Kerogen origin, evolution and structure

Preface 1 Carbon, the Earth and life: 11 Carbon and the basic requirements of life12 Chemical elements, simple compounds and their origins13 The origin of life14 Evolution of life and the atmosphere15 Major contributors to sedimentary organic matter 2 Chemical composition of biogenic matter: 21 Structure of natural products22 Carbohydrates23 Amino acids and proteins24 Lipids25 Lignins, tannins and related compounds26 Nucleotides and nucleic acids27 Geochemical implicatiions of compositional variation 3 Production, preservation and degradation of organic matter: 31 How and why organic-rich deposits form32 Controls on primary production33 Preservation and degradation of organic matter34 Depositional environments associated with accumulation of organic matter 4 Long-term fate of organic matter in the geosphere: 41 Diagenesis42 Humic material43 Coal44 Kerogen45 Catagenesis and metagenesis46 Temporal and geographical distribution of fossil organic carbon 5 Chemical stratigraphy 51 Biologically mediated transformations52 Examples of source indicators in recent sediments53 Diagenesis at the molecular level54 Source and environment indicators in ancient sediments and oil55 Thermal maturity and molecular transformations56 Palaeotemperature and age measurement57 Maturity of ancient sedimentary organic matter58 Isotopic palaeontology 6 The carbon cycle and climate 61 Global carbon cycle62 Changes in carbon reservoirs over geological time63 Palaeoclimatic variations64 Isotopic excursions at period boundaries65 Human influence on the carbon cycle 7 Anthropogenic carbon and the environment 71 Introduction72 Halocarbons73 Hydrocarbon pollution in aquatic environments74 Endocrine disrupting chemicals75 Environmental behaviour of selected xenobiotic compounds76 Factors affecting the rate of anthropogenic components Appendix 1: SI units usedAppendix 2: SI unit prefixesAppendix 3: Geological timescaleReferencesIndex

An introduction to organic geochemistry

How much dissolved oxygen was present in the mid-Proterozoic oceans between 1.8 and 1.0 billion years ago is debated vigorously. One model argues for oxygenation of the oceans soon after the initial rise of atmospheric oxygen 2.3 billion years ago. Recent evidence for H 2 S in some mid-Proterozoic marine basins suggests, however, that the deep ocean remained anoxic until much later. New molybdenum isotope data from modern and ancient sediments indicate expanded anoxia during the mid-Proterozoic compared to the present-day ocean. Consequently, oxygenation of the deep oceans may have lagged that of the atmosphere by over a billion years.

https://ebme.marine.rutgers.edu/HistoryEarthSystems/HistEarthSystems_Fall2010/Week5_Oct1_2009anbar%20mo%20paper.pdf

Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans.

The oldest widely accepted evidence for oxygenic photosynthesis on Earth comes from hydro-carbon biomarkers extracted from 2.7-billion-year-old shales in the Pilbara Craton of Australia, thought to be evidence of eukaryotes and photosynthetic cyanobacteria. This early date has caused controversy because of the long delay between this earliest appearance of oxygen-producing cyanobacteria and the 'great oxidation event' that caused the rise of atmospheric oxygen some 300 million years later. New work by Rasmussen et al. shows that the organic biomarkers are not of Archaean age and must have entered the rocks later, some time after about 2.2 billion years ago. The earliest unambiguous fossil evidence for eukaryotes and cyanobacteria thus reverts to 1.78–1.68 and 2.15 billion years, respectively. The oldest widely accepted evidence for oxygenic photosynthesis comes from hydrocarbon biomarkers extracted from 2.7-billion-year-old shales in the Pilbara Craton, Australia, thought to be evidence of eukaryotes and photosynthetic cyanobacteria. But evidence now shows that the organic biomarkers were not indigenous to the rocks containing them, and must have entered the rocks after ∼2.2 Gyr ago. The earliest unambiguous fossil evidence for eukaryotes and cyanobacteria thus reverts to 1.78–1.68 and 2.15 Gyr, respectively. The evolution of oxygenic photosynthesis had a profound impact on the Earth’s surface chemistry, leading to a sharp rise in atmospheric oxygen between 2.45 and 2.32 billion years (Gyr) ago1,2 and the onset of extreme ice ages3. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from ∼2.7-Gyr-old shales in the Pilbara Craton, Australia, which contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria4,5,6,7. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10–20‰) of 13C relative to bulk sedimentary organic matter5,8. Here we present micrometre-scale, in situ 13C/12C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that originally yielded biomarkers. Our results show that both kerogen and pyrobitumen are strongly depleted in 13C, indicating that indigenous petroleum is 10–20‰ lighter than the extracted hydrocarbons5. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism ∼2.2 Gyr ago9 and thus do not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon. The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78–1.68 Gyr ago and ∼2.15 Gyr ago10,11, respectively. Our results eliminate the evidence for oxygenic photosynthesis ∼2.7 Gyr ago and exclude previous biomarker evidence for a long delay (∼300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45–2.32 Gyr ago1.

