Geol Supplementary Notes 463-RWR-3,4.5 GEOL RWR-3 FORMATION OF PETROLEUM, 1

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1 GEOL RWR-3 FORMATION OF PETROLEUM, 1 Recommended sections to read in the textbook: Chapters 5 and 11 cover the relevant material, but provide more detail than is covered in class. Both inorganic and organic theories have been suggested to explain the origin of oil and natural gas. Although some gases (especially CH 4 ) and individual components in petroleum may originate by inorganic processes, the weight of evidence strongly supports an origin in organic matter produced at and near the earth s surface. Inorganic hypotheses Cosmic origins? Consolidation of H and C during consolidation of the Earth? Carbonaceous chondrites and space dust contain hydrocarbons evidence of primary organic source? Petroleum should be more widespread in space and time if there was a cosmic source. Reactions of metal carbides within the Earth? FeC 2 + 2H 2 O = C 2 H 2 [acetylene] + Fe(OH) 2 Al 4 C H 2 O = 3CH 4 + 4Al(OH) 3 Fischer-Tropsch reaction: CO 2 + H 2 = CO + H 2 O, then CO + 3H 2 = CH 4 + H 2 O There is no evidence that metal carbides exist in the mantle. Hydrocarbons in igneous rocks as evidence? Hydrocarbons, including bitumens, can be found in igneous rocks: 1. In vesicles and inclusions in alkaline igneous rocks (e.g. Arendal, Norway). Origin is controversial. 2. In thermal aureoles around basic intrusions in sediments. Can be explained by distillation of kerogen in surrounding sediments due to heat of the intrusion petroleum may be incorporated in the igneous rocks as they cool. 3. In weathered and fractured igneous rocks. Normally explained by hydrocarbon migration into the rocks from a sedimentary (organic) source. Mantle degassing? Polymerization of inorganic gases such as CH 4 that are produced in the mantle. It is difficult to produce the range of complex hydrocarbons by polymerization; also problems of permeability and porosity in lower crust. 1

2 Main problems with inorganic theories of petroleum genesis: Poor correlation between petroleum and volcanism Paucity of Precambrian oil Isotopic evidence favours organic origin Petroleum is optically active linked to organic origin Presence of homologous series Geological association with sedimentary basins GENERAL MODEL FOR ORIGIN AND MATURATION OF PETROLEUM (MODIFIED FROM FIG. 4.1 IN HUNT, 1996) Diagenesis Hydrocarbons synthesized by organisms LIFE (Photosynthesis and the food chain) 25 C Lipids, proteins, carbohydrates, etc. Catagenesis minor change 50 C Bacterial and chemical action Kerogen PETROLEUM Maturation Light oil Gases 200 C Heavy oil Pyrobitumens Metagenesis Methane Graphite 2

3 GEOL RWR- 4 and 5 SOURCES AND ENVIRONMENTS OF ORGANIC MATTER (Which depositional settings produce good source rocks?) Sources of organic matter Organic matter may be allochthonous (derived, detrital, washed in) or autochthonous (produced in the depositional environment). Allochthonous organic matter Terrestrial plant and animal debris Spores and pollen (eolian or waterborne) Recycled (old) kerogen from sedimentary rocks Autochthonous organic matter Phytoplankton (algae, diatoms, etc.) primary C producers by photosynthesis Zooplankton (copepods, foraminifera, etc.) Fish (nekton) Benthos (corals, sponges, etc.) Bacteria ORGANIC MATTER (OM) ACCUMULATION IN DIFFERENT DEPOSITIONAL ENVIRONMENTS Deserts (< 0.05% OM) Waxy organic matter Almost all converted to CO 2 and H 2 O Almost no source-rock potential (but sandstones in deserts may have high reservoir potential) Abyssal Ocean Plains (< 0.1% OM) Pelagic muds and oozes Oozes may be calcareous (e.g., from coccoliths, foraminifera) or siliceous (e.g., from diatoms, radiolaria) In the deepest, central parts of the oceans, bottom waters are undersaturated with respect to CaCO 3 and amorphous silica: oozes cannot form (shells dissolve); only detrital clays can accumulate Most OM produced is consumed in water column and recycled OM that sinks through the water column to reach the ocean floor may then be consumed by benthic organisms Fecal pellets allow rapid delivery of OM to the seabed Nutrients are not abundant in the central part of the oceans, so primary productivity is often low 3

