archaeal (Miscellaneous Crenarchaeotic Group, Methanosarcinales and Methanobacteriales) sequences were similar to those previously detected in both marine and terrestrial subsurface environments, reflecting the changing depositional conditions of the formation. However, the presence of ANME sequences had not been previously found in the terrestrial subsurface. A large proportion of the bacterial 16S rDNA diversity was cultured (43% of commonest genera). Prokaryotic populations and activity changed with lithology and depth and substrates (formate, acetate, oxalate) may diffuse from high-carbon, lignite/coaly layers to support bacterial populations in adjacent sandy or clay-silt layers.

Influence of maturity on distributions of benzo- and dibenzothiophenes in Tithonian source rocks and crude oils, Sonda de Campeche, Mexico

Abstract The Sureste Basin is the most prolific oil-producing area of Mexico, extending from the mainland into the Sonda de Campeche offshore region. The Tithonian sedimentary sequence is the most important source of hydrocarbons. It contains sulphur-rich kerogen which progressively cracks to form petroleum beginning at 0.53% Rr. Maturity increases from northeast to southwest across the region. Sulphur-compounds in the Tithonian source rocks and in the petroleums they have generated have been studied by gas chromatography. Alkylbenzothiophenes are present in highest abundance at low maturity whereas the alkyldibenzothiophenes are most abundant at higher stages of maturity. The relative abundance of specific C3-benzothiophene isomers are especially sensitive to changes in maturity of sulphur-rich Type-II organic matter. A novel maturity parameter, the C3-benzothiophene index (C3BTI), shows a very good correlation to vitrinite reflectance values (R2≅0.96, n=10). The methyldibenzothiophene ratio MDR′ also has a wide dynamic range. Application of C3BTI and MDR′ to the study area strongly supports a model involving localised vertical migration avenues, though several possible scenarios may explain the filling history of reservoirs in the southwest part of the Sonda de Campeche.

Predicting the generation of heavy oils in carbonate/evaporitic environments using pyrolysis methods

Abstract A series of low maturity kerogen and asphaltene samples of a sulphur-rich Mediterranean carbonate source rock sequence, as well as two associated oil samples were analysed by different pyrolysis methods. The source rock was characterised by the occurrence of four different lithotypes, three of which possessed source potential. Elemental analysis revealed that each source lithofacies contained a distinct kerogen type comprising Type I-S for shale intercalations, Type II-S in the laminated lithofacies and low sulphur Type II in the pure carbonate portions of the source rock. Pyrolysis-gas chromatography using coupled FID and HECD detectors provided a more detailed insight into kerogen/asphaltene composition than afforded by Rock-Eval pyrolysis. Bulk kinetic analysis demonstrated that asphaltenes and Type I-S kerogens were the most labile species present in the source rocks. Type II-S kerogens, often quoted as the major source of heavy oils, were the most stable of the sulphur-rich organic matter types. The occurrence of various kerogen types with differing stabilities in the carbonate source rock analysed, as determined by bulk pyrolysis-FID, implies that peak hydrocarbon generation in the source rock sequence is not confined to a single temperature, but rather to a relatively large temperature range of roughly 20°C. Combination of bulk hydrocarbon generation kinetics and a multistep pyrolysis approach demonstrated that for carbonate evaporitic environments it is necessary to differentiate between early and late maturity products. This is because the lower energies (46–49 Kcal/mol), which may be attributed to SC bond cleavage reactions, are empirically of greater importance, especially in view of the fact that they may represent the main generation stage under natural conditions.

Prokaryotes stimulate mineral H2 formation for the deep biosphere and subsequent thermogenic activity

The deep subseafloor biosphere contains two-thirds of Earths prokaryotic biomass this may indicate the presence of novel mechanisms of energy generation as temperatures increase in the subsurface. In sediment slurry experiments (0–100 °C) with a range of common minerals and rocks (including basalt and quartz), there is significant H2 formation at elevated temperatures, but only in the presence of prokaryotes. This stimulates further prokaryotic activity, typical of deep sediments (sulfate reduction, acetogenesis, and CO2 production, plus continuing methanogenesis), and Bacteria and Archaea representative of many deep sediment types develop. H2 and acetate formation is particularly stimulated above 70 °C. This prokaryotic activity even enhances reactions when temperatures are raised to thermogenic levels (∼125–155 °C), including hydrocarbon generation. Mechanochemistry may be important for mineral H2 formation; this is enhanced by prokaryotes (biomechanochemistry), and subsurface stress and fracturing, which is widespread on Earth.

