Tanja Barth

Deep marine biosphere fuelled by increasing organic matter availability during burial and heating

Deep-sea sediments become apparently more hostile to life with increasing depth as temperature and pressure rise, and organic matter becomes increasingly recalcitrant. Demonstrations of high bacterial populations in deep sediments, may thus appear enigmatic. How, then, can the continued presence of active bacterial populations in deep sediments that are over 10 million years old be explained? Although volatile fatty acids, particularly acetate, are important intermediates in the anaerobic degradation of organic matter,, their concentrations are kept very low in sediments (<15 µM) by rapid bacterial consumption,. Here we show that heating surface coastal marine sediments to simulate increasing temperature during burial produces an increase of over three orders of magnitude in acetate concentration and increases bacterial activity. We found that pore-water acetate concentration at two sites in the Atlantic Ocean increased at depths below about 150 m and was associated with a significant stimulation in bacterial activity. Comparing these acetate concentrations to in situ temperatures confirmed that there was a notable generation of acetate associated with temperature increases during burial. This was supported by heating experiments with deep sediments. Thus, acetate generation from organic matter during burial may explain the presence of a deep bacterial biosphere in marine sediments, and could underpin an even deeper and hotter biosphere than has previously been considered.

Organic acids and inorganic ions in waters from petroleum reservoirs, Norwegian continental shelf: a multivariate statistical analysis and comparison with American reservoir formation waters

Abstract A data base consisting of organic acid anion composition, reservoir depth, temperature and pressure, and inorganic ion composition of 22 samples of produced formation waters from different wells on the Norwegian continental shelf is presented. Correlation between the different properties of the samples was explored by multivariate statistical analysis, with special emphasis on the organic acid concentrations. Organic acid concentrations, pH and alkalinity are positively correlated with each other and in general negatively correlated with the inorganic ion concentrations. No significant correlation was found between organic acid levels and reservoir temperature. Alkalinity values can be predicted from the remaining ionic composition of the samples with reasonable accuracy, while surface-measured pH cannot be accurately predicted. A comparative statistical analysis was performed on an American formation water data set. These waters separate into statistically different classes based on reservoir types. Strong similarities are found for the oil reservoir waters in the two data sets.

Generation of organic compounds by hydrous pyrolysis of Kimmeridge oil shale—Bulk results and activation energy calculations

Abstract Hydrous pyrolysis of immature Kimmeridge oil shale has been performed at temperature ranging from 200 to 350°C. The produced gases, bitumen and aqueous organic acids have been quantified. Postulating a unimolecular reaction type that obeys first-order kinetics, the approximate apparent activation energies for the different reactions have been calculated using the Arrhenius relationship. Plots of the logarithm of inverse time versus the inverse temperature show a linear relationship with correlation coefficients higher than 0.970. The implications of the results are discussed with regard to reaction pathways suggested in literature, especially for reactions involving organic acids.

The effect of crude oil acid fractions on wettability as studied by interfacial tension and contact angles

Abstract Naphthenic acids have been extracted from three North Sea crude oils with different acid numbers. The acid fractions were diluted with toluene to their original crude oil concentrations, and also further diluted and compared at equal concentrations (milligram acid/gram oil). The toluene phases were then equilibrated with an aqueous phase, and the interfacial tension (IFT) and contact angles were measured as a function of equilibrium pH and acid concentrations. The aqueous phase used was 0.5 M NaCl(aq), and the contact angle experiments were performed on silicate glass cover slips. The results show a correspondence between the decrease in interfacial tension as a function of pH and the acid number of the original crude oils. Generally, low contact angles were observed at high and low pH values, whereas angles up to 47° were observed in the intermediate pH range. The contact angles did not show a direct correspondence with the acid numbers but gave the same trend as the corresponding whole crude oils (presented in a previous work). Observations from both interfacial tension and contact angle experiments emphasize the importance of acid structures, or acid types, present in the fractions compared to the acid concentrations. Acid characterization by gas chromatography with a mass sensitive detector (GC-MS) showed significant differences in the molecular structures for the acids dominating each of the three fractions. Fractions enriched in alkyl acids, phenols and cyclic acids were found to have impact on the wetting properties of the silicate surface, whereas fractions enriched in more complicated aromatic ring structures with high degree of carboxylic compounds did not affect the contact angle. FT-IR spectra of the acid extracts confirmed the results from the GC-MS analysis and showed correspondence to the acid numbers of the three crude oils.

