ĐỊA CHẤT DẦU KHÍ ( PETROLEUM GEOLOGY ) - CHƯƠNG 5 pptx

1.2-Hydrocarbons and Kerogen Type • The macerals and amorphous particles in kerogen affect its ability to generate hydrocarbons.. Mostly inertinite; some amorphous decomposition products

CHAPTER 05

GENERATION AND MIGRATION

OF HYDROCARBON

1- GENERATION OF HYDROCARBON

1.1-Petroleum Source Material

1.1.1-Formation and Preservation of Organic

Matter

• In the nineteenth century, it was widely believed

that petroleum had a magmatic origin and that it

migrated from great depths along subcrustal faults

• But the overwhelming evidence now suggests that the original source material of petroleum is organic matter formed at the earth's surface.

• The process begins with photosynthesis, in which plants, in the presence of sunlight, convert water and carbon dioxide into glucose, water and

oxygen:

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

• Photosynthesis is part of the larger-scale carbon

cycle ( Fig 01 ) Ordinarily, most of the organic

matter produced by photosynthesis gets recycled back to the atmosphere as carbon dioxide This can occur through plant and animal respiration, or

through oxidation and bacterial decay when

organisms die

Fig 01-CARBON CYCLE

1.1.2-Preservation and Organic Productivity

• All organic matter in the ocean is originally formed through photosynthesis The main producers are phytoplankton, which are

microscopic floating plants such as diatoms, dinoflagellates and the blue-green algae

Bottom-dwelling algae are also major

contributors in shallow water, shelf

environments.

1.1.3-Preservation and Organic Destruction

• Areas of high productivity are not necessarily those best suited for preservation Destruction

of organic matter must also be prevented

Complete biological recycling of organic

carbon is retarded by anything that limits the supply of elemental oxygen.

• This occurs most favorably in either one of two settings: rapid rate of deposition; and stratified, oxygen-poor water bodies with anoxic bottoms

• First, rapid deposition may be necessary to keep the

organic material from being destroyed

• Preservation is also favored by density stratification,

which produces oxygen-poor bottom waters

• Water stratification and oxygen depletion are well known in the modern Black Sea,

• The Eocene-age lakes of Utah, Colorado and Wyoming,

in which the Green River oil shale formation was

deposited, have been interpreted as seasonally stratified water bodies which at a later stage become permanently stratified (Fig 02)

Fig 02

• In the present-day world's oceans, there is a zone

of maximum oxygen depletion at a depth of about

200 meters, with oxygen more abundant in the

shallow surface waters and again at deeper levels

(Figure 03)

Fig 03

1.1.4-Diagenesis of Organic Matter

• There are three important stages in the burial and

evolution of organic matter into hydrocarbons:

– diagenesis;

– catagenesis;

– and metagenesis

Diagenesis of Organic Matter

Diagenesis of organic matter begins as soon as sediment is

buried However, the point at which diagenesis ends is

subject to how the term is used Some geologists use the term in a restricted sense to include only processes that

occur as sediment consolidates into sedimentary rock

Others expand the realm of diagenesis to include all

processes extending up to, and blending imperceptibly

into, regional metamorphism

In this discussion, diagenesis is defined on the basis of

organic matter, and it includes all changes that occur up to the stage of petroleum generation

• Freshly deposited muds are unconsolidated and may contain more than 80% water in their

pores These muds compact very quickly Most

of the porosity is lost in the first 500 meters of burial (Figure 04) After that, compaction to

form mudstones or shales continues much more slowly.

Fig 04

1.1.5-Kerogen Components

• Under the microscope, kerogen appears as disseminated organic fragments Some of this material is structured It is recognizable as plant tissue fragments, spores, algae, and other pieces with a definite biological organization These plant-derived structured fragments can be grouped into

distinct biological units called macerals Macerals in

kerogen are equivalent to minerals in rocks

• Three major maceral groups are important: vitrinite,

exinite and inertinite

Kerogen Components

• Vitrinite is the dominant maceral type in many kerogens and is the

major component of coal It is derived almost entirely from woody

tissue of the higher land plants Because it is derived from lignin and

is difficult to break down, vitrinite can appear in almost any

depositional environment, marine or nonmarine, and is generally the most abundant type of structured particle.