Techniques for Contamination Assessment During Drilling for Terrestrial Subsurface Sediments

Details about the procedures for drilling a ca. 150 m long drill core in a terrestrial setting under contamination controlled conditions are presented. Different to previous studies we only used commercially available drilling equipment to reduce the cost of operation significantly. The goals were (1) to minimize, (2) to monitor and, if possible, to quantify the contamination of the recovered sediments, and (3) to identify the different sources of contamination. Both the potential contamination of the sample material by surface microorganisms and non-indigenous material was assessed. To estimate the infiltration of drill mud into the core, fluorescent microspheres, having about half the size as microorganisms, were added to the mud. The drilling technique used was mud rotary drilling. With the exception of the very beginning of the drilling operations, the drill mud was devoid of any allochthonous hydrocarbons potentially derived from the drilling equipment or drill additives, and its biomarker composition reflected the varying organo-facies that were penetrated. Due to the lack of allochthonous hydrocarbons in the drill mud, its infiltration into the sediment cannot be traced by organic geochemical biomarker analysis. Microspheres proved to be a sensitive tool for the assessment of infiltration of drill mud into the core. The concentration of microspheres in the drill mud decreased continuously during the drilling, most probably caused by seepage of mud through leaks and attachment of spheres to the surface scum in the mud pit. Microscopic enumeration of the microspheres showed great variability in the depth of penetration of mud into the core, apparently unaffected of lithology. The sampling of the core material in the laboratory was carried out inside an anaerobic chamber. Several techniques for subsampling were used, according to sediment properties. The overall results indicate that, if strict contamination control protocols are employed, it is possible to recover uncontaminated samples at reasonable cost with commercially available drilling equipment.

Bulk composition and phase behaviour of petroleum sourced by the Bakken Formation of the Williston Basin

Abstract The Bakken Formation is currently regarded primarily as a self-contained, unconventional petroleum system. While previously viewed as a source for oil occurring in overlying formations, it is now predicted that resources of more than 3.5 billion barrels of oil are trapped intraformationally. New insights into the formation9s open v. closed nature are presented here using the physical properties of natural petroleum, source rock characteristics and the numerical modelling of phase behaviour. In the mature western part of the basin petroleum accumulations have been postulated to be continuous in nature, characterized by very short migration distances of indigenous hydrocarbons. This necessitates that the composition and therefore physical properties of the generated hydrocarbons must be controlled by the maturity of the source rock in the immediate vicinity. This assumption is not supported by the clustering of higher gas–oil ratios and lighter oil gravities along the locations of the anticlines in the basin. We have used open and closed system pyrolysis techniques to predict the bulk composition of the petroleum generated at different transformation stages, both cumulatively and instantaneously. Based on these predictions the Bakken would contain dominantly undersaturated fluids throughout the basin. Differences in predicted GORs of cumulative and instantaneous models support the conclusion that the reported hydrocarbon compositions cannot completely be explained by a tight self-contained petroleum system. The observed variability of in-place hydrocarbon compositions is readily explained by lateral migration of petroleum in the main middle Bakken carrier, and vertical leakage of emplaced hydrocarbons from the fractured reservoir at anticline locations. This has resulted in the loss of the early generated petroleum, and led to a present-day dominance of late generation products. These results reveal that the Bakken Formation is a partly open petroleum system, at least along the major anticlines of the Williston Basin.

Hydrocarbon Generating Potentials of Benue Trough Coals

Benue trough is an inland coal bearing basin that has been only lightly explored for petroleum. An organic geochemical analysis of coal samples from Kumo and Lamza in the upper part of the trough and from Lafia-Obi in the lower part was undertaken to assess their hydrocarbon generative potentials. The highest %Ro and Tmax of 0.9–1.1 % and 449–455 °C was found in Lafia-Obi samples, next was Lamza with 0.5–0.7 % and 434–438 °C and Kumo with 0.4–0.6 % and 415–429 °C. The HI of Lafia-Obi, Lamza and Kumo are within the ranges of 136–246 mg HC/g TOC, 184–269 mg HC/g TOC and 167–322 mg HC/g TOC respectively. The Odd-over-Even-Predominance (OEP) values and pristane/phytane (Pr/Ph) ratios of the samples suggest that they are dominantly terrestrial in nature and were deposited in an oxic to suboxic environment. Ratios of 20S/(20S + 20R)ααC29 cholestanes, 20S/(20S + 20R)ββC29 cholestanes, βα/(βα + αβ) C30 hopanes, Ts/Ts + Tm C27 hopanes and other maturity parameters suggest that Lafia-Obi is highest in maturity (peak oil window), followed by Lamza (early oil window) and then Kumo (immature).

Shale Gas – eine neue Energiequelle für Europa? Die GFZ-Forscher Brian Horsfield und Hans-Martin Schulz im Interview

Brian Horsfield und Hans-Martin Schulz sind Mitarbeiter des Deutschen GeoforschungsZentrums in Potsdam (GFZ). Gemeinsam haben sie das Projekt GASH (Gas Shales in Europe) ins Leben gerufen und koordinieren auch alle Aktivitaten im Rahmen des neuen Forschungsprogramms. In einem Interview nehmen die beiden Wissenschaftler ausfuhrlich Stellung zu der ungewohnlichen Erdgasquelle aus Schiefer.