rdöls - Organische Geochemie · 2018-10-22 · 3. In weathered and fractured igneous rocks. Formed by hydrocarbon migration into the rocks from a sedimentary (organic) source. Mantle

$Page 1: rdöls - Organische Geochemie · 2018-10-22 · 3. In weathered and fractured igneous rocks. Formed by hydrocarbon migration into the rocks from a sedimentary (organic) source. Mantle$

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Kapitel 1 - Bildungsbedingungen des Erdöls

Origin of Petroleum

Inorganic hypotheses

Cosmic origins?

Reaction of elemental H and C during consolidation of the Earth? Carbonaceous chondrites and space dust contain hydrocarbons. Can this be taken as evidence of a primary organic source?

If so, petroleum should be more widespread in space and time given that 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

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

Fischer-Tropsch reaction:

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

minimum 250°C at 25 bar pressure in presence of metal catalysts

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Origin of Petroleum

Hydrocarbons in igneous rocks as evidence?

Hydrocarbons (HC), including bitumen, 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 HC can derive from kerogen in surrounding sediments heated by the intrusion — petroleum may be incorporated in the igneous rocks as they cool.

3. In weathered and fractured igneous rocks. Formed by hydrocarbon migration into the rocks from a sedimentary (organic) source.

Mantle degassing?

Polymerization of inorganic gases such as CH4 that are produced in the mantle. The range of complex HC is unlikely to derive from polymerization; permeability and porosity in lower crust are too low. Isotopic evidence (mantle d13C -5 to -8 ‰) argues against a primordial

carbon source.

Serpentinization of ultramafic rocks?

Widespread process to generate CH4 with a potential to release minor amounts of higher (C2 to C5) analogues. Debated in current research.

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Origin of Petroleum

Inorganic hypotheses

Main problems with inorganic theories of petroleum genesis:

Poor correlation between petroleum and volcanism;

Geological association with sedimentary basin;

Paucity of Precambrian oil;

Isotopic evidence favours organic origin;

Petroleum is “optically active” – linked to organic origin;

Presence of homologous series.

Organic hypotheses

Compelling evidence from geological, biochemical and isotopic evidence suggests an origin of petroleum and gas from biological precursor molecules that have been diagenetically altered.

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Origin of PetroleumOrganic hypotheses

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Origin of Petroleum

Organic hypotheses

SOURCES AND ENVIRONMENTS OF ORGANIC MATTER

(which depositional settings produce good source rocks?)

Sources of organic matter

Organic matter may be allochthonous (detrital, washed in) orautochthonous (produced in the depositional environment).

Allochthonous organic matter

• Terrestrial plant, microbial, and animal debris

• Spores and pollen (eolian or waterborne)

• Recycled (old) kerogen from sedimentary rocks

Autochthonous organic matter

• Phytoplankton (algae, cyanophytes, etc.) – primary photosynthetic C-producers

• Zooplankton (copepods, foraminifera, etc.)

• Fish (nekton)

• Benthos (corals, sponges, etc.)

• Bacteria, Archaea, Virusses?

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ORGANIC MATTER (OM) ACCUMULATION IN DIFFERENT DEPOSITIONAL ENVIRONMENTS

Deserts (< 0.05% OM)

• Waxy organic matter

• Almost all organic matter converted to CO2 and H2O

• Almost no source-rock potential (desert sandstones 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 deep, central parts of the oceans, bottom waters are undersaturated with CaCO3 and amorphous silica: oozes cannot form (shells dissolve); only detrital clays can accumulate

• Most OM produced is consumed within the 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 (0.2–0.5% OM)

• Adequate productivity – nutrients often supplied from the land; abundant oxygen

• Waves and currents may produce coarse sediments

• High oxygenation of the permeable sediment will lead to early biodegradation

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

Coastal Swamps (10 – 100% OM)

• High vegetation; stagnant

• Peat produced (coal + methane)