4 High Energy Coasts ( % OM) Adequate productivity nutrients often supplied from the land; abundant oxygen Waves and currents may produce coarse sediments High oxygenation of the permeable sediment can lead to early biodegradation (biological breakdown of organic matter to CO 2 and water Low Energy Coasts (0.5 5% OM) High productivity Muds or carbonate muds deposited Can produce good source material if rate of biogenic decay of OM is not too high Distal Floodplains and Deltas (0.5 > 10% OM) Mainly clay sedimentation Organic matter is mainly terrestrial (produces Type III kerogen) Yields much coal and gas, but little oil Silled Basins, Enclosed Seas (< 2 > 10% OM) High productivity Clays Often anoxic Can produce highly favourable source rocks Epeiric (Epicontinental) Seas (< 1 - > 10%) Muddy sediments Can be very favourable if circulation is restricted Lakes, Coastal Lagoons (< 1 - > 10%) Favourable if: Low clastic input Clay sedimentation Stratified waters Most are not stratified May be eutrophic (algal blooms) Coastal Swamps (10 100%) High vegetation; stagnant Peat produced (coal + methane) 4

5 PRODUCTION AND ACCUMULATION OF ORGANIC MATTER Most oil is biological in origin and derived from organic matter in sediments. Marine organic matter is formed in the photic zone by phytoplankton (primary producers) that fix carbon through photosynthesis. The highest productivity occurs in the uppermost 50 m of the ocean, declining with depth as sunlight penetration decreases. Solar Energy Photic (less dense) Phytoplankton (algae) fix carbon photosynthetically Sea surface Bacteria, zooplankton and animals consume organic matter MOST (90%) OF THE ORGANIC MATTER AND NUTRIENTS ARE RECYCLED About 10% OM reaches seafloor Fecal material Aphotic (dense) Benthic organisms consume OM Microbial diagenesis Most organic matter [C] fixed by photosynthesis in upper m is recycled in the water column by passing through the food chain. Phytoplankton (diatoms, algae: primary producers of OM) are oxidised or eaten by zooplankton. Both types of plankton are then consumed by other higher organisms. They defecate, producing pellets that contain the indigestible part of the organic matter. The pellets sink relatively quickly to the bottom, whereas plankton are commonly degraded in the water column. The organic matter that arrives on the ocean (or lake) floor can then be consumed by benthic organisms. Only a few percent of the organic matter produced is buried in sediments, especially in the deepest parts of the oceans. High organic productivity in the oceans depends mainly on adequate sunlight (for photosynthesis) and availability of nutrients. In surface waters, sunlight generally is not a limiting factor except seasonally (winter) at high latitudes. Nutrients (mainly N and P) have a very heterogeneous distribution in marine waters. The highest concentrations are commonly found in coastal regions, where they are land-derived (e.g., soil erosion with leaching to rivers), and in zones of upwelling. Upwellings are present mainly on the western margins of the 5

6 continents (e.g., offshore Peru, Chile, Namibia, etc.), and in areas of oceanic divergence, as for example in the equatorial Pacific. In polar regions, cold oxygen and nutrient-rich water sinks to great depths and flow slowly toward low latitudes. In areas with strong prevailing land winds, that cold water may well up to the surface. The nutrients stimulate phytoplankton growth that, in turn, sustains an abundance of zooplankton, fish, etc. At such locations, above average quantities of organic matter may reach the ocean floor. Coriolis Effect causes surface waters to veer away from the continent -- allows deeper nutrient-rich waters to well up N, P Organic-rich muds Simplified setting of a coastal upwelling On the ocean floor, organic matter will be degraded by microorganisms (mainly bacteria) and consumed by burrowing organisms. The organisms reduce the organic content of the sediments because most of the organic matter is digested. Bioturbation may stir up the sediments and allow exposure to oxygen-bearing bottom water. If the water is stagnant, with little (dysaerobic or suboxic) or no (anaerobic) oxygen, more organic matter can be preserved. 6

