10 Origin of Source rocks

How Good Source Rocks Form:

Origin of Source Rocks

• 1. Sedimentation in areas of high organic productivity • 2. Preservation of organic matter in sediment

Photosynthesis • The most important reaction on the planet (for us) chlorophyl

• CO2 ↑ + H2O → CH2O + O2↑ light

glucose

Organic Matter

Fate of Organic Matter in Oceans • • • •

Proteins Carbohydrates (sugars, starch, cellulose) Lipids (oils, fats) Lignin (in plants) More resistant to decay

• HC generation is limited by available H (reducing environment)

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Requirements for High Organic Productivity • • • •

Plankton Primary Productivity Map

Light (for photosynthesis) Temperature Nutrients Low turbidity

• Phytoplankton (Photosynthesizing algae) 90% of Organic Matter in Oceans • + Bacteria, zooplankton, fish, etc

High Productivity Moderate Productivity Low Productivity

10-year animation of Ocean Color -Chlorophyll concentrations

International Geosphere-biosphere program

Areas of High Productivity • At boundaries between polar and temperate regions • West side of continents • In the upper 100m of ocean

SeaWiFs instrument on OrbView-2 Satellite, NASA

Driving Mechanism

Ocean surface Temperature

• Upwelling of deep water brings nutrients

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Preservation of Organic Matter

The bottom of a basin

•Anoxic conditions •Rapid burial

Bioturbated Sediment

• Anoxic conditions • Rapid burial

Laminated Sediment

How do you get Anoxic conditions? • Water Stratification – Temperature – Salinity

• High organic productivity near surface

Black Sea: permanent salinity stratification

Restricted marine basin, high evaporation

Example: The Mediterranean

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Upwelling zone on shelf

Upwelling

• Namibia • Organic productivity exhausts oxygen at depth • Sea level is key

Total Organic Content (TOC) of Rocks

Anoxic conditions off Peru

Source Rocks

Evolution of Organic Matter in source rocks during burial ri a c te Ba

lD

ay ec

Methane

Yield of gas from organic matter Diagenesis

Org. Matter Inc

Kerogen

rea

sing

Catagenesis Tem

per a tu re

Bitumen

Oil + Gas +Residue

Metagenesis

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Maturation of Organic Matter 1. Diagenesis – – – – – – – –

Shallow burial Temp 0 to 50oC Bacterial decay and reduction Chemical reduction Produce free CH4, CO2, H20 Produce Kerogen H:C ratio remains constant Rocks undergoes compaction and cementation

Kerogen • Disseminated solid organic matter in sed. rocks • Insoluble in organic solvents • Made of C, H, O plus minor N and S

Phytoclasts (plant organic matter)

Amorphous Organic Matter

Humic Kerogen

Kerogen Types • Type I (Algal) – High H:C ~1.65, Low O:C – Rich in lipids (mostly saturated ) – Oil prone

Kerogen Types

• Type III (Humic) Low H:C ~0.84, High O:C Rich in aromatic compounds Plant material and coal GasProne

Type II

Type III (Humic)

Tertiary Greenland and AK

Atomic O/C

Pyrolysis Experiments

Maturation of Organic Matter • • • • •

Jr Saudi Arabia

Atomic H/C

– Intermediate H:C ~1.25, Interm O:C – Algal+Zooplancton – Oil and Gas prone

Type I (Algal) Green River

Van Krevelen Diagrams

• Type II (Liptinic)

– – – –

Maturity

Catagenesis 50 to 200oC Oil Window 60 to 120-160oC Gas Window 120 to 225oC Thermal maturation of kerogen H:C decreases Hunt, 1995

Monterrey Shale (CA)

Green River Shale (WY)

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Thermal Alteration of Kerogen

Kerogen maturation and hydrocarbon generation Gasoline

Wet Gas

Graphite

Methane

Seewald, 2003

Comparison of two Source rocks

Pyrolysis and Gas Chromatography

Pyrolysis

Pyrolysis vs depth

Hydrocarbons already in the rock

Hydrocarbons generated during pyrolysis (remnant potential)

Immature Source Rock

Mature Source Rock

Increasing temperature

Over mature Source Rock

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Geochemical log Total Organic Carbon TOC

Pyrolysis Peaks S1

S2

PaleoThermometers

Thermal maturity Ro

Tmax

The darker the toast the hotter it got.

Vitrinite Reflectance

Down hole vitrinite reflectance profile.

Vitrinite is plant organic matter, common in coal, which reflects light more strongly as it becomes more cooked.

Increasing thermal maturity

1.2% reflectance vitrinite from Lower Cretaceous Pearsall shale (TX) (near the bottom of the oil window.

Hunt, 1995

Source rock maturity measured by vitrinite reflectance of coal

Vitrinite reflectance

Take home points on Source Rocks • Required: – High organic productivity – Preservation (anoxic conditions)

• Chain of thermal maturation: Organic Matter →Kerogen →Bitumen →Oil →Gas →residue • Kerogen types: I algal → oil, ll sapropelic →oil+gas, III humic →gas • Immature, mature , overmature condition determine remaining genereation potential • Pyrolysis and vitrinite reflectance can be used to determine source rock maturity

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