Reassessing the first appearance of eukaryotes and cyanobacteria

Rising oxygen levels in the Earth's early atmosphere marked the end of a 2.5-billion-year period dominated by oceans with low levels of oxygen. But geochemical evidence suggests that for the following billion years the oceans remained largely devoid of oxygen. The discovery of molecular fossils (hydrocarbon biomarkers) in 1.6-billion-year-old sedimentary rocks from a marine basin in northern Australia now offers insights into the marine ecosystem at the time. The biomarkers record an anoxic and sulphidic world hostile to to many forms of life but supporting blooms of sulphide-breathing green and purple bacteria. The disappearance of iron formations from the geological record ∼1.8 billion years (Gyr) ago was the consequence of rising oxygen levels in the atmosphere starting 2.45–2.32 Gyr ago1,2,3. It marks the end of a 2.5-Gyr period dominated by anoxic and iron-rich deep oceans. However, despite rising oxygen levels and a concomitant increase in marine sulphate concentration, related to enhanced sulphide oxidation during continental weathering4, the chemistry of the oceans in the following mid-Proterozoic interval (∼1.8–0.8 Gyr ago) probably did not yet resemble our oxygen-rich modern oceans. Recent data5,6,7,8 indicate that marine oxygen and sulphate concentrations may have remained well below current levels during this period, with one model indicating that anoxic and sulphidic marine basins were widespread, and perhaps even globally distributed4. Here we present hydrocarbon biomarkers (molecular fossils) from a 1.64-Gyr-old basin in northern Australia, revealing the ecological structure of mid-Proterozoic marine communities. The biomarkers signify a marine basin with anoxic, sulphidic, sulphate-poor and permanently stratified deep waters, hostile to eukaryotic algae. Phototrophic purple sulphur bacteria (Chromatiaceae) were detected in the geological record based on the new carotenoid biomarker okenane, and they seem to have co-existed with communities of green sulphur bacteria (Chlorobiaceae). Collectively, the biomarkers support mounting evidence for a long-lasting Proterozoic world in which oxygen levels remained well below modern levels.

Biomarker evidence for green and purple sulphur bacteria in a stratified Palaeoproterozoic sea

Five potential source rocks have been discovered in the middle Proterozoic of the McArthur basin. The lacustrine Barney Creek Formation (McArthur Group) and the marine Velkerri Formation (Roper Group) compare favorably in thickness and potential with demonstrated petroleum source rocks in the Phanerozoic. The former contains up to 7% total organic carbon (TOC) and kerogen types I and II. TOC values in the latter range up to 6.5% and the kerogen is type II. Petrographic examination shows the organic matter is mostly lamalginite, which on maturation yields "nonfluorescent lamalginite" and bitumen. Extractable hydrocarbon yields from mature samples indicate that good to excellent source rocks are present and there is ample evidence of migration having occurred. Tmax values derived from Rock-Eval analysis may be used to define maturation levels in much the same way as in Phanerozoic sediments. At low maturation levels, the reflectivities of lamalginite and bitumen are lower than vitrinite reflectance values calculated from methylphenanthrene indices, but converge in the mature zone. This convergence reflects both the inherent lower reflectivity of hydrogen-rich macerals and the source control on methylphenanthrene indices at low maturity. Maturation levels in McArthur Group sediments vary from marginally mature to overmature with abrupt changes over short distances associated with faults. Hydrocarbon generation occurred prior to the deposition of the younger Proterozoic sediments of the Roper Group. In contrast, the source rocks in the Roper Group re marginally mature to mature throughout the study area and are overmature only where they are affected by igneous intrusions. Considerations of burial history suggest that hydrocarbon generation may have occurred during deposition of the Roper Group although an early Paleozoic timing cannot be excluded.

A. C. Cook

Papers

Petroleum Geology and Geochemistry of Middle Proterozoic McArthur Basin, Northern Australia II: Assessment of Source Rock Potential

Petroleum source rock assessment in non-marine sequences: pyrolysis and petrographic analysis of Australian coals and carbonaceous shales