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Abstract
A hydrocarbon source rock s generally considered to be a finegrained rock that, during its burial and heating, generates and releases enough fluids to form commercial accumulations of oil or gas (Fig. 9.1a). Back in 1981, Kirkland and Evans made the observation that some 50 % of the world’s oil sequestered in carbonate reservoirs may be associated with mesohaline micritic source rocks. Heresy or not, the notion that much of the oil in carbonate reservoirs, sealed by evaporite salts, may have been sourced in earlier less saline, but still related, evaporitic (mesohaline) conditions, is worthy of consideration. The association between mesohaline waters, the accumulation of organic-rich sediments and the evolution of the resulting evaporitic carbonates into source rocks has been noted by many, including: Woolnough (1937), Sloss (1953), Moody (1959), Dembicki et al. (1976), Oehler et al. (1979), Malek-Aslani (1980), Kirkland and Evans (1981), Jones (1984), Hite et al. (1984), Eugster (1985), Sonnenfeld (1985), Ten Haven et al. (1985), Warren (1986), Evans and Kirkland (1988), Busson (1991), Edgell (1991), Beydoun (1993), Benali et al. (1995), Billo (1996), Aizenshtat et al. (1998), Carroll (1998), Schreiber et al. (2001), Love et al. (2007), Schnyder et al. (2009), Warren (2011), Comer (2012).
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The term diagenesis, as used by petroleum geochemists, describes processes of organic modification or alteration in the presence of pore waters where circulation is driven by surface processes. This usage is much more restrictive than the sedimentological use of the term which encompasses all alteration or modification from the time of sediment deposition until the onset of metamorphism. In order to minimise any possible confusion, the term eogenesis is preferred by some workers, but this is a different meaning to eogenetic as used by Choquette and Pray (1970).
Halotolerant organisms can tolerate high salt concentrations but grow better at somewhat lower salinities. Halophilic organisms grow best at very high salt concentrations and can be killed by lower salinities. Haloxene organisms cannot tolerate high concentrations of salts.
Autotrophs (literally “self feeders”) are organisms capable of producing organic compounds from simple inorganic compounds (producers not consumers). Autotrophs use carbon dioxide (CO2) as a sole source of carbon for growth and obtain their energy from light (photosynthetic autotrophs or photoautotrophs) or from the oxidation of inorganic compounds (chemosynthetic autotrophs or chemoautotrophs) (Fig. <InternalRef RefID="Fig11" >9.11</Internal Ref>).
Photoautotrophs use light as a source of energy and CO2 as a source of carbon (oxygenic photosynthesis).
Chemoautotrophs use endogenous light-independent reactions to obtain energy, these reactions involve inorganic molecules and an electron donor other than water and do not release oxygen.
Lithoautotrophs depend upon inorganic compounds as electron donors for energy production.
Heterotrophs (literally “feeders on others”) use organic molecules synthesized outside their body as a source of energy and carbon (consumers, detritovores, decomposers). They are saprophytes, obtaining their nutrients from dead organic matter. Most chemotrophs are autotrophic, but some are heterotrophs (chemoheterotrophs), which use inorganic oxidation for energy but use organic matter for carbon as well as supplemental energy. Photosynthetic bacteria have the biochemistry for either anoxygenic photosynthesis (non O2-producing) or oxygenic photosynthesis (O2-producing). Most photosynthetic bacteria are autotrophs that fix CO2 (photoautotrophs), but some rely on organic matter for their carbon (photoheterotrophs). Adaptive prokaryotes switch their modes of metabolism depending on environmental conditions (Peters et al. 2005).
Pink to purple colours that typify many hypersaline water bodies comes from concentrations of mostly carotenoid pigments present in the cytoplasm of various halophilic microorganisms. Most haloarchaea are red due to a high content of C-50 carotenoids of the bacterioruberin series in the cell membrane (Fig. <InternalRef RefID="Fig13" >9.13</Internal Ref>). Photosynthetic cyanobacteria and eukaryotes (e.g., unicellular green algae of the genus Dunaliella) contribute to the pigmentation of the hypersaline waters thanks to the presence of chlorophylls and C-40 carotenoids (mostly all-trans- and 9-cis- β-carotene). Chloroplasts in D. salina and D. parva accumulate large quantities of this β-carotene at their peripheries in the form of droplets (plastoglobuli) and so blooms appear brown-red, not green like most other alga and cyanobacteria. The carotene seems to act as photo-protective ‘sun-screen’ to protect chlorophyll integrity and shield cellular DNA from the high levels of irradiance that characterizes the normal habitat of halophilic Dunaliella. It may also act as a ‘carbon sink’ to store excess carbon produced during photosynthesis under conditions where growth is nutrient-limited but photosynthetic carbon fixation must continue.