Interactions between organic acids anions in formation waters and reservoir mineral phases

Abstract Formation waters from the Norwegian continental shelf have been analysed for short-chain organic acid anions. Acetic acid is the major component, and the observed levels are in the range 0.3–20 mM in uncontaminated formation water samples. Studies of individual reservoirs and formations over time and area show a quite uniform distribution of the acids within each unit for large volumes of fluid. The organic acid anion levels are generally low in reservoirs containing biodegraded oil. Statistical analysis has been used to find correlations between the organic and inorganic ion compositions and the reservoir conditions and shows a dominating negative correlation between the organic acid anion levels and the levels of inorganic ions, indicating that the main effect of the organic ions is to increase the stability of mineral phases. Laboratory experiments have also been performed and show similar effects for sandstone samples. Under oxidizing conditions the buffering capacity of the organic acid species is also important, as it prevents the development of extreme pH conditions caused by pyrite oxidation. In carbonate systems few systematic effects were observed. The results, both for the natural systems and the laboratory experiments, show that the presence of organic acids in the formation waters does not contribute to the development of secondary porosity in the reservoir rock. The observed relationships rather indicate decreasing concentration levels of inorganic ions in the formation waters with increasing levels of organic anions and thus an increase in mineral stability.

Organic acids from source rock maturation: generation potentials, transport mechanisms and relevance for mineral diagenesis

Abstract Hydrous pyrolysis has been used to simulate maturation of source rocks with respect to the generation of organic acids and CO2. The results from pyrolysis experiments on eight different source rocks at this laboratory are given and compared with published data. The investigation show levels of generation for acetic acid equivalent to 0.2–1.2% of the TOC of the source rock and total acids at a level of 1–2% of the TOC. The generated CO2 is equivalent to 1–10% of the TOC. The relative amounts of CO2 and organic acids and the maturity dependence of the yield are a function of the source rock type. The amounts of acids generated are sufficient to give significant concentrations in the fluid phases of the source rock and, after migration, in adjacent rocks. Reaction and equilibration with minerals will occur at the first contact, which in most cases will be in or close to the source rock. The organic acids may initially be dissolved in the oil phase, but their high aqueous solubilities will make them diffuse rapidly out of the oil phase and into the surrounding water phase as soon as the oil has migrated into porous carrier beds. The migration of the petroleum phase is driven by buoyancy and there is no equivalent drive for water flow from source rock to reservoir. If the distance between source and reservoir s large, the organic acids found in the reservoir formation waters must therefore have been transported in the oil phase or generated in situ from emplaced oil or disseminated kerogen. The high water solubility of the acids will, however, limit the distances they can be transported in the oil phase. In situ generation in the reservoir must therefore be seriously considered.

Contribution of dissolved organic species to the carbon and energy budgets of hydrate bearing deep sea sediments (Ocean Drilling Program Site 997 Blake Ridge)

Abstract Pore water extracted from sediments penetrated on Leg 164 of the Ocean Drilling Program at the Blake Ridge (West Atlantic) were analysed for acetate, total dissolved organic carbon, bromide and iodide, to help explain the occurrence of subsurface maxima in bacteria biomass and activity reported previously from a nearby site. The high concentrations of these organic matter decomposition by-products in the pore waters from sediments with moderate concentrations of sedimentary organic matter can convincingly be modelled as resulting from upward migration of pore water. The amount of acetate and unidentified DOC transported by the pore water contribute significantly to the pool of metabolizable carbon, and may be the most important substances in energetic terms.

