10_Origin of Source Rocks

8/13/2019 10_Origin of Source Rocks

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Origin of Source Rocks

How Good Source Rocks Form:

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)

CO2 + H2O CH2O + O2

chlorophyl

light

glucose

Fate of Organic Matter in Oceans Organic Matter

Proteins

Carbohydrates (sugars, starch, cellulose)

Lipids (oils, fats)

Lignin (in plants)

HC generation is limited by available H

(reducing environment)

More resistant to

decay


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Requirements for High Organic

Productivity

Light (for photosynthesis)

Temperature Nutrients

Low turbidity

Phytoplankton (Photosynthesizing algae)

90% of Organic Matter in Oceans

+ Bacteria, zooplankton, fish, etc

Plankton Primary Productivity Map

International Geosphere-biosphere program

High Productivity

Moderate Productivity

Low Productivity

10-year animation of Ocean Color

-Chlorophyll concentrations

SeaWiFs instrument on OrbView-2 Satellite, NASA

Areas of High Productivity

At boundaries between polar and

temperate regions

West side of continents

In the upper 100m of ocean

Driving Mechanism

Upwelling of deep water brings nutrients

Ocean surface Temperature


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Preservation of

Organic Matter

Anoxic

conditions

Rapid

burial

Anoxic conditions

Rapid burial

The bottom of a basin

Bioturbated

Sediment

Laminated

SedimentHow 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

Namibia

Organic productivityexhausts oxygen at

depth

Sea level is key

Upwelling

Anoxic conditions off Peru TotalOrganic

Content

(TOC) of

Rocks

Source

Rocks

Evolution of Organic Matter in source rocks

during burial

Org. Matter

Kerogen

Bitumen

Oil + Gas +Residue

Bacte

rialD

ecay Methane

IncreasingTemperature

Yield of gas from organic matter

Diagenesis

Catagenesis

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

Amorphous Organic MatterPhytoclasts (plant organic matter)

HumicKerogen

Kerogen Types Type I (Algal)

High H:C ~1.65, Low O:C

Rich in lipids (mostly saturated )

Oil prone

Type II (Liptinic) Intermediate H:C ~1.25, Interm O:C

Algal+Zooplancton

Oil and Gas prone

Type III (Humic) Low H:C ~0.84, High O:C

Rich in aromatic compounds

Plant material and coal GasProne

Kerogen

TypesVan Krevelen

Diagrams

Atomic O/C

AtomicH/C

Type I (Algal)

Type II

Type III

(Humic)

Maturity

GreenRiver

Jr Saudi

Arabia

Tertiary

Greenland

and AK

Maturation of Organic Matter

Catagenesis 50 to 200oC

Oil Window 60 to 120-160oC

Gas Window 120 to 225o

C Thermal maturation of kerogen

H:C decreases

Pyrolysis Experiments

Monterrey

Shale (CA)

Green River

Shale (WY)

Hunt, 1995


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Kerogen maturation and hydrocarbon

generation

Seewald, 2003

Thermal

Alteration of

KerogenGasoline

Wet Gas

MethaneGraphite

Comparison of two Source rocks Pyrolysis and Gas Chromatography

Pyrolysis

Hydrocarbons

already in the rock

Hydrocarbons generated

during pyrolysis (remnant

potential)

Increasing temperature

Pyrolysis vs depth

Immature Source

Rock

Mature Source Rock

Over mature Source Rock


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Geochemical log

TOC TmaxS2S1 Ro

Pyrolysis PeaksTotal Organic

CarbonThermal maturity

Paleo-

Thermometers

The darker the toast

the hotter it got.

Vitrinite Reflectance

Vitrinite is plant

organic matter,

common in coal,

which reflects light

more strongly as it

becomes more

cooked.

1.2% reflectance vitrinite from Lower

Cretaceous Pearsall shale (TX) (near the

bottom of the oil window.

Down hole

vitrinite

reflectance

profile.

Increasing

thermal

maturity

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 MatterKerogenBitumenOilGasresidue

Kerogen types: I algal oil, ll sapropelicoil+gas, III humicgas

Immature, mature , overmature conditiondetermine remaining genereation potential

Pyrolysis and vitrinite reflectance can be used todetermine source rock maturity

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10_Origin of Source Rocks