Chlorophylls absorb red and blue wavelengths much more strongly than they absorb green wavelengths, so chlorophyll-bearing cyanobacteria and most photosynthesizing plants appear green. In contrast, the carotenoids and phycobiliproteins strongly absorb green wavelength, so microbes and algae with large amounts of carotenoid pigments appear yellow to brown, those with large amounts of phycocyanin appear blue, and those with large amounts of phycoerythrin appear red.
Pigment levels can indicate the stratification of the microbial community in any photoresponsive biomass in a brine column. Red wavelengths (long wavelengths) in white light are absorbed in the first few metres of a brine column or the uppermost millimetre or two of a microbial mat (chlorophyll utilizers flourish). The blue and green wavelengths (shorter) reach deeper into the brine column.
Halophilic archaea were first investigated microbiologically as a common cause for spoilage of salted fish, with their carotene imparting a characteristic red colour. None of the halophilic archaea have been proved to cause disease, so their effects on salted foodstuffs are largely aesthetic. Haloarchaeal spoilage explains the red colour and the foul smell in “red herring.” The phrase “throw in a red herring” means to mislead. Spoiled salted fish were once used by poachers to distract hunting hounds. Poachers would interpose themselves between the prey and the hunting party and drag a sack of red herring across the trail to mislead the dog pack, which would follow the scent trail left by the red herring and not the prey. This would give the poachers the opportunity to bag the prey.
The euphotic (photic) zone describes the upper part of a water mass exposed to sufficient sunlight for photosynthesis to occur. Below is the aphotic zone. The lit zone can extend down to a few cm in a turbid water body, or as much as 200 m in clear waters (Fig. <InternalRef RefID="Fig12" >9.12</Internal Ref>).
Thiotrophic describes an organism that oxidizes sulphur compounds as a major part of its metabolism. Purple and green sulphur bacteria are generally not called thiotrophic because they gain energy by photosynthesis and are thus classified as phototrophic, even though sulphide is required for this process.
Dissimilatory sulphate reducers release large amounts of hydrogen sulphide (H2S and HS) to the ambient environment, as products of their energy metabolism and so cause the characteristic smell of rotten eggs in the host sediment. The process is carried out by a heterogeneous group of bacteria and archaea that occur in anoxic environments with temperatures up to 105 °C.
Assimilatory sulphate reduction is conducted by some bacteria in order to synthesize sulphur-containing cell components. It is another form of intracellular sulphate reduction, it is a metabolic process set that reduces sulphate to typically produce sulphide groups attached to amino acids, which are assimilated as useful components into the cell.
The term mono, used in the name Mono Lake, California, is likely a Yokut Indian word for naturally salted brine-fly pupae, a local delicacy before European settlement. However the Yokut people lived 200 miles north of Mono Lake, they harvested brine flu in Owens Lake, not Mono Lake. Prior to European settlement a small group of people (≈200 persons) calling themselves Kutzadika’a inhabited the Mono Lake region. The name Kutzadika’a, roughly translated, means “fly eaters” and Kutsavi was the Kutzadika’a term for brine fly, not Mono. Numbering around 200 individuals, the Kutzadika’a spent most of the year in the Mono Basin following an annual cycle of food gathering that seasonally focused on brine fly.
The term halobacteria (as opposed to the family Halobacteriacea, which exclusively encompasses the halophilic archaea) is a terminological carry-over from the old two-part morphology-based classification of life (see earlier discussion of Woese and his influence on microbial nomenclature and classification). When trying to make sense of the taxomic complexities that typify the microbial realms one should always remember that in contrast to eukaryotic nomenclature, there is no such thing as an official classification of prokaryotes. It remains a matter of scientific judgment and general agreement. The closest approximation to an “official” classification of prokaryotes would be one that is widely accepted by the community of microbiologists. The debate continues.
9.6 Isoprenoids are a major class of nonsaponifiable lipids that occur in plants, animals, and bacteria and are characterized by chains of modular groups of five carbon atoms in which the typical pattern has four of the carbon atoms in a linear chain and a single carbon attached at the carbon one position removed from the end of the chain. The term isoprenoid is derived from the name of the five-carbon, doubly unsaturated branched hydrocarbon isoprene, which could in principle be the simplest monomeric chemical precursor for this class of compounds.
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Isoprenoids are also known as terpenes. Terpenes are usually grouped according to the number of isoprene (C5H8) units in the molecule: monoterpenes (C10H16) contain two such units; sesquiterpenes (C15H24), three; diterpenes (C20H32), four; triterpenes (C30H48), six; and tetraterpenes (C40H64), eight. The carotenoid pigments are the best known tetraterpenes in the geological realm.