Thermal generation of carbon dioxide and organic acids from different source rocks

Carbon dioxide and organic acids are claimed to play an important role in sandstone diagenesis. In the present work hydrous pyrolysis was used for a qualitative comparison of the relative potential of different source rocks to generate organic acids and carbon dioxide. In addition, the carbon isotopic composition of the generated carbon dioxide was determined. Experiments with three immature source rocks are described; a Miocene brown coal from Germany, a coal from the Middle Jurassic Ness Formation in the northern North Sea and a rich marine type II source from the Late Jurassic Kimmeridge Clay Formation onshore U.K. Both coal samples are typically humic. A stepwise heating program from 150 to 365°C was used. Closed Pyrex tubes containing immature coal (%Rmm = 0.37) were heated in order to measure vitrinite reflectance evolution. The experimental results were related to vitrinite reflectance, as a maturity reference parameter, and to a kinetic model using kinetic parameters for carbon dioxide generation (Burnham A. K. and Sweeney J. J. (1989) Geochim. Cosmochim. Acta53, 2649–2657). Differences in the yiels and carbon isotopic composition were obtained between the marine type II source rock and the coals. Organic acid generation converted between 1 and 3% of the organic carbon. Carbon dioxide was the major oxygen containing component, with molar yields 5–10 times that of organic acids. The experimental yields of organic acids and carbon dioxide were compared to a first order kinetic model for carbon dioxide generation, and a reasonable correspondence in the transformation ratio curves was observed.

Quantitative determination of thermal maturity in sedimentary organic matter by diffuse reflectance infrared spectroscopy of asphaltenes

Infrared spectroscopic analysis of the asphalthenes extracted from an artificially matured Kimmeridge clay sediment (kerogen type II) reveals an excellent correlation (0.997) between temperature and a maturity parameter based on two peaks at approximately 1600 and 1700 cm−1. The change in intensity of the two peaks is discussed in terms of chemical changes of kerogen during the maturity range investigated in the present work (late diagenesis and early catagenis, i.e. %Ro increasing from approximately 0.5 to 0.85%). Comparison with a maturity index based on the methylphenanthrene distribution shows that the spectroscopic determined parameter correlates best with temperature in the maturity range examined in this work.

Compound-specific carbon isotope analysis of natural and artificially generated hydrocarbons in source rocks and petroleum fluids from offshore Mid-Norway

Stable carbon isotope signatures of individual hydrocarbon components provide important information on the source rock facies of oils as well as their asphaltene fractions, despite maturity variations and secondary processes such as phase fractionation and biodegradation. Our study is based upon petroleum fluids and source rock samples from offshore Mid-Norway, where the main source rock is the marine shales of the Upper Jurassic Spekk Formation. An alternative source is provided by coals and coaly shales of the Lower Jurassic Are Formation. In terms of maturity the source rock samples are distributed throughout the main phase of oil generation. In this study compound-specific carbon isotope analysis has been performed on thermal extracts and kerogen pyrolysates of the source rocks, as well as on the oils and the pyrolysis products of their asphaltenes. We demonstrate that trends of absolute isotope data and specific isotopic fingerprints after normalizing the data are useful for petroleum-source rock correlations. The carbon isotope ratios of the n-alkanes separate the Are from the Spekk Formation samples in both thermal extracts and kerogen pyrolysates, becoming less negative with increasing terrestrial input. The Spekk samples also tend to become isotopically heavier with increasing maturity, while the Are samples do not show any systematic difference with increasing maturity. The carbon isotope ratios of the n-alkanes also discriminate between the oils as well as the asphaltenes from the fluids. The individual components generated by pyrolysis of kerogens are isotopically heavier than those from the thermal extracts. This is expected from models of kinetic isotope effects which predict that the isotopically lightest products are generally released at an earlier stage in the maturation process. A similar isotopic difference is also observed between the pyrolysis products of asphaltenes and the original oils derived from the same source rock type. However, in a sample where the oil and its asphaltene fraction originate from two different source rocks, the opposite relationship between the carbon isotope composition of the oil and its asphaltene fraction is observed. Bulk isotope data of kerogen concentrates, pyrolysates and the residue after pyrolysis of the source rock samples as well as bulk isotope data of whole oils are also presented. We show that besides separating Are from Spekk, the bulk carbon isotope values of the Spekk samples are considerably heavier than the n-alkanes from the thermal extracts, whereas those of the Are samples are almost equal.