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Silled Basins, Enclosed Seas (< 2 – > 10% OM)

• High productivity, nutrient traps

• Low energy-deposition of clays

• Often anoxic and thus low degree of biodegradation

• Can produce highly favourable source rocks

Epeiric (Epicontinental) Seas (< 1 - > 10% OM)

• Muddy sediments

• Can be very favourable if circulation is restricted

Lakes, Coastal Lagoons (< 1 - > 10% OM)

Favourable if:

• Low clastic input

• Clay sedimentation

• Stratified waters, although most are not stratified

• May be eutrophic (algal blooms)

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Survival rate of OM < 0.1%

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Oil is biological in origin and derived from organic matter in sediment. Marine organic matter [OM] is formed in the photic zone by phyto-plankton (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.

Most OM fixed by photosynthesis in upper 100–150 m is immediately recycled in the water column by passing through the food chain. Phytoplankton (cyanos, archaea, algae: primary OM producers) are consumed or oxidized by zooplankton (e.g. copepods).

Both types of plankton are then utilized by higher organisms, which produce fecal pellets that contain the indigestible part of the OM. The pellets sink to the bottom rather quickly, whereas plankton iscommonly degraded in the water column. OM arriving on the ocean/lake floor can then be consumed by benthic organisms.

Only a few percent (mostly <0.1%) of the organic matter produced is buried in sediments, especially in deeper parts of the oceans.

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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.

Upwelling zones are present mainly on the western margins of thecontinents (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 flows slowly toward low latitudes. In areas with strong and prevailingly off-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.

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In oxygenated waters most OM is consumed (broken down to CO2 and H2O). 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 OMsurvives biodegradation.

The OM that survives is usually hydrogen-poor. Therefore, it is more gas prone.

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Where a 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 diverse and efficient than aerobic microbes.

More OM survives because of thelack of biogenic activity. It tends to be rich in hydrogen and lipids, and is oil prone. The OM accumulates in laminated/parallel bedded (nobioturbation) black muds andshales.

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On ocean floors, organic matter will be degraded by microorganisms (mainly bacteria) and consumed by burrowing organisms. These 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 or anoxic) oxygen, more organic matter can be preserved.

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 salinewaters. 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.

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ORGANIC MATTER (OM) ACCUMULATION IN DIFFERENT DEPOSITIONAL ENVIRONMENTS - Stratification

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 enzymatically mediated by microbes. This process can occur within the water column, on the seabed and within the sediment. 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.

The other setting where salinity/density stratification plays a major role is in silled marine basins (e.g. modern Black Sea), fjords where marine deep water is covered by freshwater from continental run-off, and deep stratified lakes (e.g., Lake Tanganyika, in E. Africa).

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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 (Bosporus) 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 oil source rocks. Most of the organic matter derives from plankton, or in nearshore areas from terrestrial influx.

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During periods of high sea-level, the zone of oxygen depletion may be displaced onto shallow seas covering the continents (i.e., epeiric or epicontinental seas). This can result in deposition of favourable source rocks.

Many, but not all, of the world’s best petroleum source rocks were formed during marine transgressions.

All of the depositional systems mentioned above ultimately require a continuous re-supply of nutrients to maintain surface water primary biological productivity. It is often difficult to comprehend, how such a nutrient influx can persists for the hundreds of thousands or even millions of years to accumulate thick source rock sequences. 20 m of source rock deposition will require 200ka at a sedimentation rate of 0.1m/ka (after compaction) or every meter of source rock takes 10,000 years to accumulate.

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ORGANIC MATTER (OM) ACCUMULATION IN DIFFERENT DEPOSITIONAL ENVIRONMENTS – Oceanic OMZ

A zone of biologically-induced oxygen depletion (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.

