Hydrocarbon Generation and Migration

What is oil and gas? Where does it come from?

Origin: Planktoncache.eb.com/eb/image?id=93510

en.wikipedia.org/wiki/Image:Copepod.en.wikipedia.org/wiki/Image:Ceratium_hirundinella.jpg

• Most oil and gas starts life as microscopic plants and animals that live in the ocean.

Plant plankton Animal plankton

10,

000

of t

hese

bug

sw

ould

fit

on a

pin

head

!

However, under certain conditions there may be very little oxygen on the sea floor.

This may be because the ocean is deep and stagnant and oxygen has not been mixed down from the surface waters.

No animal life can survive where the sea bed is completely lacking oxygen. Without animals to eat the dead plankton, the organic mush builds up on the sea bed.

Introduction

When plankton dies it slowly settles to the sea bed where it forms an organic mush. Usually there are lots of animals living on the sea floor that feed on this material. One important group is the polychaete worms. These are detritivores, which means they eat the dead and decay remains of other organisms

On the sea bed

upload.wikimedia.org/wikipedia/en/0/04/Plankton.jpg

When the plankton dies it rains down on sea bed to form an organic mush

Sea bed

en.wikipedia.org/wiki/Image:Nerr0328.jpg

If there are any animals on thesea bed these will feed on theorganic particles

Origin: Black Shaleupload.wikimedia.org/wikipedia/en/0/04/Plankton.jpg

• However, if there is little or no oxygen in the water then animals can’t survive and the organic mush accumulates

• Where sediment contains organic matter, it eventually forms a rock known as a Black Shale

© Earth Science World Image Bank

Interpretation of Total Organic Carbon (TOC)

(based on early oil window maturity)HydrocarbonGenerationPotential

TOC in Shale(wt. %)

TOC in Carbonates(wt. %)

Poor

Fair

Good

Very Good

Excellent

0.0-0.5

0.5-1.0

1.0-2.0

2.0-5.0

>5.0

0.0-0.2

0.2-0.5

0.5-1.0

1.0-2.0

>2.0

Organic Matter in Sedimentary Rocks

Reflected-Light Micrographof Coal

Vitrinite

Kerogen

Disseminated Organic Matter inSedimentary Rocks That is Insolublein Oxidizing Acids, Bases, andOrganic Solvents.

Vitrinite

A nonfluorescent type of organic materialin petroleum source rocks derived primarily from woody material.

The reflectivity of vitrinite is one of thebest indicators of coal rank and thermalmaturity of petroleum source rock.

Origin: Cooking

www.oilandgasgeology.com/oil_gas_window.jpg

As source rock is buried, it is heated.

Kerogen

Gas

Oil

Organic matter is first changed by the increase in temperature into kerogen,which is a solid form of hydrocarbon

Around 90°C, it is changed into a liquid state, which we call oil

Around 150°C, it is changed into a gas

A rock that has produced oil and gas in this way is known as a Source Rock

Schematic Representation of the Mechanism

of Petroleum Generation and Destruction

(modified from Tissot and Welte, 1984)

Organic Debris

Kerogen

Carbon

Initial Bitumen

Oil and Gas

Methane

Oil Reservoir

MigrationThermal Degradation

Cracking

Diagenesis

Catagenesis

Metagenesis

Pro

gre

ssiv

e B

uri

al a

nd

Hea

tin

g

Type of hydrocarbon

Sweet• The term "sweet" originates from the fact that a

low level of sulfur provides the oil with a mildly sweet taste and pleasant smell.

• High-quality, low-sulfur crude oil is commonly used for processing into gasoline and is in high demand, particularly in the industrialized nations.

• "Light sweet crude oil" is the most sought-after version of crude oil as it contains a disproportionately large fraction that is directly processed (fractionation) into gasoline (naphtha), kerosene, and high-quality diesel (gas oil).

Producers of sweet crude oil include:The Far East/Oceania: Australia Asia/Pacific: Brunei China India Indonesia Malaysia New Zealand Vietnam The Middle East Kurdistan North America: United States (Pennsylvania)

SourPetroleum containing higher levels of sulfur is called sour crude oil.

Sour crude oil is crude oil containing a high amount of the impurity sulfur (level in the oil is more than 0.5% ).

The impurities need to be removed before this lower quality crude can be refined into petrol, thereby increasing the cost of processing - higher-priced gasoline than that made from sweet crude oil.

Usually processed into heavy crude oil such as diesel and fuel oil rather than gasoline to reduce processing cost.

The majority of the sulfur in crude oil occurs bonded to carbon atoms, with a small amount occurring as elemental sulfur in solution and as hydrogen sulfide (H2S) gas.

Sour oil can be toxic and corrosive, especially when the oil contains higher levels of hydrogen sulfide, a t low concentrations has the smell of rotten eggs.

Sour crude oil needs to be stabilized by having hydrogen sulfide gas (H2S) removed from it before being transported by oil tankers for safety reasons

• Since sour crude is more common than sweet crude in the U.S. part of the Gulf of Mexico, Platts has come out in March 2009 with a new sour crude benchmark (oil marker) called "Americas Crude Marker (ACM)".

• Dubai Crude (and Oman Crude) both sour crude oils have been used as a benchmark (crude oil) oil marker for Middle East crude oils for some time.

• The major producers of sour crude oil include:

North America: Alberta (Canada), United States' portion of the Gulf of Mexico, and Mexico.

South America: Venezuela, Colombia, and Ecuador.

Middle East: Saudi Arabia, Iraq, Kuwait, Iran, Syria, and Egypt.

Reservoirrock

Seal

Migration route

Oil/watercontact (OWC)

Hydrocarbonaccumulation

in thereservoir rock

Top of maturity

Source rock

Fault(impermeable)

Generation, Migration, and Trapping of Hydrocarbons

• Secondary migration is any movement in carrier rocks or reservoir rocks outside the source rock or movement through fractures within the source rock.

•Tertiary migration includes leakage, seepage, dissipation and alteration of petroleum as it reaches the Earth’s surface