Aliphatic hydrocarbons are any chemical compound belonging to the organic class in which the atoms are not linked together to form a ring. They are divided into three main groups according to the types of bonds they contain: alkanes, alkenes, and alkynes. Alkanes (n-alkanes) have only single bonds and a continuous chain structure, alkenes contain a carbon-carbon double bond, and alkynes contain a carbon-carbon triple bond. Aromatic hydrocarbons are classified as either arenes, which contain a benzene ring as a structural unit, or non-benzenoid aromatic hydrocarbons, which are characterized by special stability but which lack a benzene ring as a structural unit.
Hopanoids are a group of compounds (triterpenoids) produced by prokaryotic organisms, and the diagenetic alteration products of these compounds (found in oils, rock extracts and sediment extracts). Just as steroids (steranes) are a useful group of biomarkers for identifying input from various eukaryotic organisms (e.g., plants and animals), an analogous group of compounds, hopanoids, are a useful group of biomarkers for identifying input from various bacteria. Hopanoids serve the same function in bacteria as sterols do in eukaryotes: they act as cell wall rigidifiers. In petroleum and its source rocks, hopanoid biomarkers exist as a subset of a group of compounds called triterpanes (isoprenoids).
Israelis living in desert kibbutz communities construct artificial black plastic line pans in the desert, fill them with stratified waters and plumb household water pipes through the bottom brine in the pans to create a domestic water heater.
An aviator once described Lake Nakuru as “a crucible of pink and crimson fire,” with a million flamingos painting an astonishing band of colour that burst into pieces as the birds took flight.
Aka a “Chicken Little” or “the sky is falling” attitude to the world: such reportage, so popular in much of the world’s newsprint and in the religious extremes, be it reporting environmental Armageddon, the “end of peak oil,” or the coming of the next prophet, sells more newspapers and gets more zealous believers than any “good news” stories or fact-based analysis and discussion. In the prevailing zeitgeist, being a skeptic and advocating testing of all hypotheses (the basis for scientific endeavour) is considered a negative position; The questions; Leading questions, so common with the popular press, like; “When did you last beat you wife?” and “Are you a climate skeptic?” and your ability to reply without prejudicing popular perceptions is key. Otherwise when you answer either question with statement like “I don’t beat my wife” or “The proposition that climate change since the start of the Industrial Age ties mostly to anthropogenic causes, is still equivocal,” typically puts you in the same place in the minds of the true believers Prejudice be it green, left wing, right wing, religious or otherwise will always color the perceptions of such a listener.
Mixolimnion: the upper, low density, freely circulating water layer of a meromictic lake.
Monimolimnion: the deep, usually salty, layer of a meromictic lake typically it is a perennially stagnant or noncirculating water mass.
Meromixis: a condition in a lake (meromictic lake) where the bottom noncirculating water mass is adiabatically isolated from the upper water mass.
Holomixis: a condition in a lake describing complete overturn and mixing of the lake water mass.
Hypoliminion: is the lowermost water mass in a lake, characterised by a generally uniform temperature (or density) profile.
Eutrophic lake: a lake characterised by an abundance of nutrients and a seasonal deficiency of oxygen in the hypolimnion. Waters are usually shallow and sediments are organic-rich laminites.
Oligotrophic lake: a lake characterised by a deficiency in nutrients. The bottom sediments typically contain low levels of organics and the water column is deep.
Monomictic Lake: A lake which undergoes one period of complete mixing during the year separated by one period of thermal stratification.
Small amounts of hydrogen sulphide occur in crude petroleum, but natural gas can contain up to 90 %. At very low concentrations of less than 10–100 ppm it smells like rotten eggs and there is a common misapprehension that its foul odour is a warning. At 100 ppm the gas kills the sense of smell in 3–15 min and will cause you to cough or your eyes to water. Over 100 ppm your eyes and throat may begin to sting. At 200 ppm, your eyes and throat will begin to burn and you will get headaches. Only 600 ppm, or 0.06 of 1 % will cause death, if you are not treated very quickly. Over 1,000 ppm causes respiratory paralysis and a sudden agonizing death from asphyxiation.
H2S is heavier than air, invisible, highly explosive and can destroy steel and rubber seals very quickly. Modern drill rig floors are all fitted with gas sniffer warning systems.
Hydrogen sulphide is considered a broad-spectrum poison, meaning that it can poison several different systems in the body, although the nervous system is most affected. The toxicity of H2S is comparable with that of hydrogen cyanide or carbon monoxide. It forms a complex bond with iron in the mitochondrial cytochrome enzymes, thus preventing cellular respiration.