(TOC: <1%)

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ORGANIC MATTER (OM) AND SOURCE ROCK FABRIC

Optical micrograph of a stacked succession of sharp-based, thin-bedded (A to D) clay-size-rich mudstones from the Jet Rock Mbr. with 3.5% TOC. Basal lamina sets within the indivi-dual beds show discontinuous wavy geometry, contain abundant pellets (p) and organomineralic aggregates (oma). Gradationally overlying the basal

laminaset are homogenized laminae, whereby bed C has a discontinuous but distinctive silt lag at its base.

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Optical micrograph of burrow-mottled (individual burrows are out-lined by dotted lines and labeled ‘‘b’’), clay-size-rich mudstone that contains many organomineralic aggregates (oma) and fecal pellets (p). Despite of obvious bioturbation this Jet Rock sample contains 3.2% TOC. Note decline in omas and thus brighter colurs due to OM loss in “b”.

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Bedding-parallelthin section of a clay-size-rich mudstone with prime examples of prominent organomineralic aggregates (oma). Note that individual omas contain intimate mixtures of red translucent amorphous organic matter, clay minerals, and opaque pyrite. This Jet Rock sample contains 4.2% TOC.

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Backscattered electron image of a Jet Rock with 6% TOC. The sample contains severalcoccolith-rich pellets (p) and a clay-rich pellet (cp). These pellets are enclosed by a fine-grained matrix that is composed ofclay minerals and very small pyrite framboids (fr). Heavily compacted omas that comprise intimate mixtures of organic matter, clay, and pyrite are outlined.

frfr

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Organic-matter and silt-bearing, clay-size-rich mudstone of the Kimmerdige Clay Fm. containing 14.4% TOC, which causes the red colours. Much of the silt-size debris visible is organized into the tests of white agglutinated foraminifera (af). Large numbers of omas are present in the matrix. The matrix contains small pellets (p) in addition to clay-size material and pyrite.

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Backscattered electron image of Kimmerdige Clay Fm., comprisingmixtures of wispy, organomineralic fragments (oma) that are intimately associated with dispersed clay minerals, silt-size quartz grains (q), and calcareouscoccolith debris. Much of the coccolith detritus is contained in fecal pellets (outlined and labeled p).

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Summary

• 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 must exceed the rate of destruction.

Favorable settings are:

• Basins with rapid fine-grained sedimentation in regions of moderate to high productivity (thick but lean SR);

• Restricted basins with slow fine-grained sedimentation, but with bottom water (and sediment) anoxicity (thin but rich SR).

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.

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COMPOSITION OF LIVING ORGANISMS

The main components of living organisms:

Carbohydrates

• Sugar chains

• Rapidly break down; geologically unstable

• Include cellulose, starch, and chitin

Lignin

• Structural component of higher plants

• More resistant to decay

• Contains aromatic (polyphenolic) rings

Tannin

• Contains functionalized aromatic rings

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Proteins

• Amino-acid polymers

• Contain most N-compounds in organic matter

• Rapidly decomposed

• Include enzymes, heme/chlorophyll, structural components of algae, shells, corals, sponges, etc.

Lipids

• Insoluble in water

• Cell membranes of all organisms

• Animal fats and vegetable oils

• Include spores, fruit, waxes, etc.

• Many contain paraffin chains

Plant and animal pigments

Essential oils and Resins

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Diagenetic loss mainly as H2O or as CO2

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Evolution of Organic Matter to Kerogen

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DIAGENESIS OF ORGANIC MATTER

10 ka – 1 ma

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During early diagenesis, the composition of the organic matter changes, and its potential to produce petroleum is partly determined.

Microbial degradation begins in the water column:

Less than 10% (most often <0.1 %) of the organic matter settles to the bottom.

Much of that arrives in fecal pellets.

Much of it is degraded in the water column, initially by oxidation.


