1_formation of Petroleum

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Geol 463.3 Supplementary Notes 463-RWR-3,4.5

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GEOL 463.3RWR-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 CH4) and individual components in petroleum mayoriginate by inorganic processes, the weight of evidence strongly supports an origin in organic

matter produced at and near the earths surface.

Inorganic hypotheses

Cosmic origins?Consolidation of H and C during consolidation of the Earth? Carbonaceous

chondrites and space dust contain hydrocarbonsevidence 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?

FeC2 + 2H2O = C2H2[acetylene] + Fe(OH)2

Al4C3 + 12H2O = 3CH4 + 4Al(OH)3

Fischer-Tropsch reaction:

CO2 + H2 = CO + H2O, then CO + 3H2 = CH4 + H2O

There is no evidence that metal carbides exist in the mantle.

Hydrocarbons in igneous rocks as evidence?Hydrocarbons, including bitumens, can be foundin 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 intrusionpetroleummay be incorporated in the igneous rocks as they cool.

3. In weathered and fractured igneous rocks. Normally explained by hydrocarbonmigration into the rocks from a sedimentary (organic) source.

Mantle degassing?Polymerization of inorganic gases such as CH4that are produced in the

mantle. It is difficult to produce the range of complex hydrocarbons by polymerization; alsoproblems of permeability and porosity in lower crust.


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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)

LIFE

(Photosynthesis andthe food chain)

Lipids, proteins,carbohydrates, etc.

Hydrocarbonssynthesized by

organisms

Bacterial andchemical action

Kerogen

minorchange

Catagenesis

Diagenesis

Maturation

Light oil

Gases

Methane

Heavy oil

Pyrobitumens

GraphiteMetagenesis

50C

200C

25C

PETROLEUM


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GEOL 463.3RWR- 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 DIFFERENTDEPOSITIONAL ENVIRONMENTS

Deserts (< 0.05% OM)

Waxy organic matter Almost all converted to CO2and H2O 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 toCaCO

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


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High Energy Coasts (0.20.5% 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 CO2and water

Low Energy Coasts (0.55% OM)

High productivity Muds or carbonate muds deposited Can produce good source materialif 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)


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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 throughphotosynthesis. The highest productivity occurs in the uppermost 50 m of the ocean, declining

with depth as sunlight penetration decreases.

Solar Energy

Sea surface

Photic(less dense)

Aphotic(dense)

Phytoplankton (algae) fix carbon photosynthetically

Bacteria, zooplankton andanimals consume organic matterMOST (90%) OF THE ORGANICMATTER AND NUTRIENTS ARE

RECYCLED

Benthic organisms consume OM

About 10% OMreaches seafloor

Microbial diagenesis

Fecalmaterial

Most organic matter [C] fixed by photosynthesis in upper 100150 m is recycled in the watercolumn 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 otherhigher organisms. They defecate, producing pellets that contain the indigestible part of the

organic matter. The pellets sink relatively quickly to the bottom, whereas plankton arecommonly 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 commonlyfound 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


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continents (e.g., offshore Peru, Chile, Namibia, etc.), and in areas of oceanic divergence, as forexample 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.

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 allowexposure 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.

Coriolis Effect causes surface waters to veeraway from the continent -- allows deepernutrient-rich waters to well up

N, POrganic-rich

muds


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OXIC vs. ANOXIC WATERS

In anaerobic muds, sulphate-reducing bacteria may use much of the organic matter, and

precipitate sulphides (e.g. FeS2). If the sediments have little free iron or other metals, moresulphur 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 CO2and H2O). Most OM that reachesthe 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-

poortherefore, more GASPRONE.

OXIC

ANOXIC

Where the water column isstratified, the bottom waters may

become depleted in oxygen.

OM sinking into anoxic waterscan be degraded only by

anaerobic microbes: these are less

efficient than aerobic microbes.

More OM survives because of the

lack of biogenic activity. It tendsto be rich in H and lipids, and is

OIL PRONE. The OMaccumulates in laminated (no

bioturbation) black muds and

shales.


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STRATIFICATIONin water masses may result from several processes. Surface waters are

generally warmer and less dense than colder bottom waters, only overturning (i.e. exchangingwith bottom waters) when they cool to the same temperature (commonly 4C: maximum density

of water). Tropical waters are often permanently stratified. The water can also develop achemical 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 notmix 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 inthe water column and sediments that deplete it of oxygen.

CH2O + O2 = CO2 + H2O

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 becomeexhausted.

A zone of biologically-induced oxygen depletionand 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 worlds best source rocks were formed during marinetransgressions.

Continental shelf

Oxygen minimumzone

deepO2

Oxic(TOC:


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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).

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 deltaicsediment 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 READPAGES 111-124 in Hunt (1996) for more details.

SILL

ANOXIC

OXIC

HALOCLINE

H2S CH4

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