7 OXIC vs. ANOXIC WATERS In anaerobic muds, sulphate-reducing bacteria may use much of the organic matter, and precipitate sulphides (e.g. FeS 2 ). If the sediments have little free iron or other metals, more sulphur will be incorporated in the OM and will eventually be enriched in the oil derived from such source beds. In oxygenated waters most OM is consumed (broken down to CO 2 and H 2 O). Most OM that reaches the substrate is then destroyed by benthic organisms, including microbes. In oxic waters, OM preservation is a function of SEDIMENTATION RATE: with rapid burial, more OM survives biodegradation. The OM that survives is usually H- poor therefore, more GAS PRONE. Where the water column is stratified, the bottom waters may become depleted in oxygen. OM sinking into anoxic waters can be degraded only by anaerobic microbes: these are less efficient than aerobic microbes. OXIC ANOXIC More OM survives because of the lack of biogenic activity. It tends to be rich in H and lipids, and is OIL PRONE. The OM accumulates in laminated (no bioturbation) black muds and shales. 7

8 STRATIFICATION in water masses may result from several processes. Surface waters are generally warmer and less dense than colder bottom waters, only overturning (i.e. exchanging with bottom waters) when they cool to the same temperature (commonly 4 C: maximum density of water). Tropical waters are often permanently stratified. The water can also develop a chemical stratification (meromixis), where less saline waters rest upon denser, more saline waters. This may happen when fresh inflow waters "float" on top of saline waters, but do not mix with them unless they evaporate to produce the same salinity and density. If the salinity and density difference is great, this condition may be stable for very long periods (> 10,000 y). The development of anoxic bottom waters, however, usually results from biological processes in the water column and sediments that deplete it of oxygen. CH 2 O + O 2 = CO 2 + H 2 O Such reactions are rapid when mediated by microbes. This process can occur in the water column and on the seabed. As organic matter sinks, oxygen is consumed. If water circulation is low, resulting from density stratification of the water column, the oxygen will eventually become exhausted. Continental shelf Oxic (TOC: <3%) Intense biological activity Anoxic (TOC: 3-10%) Oxygen minimum zone Oxic deep O 2 A zone of biologically-induced oxygen depletion and anoxicity is common in ocean waters at depths of a few hundred to 1000 m. Where this zone intersects the continents, the sediments on the seafloor may underlie anoxic waters, giving high potential for preservation of organic matter. During periods of high eustatic sea-level, the zone of oxygen depletion may be displaced onto shallow seas covering the continents (i.e., epeiric and epicontinental seas). This can result in deposition of favourable source rocks (e.g.. Devonian Bakken Formation shales in Saskatchewan). Many, but not all, of the world s best source rocks were formed during marine transgressions. 8

9 The other setting where stratification plays a major role is in silled marine basins (e.g. modern Black Sea) and deep stratified lakes (e.g., Lake Tanganyika, in E. Africa). SILL OXIC HALOCLINE H 2 S CH 4 ANOXIC Simplified setting of Black Sea. The halocline marks the boundary between normal, oxygenated seawater and the anoxic bottom waters, which have a salinity of about 20 g/l TDS (total dissolved solids). The sill (Bosphorus) is 27 m below sea level. The sediments on the floor of the Black Sea contain up to 15% TOC (Total Organic Carbon), making them excellent potential source rocks. Most of the organic matter derives from plankton. TO CONCLUDE: Most oil originates in the organic matter buried in fine-grained sediments clay (shale) and carbonate mudstones. For preservation of OM, the rate of generation should exceed the rate of destruction. Favorable settings are: Basins with rapid fine-grained sedimentation in regions of moderate to high productivity; Restricted basins with slow fine-grained sedimentation, but with bottom water (and sediment) anoxicity. Restricted marine (and lacustrine) basins mainly have planktonic organic matter that is oil-prone or sapropelic. Sites of rapid clay sedimentation are found on continental shelves, especially near sites of deltaic sediment influx. The organic matter, however, is often derived from terrestrial plants (humic), and may produce more gas than oil. Transgressive cycles are more important than regressive cycles in making good source rocks. Please READ PAGES in Hunt (1996) for more details. 9