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ANOXIC ZONE

In the anoxic zone, anaerobic microbes use enzymes to decompose proteins, carbohydrates and lipids into simpler molecules, often by fermentation and reduction. Three zones are typically present:

Nitrate Reduction

After oxygen is depleted, NO3-is used as an energy source:

6CH2O + 4NO3-= 6CO2 + 6H2O + 2N2

or 5CH2O + 4NO3-= 2N2 + 4HCO3

-+ CO2 + 3H2O

When nitrate is exhausted:

Bacterial Sulphate Reduction (BSR)

2CH2O + SO42-

= H2S + 2HCO3–

SO42-

= S + 2O2 (mainly by Desulfovibrio bacteria)

If iron is available in the sediment, H2S may combine with Fe to form pyrite. This process explains why pyrite/marcasite is so common in black, organic-rich shales (and coal, which also accumulates under reducing conditions). If no Fe is available, the H2S may migrate into the oxic zone (to become SO4

2-) or may

combine with organic molecules. This can result in an S-rich crude oil.

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ANOXIC ZONE

After sulfate is depleted:

Methanogenesis (fermentation)

Methanogenic archaea produce CH4 from the residue of the overlying zones:

e.g., CH3COOH (from cellulose) = CH4 + CO2 (acetate fermentation)

CO2 + 8H+

= CH4 + 2H2O (hydrogenotrophy or CO2 reduction)

These processes give rise to CH4, as well as CO2, H2, H2S, NH3 and P2O5.

Biogenic gas may form commercial accumulations (e.g., Western Siberia; southern Alberta) or be a hazard (fire, blowouts) during shallow drilling.

Anaerobic degradation of OM is much less efficient that aerobic (oxic) degradation. It produces a more reduced (H-rich) residue, rich in lipids.

The early microbial reactions:

• remove much of the N, S, O, and P

• lead to an early concentration of C and H in the residue.

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Kerogen

Kerogen, which is a macro-molecular complex with a polymer-like structure, is the fraction of sedimentary organic matter that does not dissolve in organic solvents, bases or mineral acids (operationally not structurally defined).

It forms in the upper few hundred metres of the sediment column from organic precursors that have been modified during diagenesis. It may include organic particles with a recognizable morphology, including algal and fungal spores, cuticles and remains of woody tissue.

Kerogen resists oxidizing acids, and can be recovered from sedimentary rocks by dissolving them in HCl or HF, and/or by using heavy liquids of different density: kerogen is lighter than minerals. Kerogen can then be studied using a range of optical and spectroscopic methods.

Although difficult to analyse, heating in an inert atmosphere (pyrolysis) will break it up into small fractions that can be analysed by Rock Eval, mass spectrometry and gas chromatography.

Kerogen is usually classified according to its “type” (Tissot and Welte, 1984), based mainly on its H/C ratio and O/C ratio. Several variations on the original classification have been published (Pepper and Corvi = A to F), but three main types (I to III) according to T&W are generally recognized:

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TYPE I (sapropelic)

• High initial H/C ratio (1.3–1.7)

• Low initial O/C ratio (<0.1)

• Derived from microbial breakdown of planktonic algae, spores, cuticles

• Enriched in lipids (e.g. fatty acids, oils, alcohols, waxes)

• Produces mainly oil, often high-wax oil, with thermal maturation

• Typical oil shale kerogen (freshwater/marine), but rare vs. Type II

TYPE II (mixed marine)

• Intermediate initial H/C ratio (1.0–1.5)

• Fairly low initial O/C ratio (0.1–0.2)

• Can be S-rich

• Commonly derived from phytoplankton, zooplankton and other marine organisms deposited under reducing conditions; also minor plant material

• Most common and richest source rocks for regular black oil.

If type II-S, low maturity released heavy oils of low value are generated.

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TYPE III (humic)

• Low initial H/C ratio (<1)

• High O/C (>0.2)

• Derived from land plant organic matter such as lignin, tannin and cellulose

• Generates abundant CO2 and methane (CH4).

• Coal (Type IV) has a similar composition and structure

Kerogen types by elemental composition

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Conversion of solid kerogen to liquid petroleum needs burial temperatures of at least 50°–70°C (which is equivalent to 1.8 – 2.5 km of burial) and a long period of (geological) reaction time.

The optimum temperature range for thermal maturation is 80–130°C, equivalent to a burial depth of about 3 – 4 km and to a typical geothermal gradient of 25–30°C.

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The alteration that begins at 50–70°C continues until the H/C ratio isabout 0.6 and the O/C ratio is less than 0.1 at about 150°C. The peakof oil production (catagenesis) is reached at about 80-120°C.

At temperatures >130°C the longer hydrocarbon chains will havealready cracked, leaving only gas, mainly methane (dry gas). During this transformation, the kerogen composition will graduallymove towards pure carbon (H/C = 0).

On completion of oil maturation:

• aliphatic structural components, derived from lipids, fatty acids andproteins, etc., will have been converted into hydrocarbons;

• there will be a reduction in molecular weight of the hydrocarbons, most often via shortening of aliphatic chain length;

• aromatization (sharing H+ in larger molecules) will have occurred;

• the number of n-paraffins will have increased;

• geochemical fossils will have been liberated.

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Temperature increases with increasing burial causing carbon-carbon bonds of the kerogen to break, i.e. cracking. Cracking leads to formation of lighter hydrocarbons from long hydrocarbon chains in the kerogen and previously formed oil. The main stage of petroleum formation is called the „oil window“.

Rr=0.5%

Rr=1.3%

Rr=0.9%

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Pr/Ph and bulk lithology as Organofacies Indicators for predicting petroleum source potential

Global map of petroleum source rock data point distribution for Pr/Ph-ratios and bulk lithology determined by XRD.

Pr/Ph and bulk lithology as Organofacies Indicators

Organofacies types A,B,C, D/E/F as established by Pepper and Corvi (1995).Kerogen type III/IV is devided into hydrogen-rich terrigenic organic matter low in lignin (D/E) and type F, corresponding to lignin-rich humic input:P&C A = marine carbonate, iron-poor and sulfur–rich (type II-S)P&C B = marine siliciclastic to marl, pyrite-rich (type II)P&C C = lacustrine basins and lagoons (Type I)P&C D/E/F = terrigenic OM, humic, oxic environment (Type III/IV)


Frequency distribution for Pr/Ph ratios classified by P&C organofacies type.

A = marine carbonate, iron-poor and sulfur–rich (type II-S)B = marine siliciclastic to marly, pyrite-rich (type II)C = lacustrine basins and lagoons (Type I)D/E/F = terrigenic OM, humic, oxic environment (Type III/IV)


Carbonate content vs. Pr/Ph ratios classified by P&C organofacies type.


Clay content vs. Pr/Ph ratios classified by P&C organofacies type. Discriminative power is lower than for carbonate and organofacies C reveals a large spread, diminishing its applicability in source assessment.


Quartz/siliciclastics vs. Pr/Ph ratios classified by P&C organofacies type. Discriminative power is poor compared to carbonate and clay (e.g. Pr/Ph ratios in the range of 1 to 3 may derive from all lithofacies types).


Ternary diagram of Carbonate/Clay/Quartz (CCQ) depicting organofacies via Pr/Ph ratios (discete datapoints) versus XRD-based organofacies type. Note matching colors in both discrimination schemes.


Ternary diagram of Carbonate/Clay/Quartz (CCQ) depicting organofacies exclusively by XRD-based separation.


Ternary CCQ-diagram showing contoured Pr/Ph values and color-coded organofacies interpretations demonstrates a good match and reasonable predictive power, in particular when using big data sets (from well-logging).

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rdöls - Organische Geochemie · 2018-10-22 · 3. In weathered and fractured igneous rocks. Formed by hydrocarbon migration into the rocks from a sedimentary (organic) source. Mantle