• Exinite macerals are mainly derived from algae, spores, pollen, and

leaf-cuticle waxes High percentages of exinite are not common, but if present, they usually imply lacustrine or shallow marine

environments.

• Inertinite macerals come from various sources that have been

extensively oxidized before deposition Charcoal, derived from woody plants, is the major recognizable type Inertinite is usually a minor

component of kerogen, and is abundant only where much of the

organic matter has been recycled.

• In addition to the structured macerals, some of

the components of kerogen are amorphous.

Amorphous particles have been so mechanically broken up and/or chemically altered by bacteria and fungi that their original maceral types and

cell structures have been obliterated -Amorphous particles are not true macerals but alteration

products, although the maceral term

"amorphinite" has sometimes been applied to

these materials.

Kerogen Components

1.2-Hydrocarbons and Kerogen Type

• The macerals and amorphous particles in kerogen affect its ability to generate hydrocarbons Oil-prone kerogens

generally are made of more than 65% exinite and

amorphous particles (Figure 05)

• Kerogens with 65% to 35% of oil-prone components will expel mostly condensate and wet gas With less than 35% oil-prone constituents, the kerogen will yield dry gas if

dominated by vitrinite and will be non-reactive and barren

if dominated by inertinite

Figure 05- Types of petroleum generated from kerogen, based on

visual analysis with reflected light microscope

• The oil-prone kerogens can be divided into two types

the algal components of exinite, and is

formed in either lacustrine or marine

environments Type I kerogen is derived

mainly from lipids and tends to produce

crudes that are rich in saturated

hydrocarbons

Mostly inertinite; some amorphous decomposition products

Fossil charcoal and other oxidized material of continental vegetation

III Coaly

Amorphous particles derived mostly from phytoplankton, zooplankton, and higher organisms; also some macerals from these groups

Decomposition in reducing environments, mostly marine

II Mixed Marine

Mostly algal components: of exinite (alginite); some amorphous material derived from algae

Algae of marine, lacustrine,boghead coal environments

I Algal

Organic Constituents Origin

Kerogen Type

Table 1 Kerogen types, their origin, and organic particle

constituents

• Type II is a kerogen derived from mixed marine sources

Its particles are mostly amorphous and result from the

decomposition of phytoplankton, zooplankton, and some higher animals Its chemical nature is intermediate between Types I and III Type II kerogens tend to produce

naphthenic and aromatic-rich oils, and they yield more gas than Type I

• Type III or coaly kerogen, is rich in vitrinite macerals,

and therefore has a very low capacity to form oil It mainly generates dry gas Any oils generated from Type III

kerogens are mostly paraffinic waxy crudes derived from its exinite and amorphous constituents

• There is a fourth kerogen type which is extremely rare It is rich in

inertinite macerals and produces very low hydrocarbon yields

Inertinite is, as its name implies, inert and has practically no ability

to generate either oil or gas (Figure 05)

• Sedimentary rocks commonly contain mixtures of the kerogen

types Many oil shales contain dominantly Type I, the algal

kerogens Coals and some nearshore clastic source rocks, such as those found in deltas, contain mainly Type III, coaly kerogen In

some cases, coal deposits can be direct contributors to significant natural gas accumulations, as for example the Carboniferous coals

of the North Sea Many marine source rocks have either Type I algal

or Type II mixed marine kerogen, with Type II the more common For example, some of the excellent source rocks of Iran contain

mostly Type I, algal kerogen, while the Paleozoic source rocks of the North African Sahara have Type II, mixed marine kerogen

Chemical Changes with Kerogen Maturation

• In the stage of diagenesis, prior to the generation of oil and gas, each of the kerogen types has a unique

called a Van Krevelen diagram ( Figure 07, and

Figure 08)

Figure

06-Kerogen types

Figure 07

• Of particular importance is the H/C ratio, which

decreases rapidly as hydrogen-rich molecules are

cracked off as oil or gas

• Remember that the highest possible organic H/C ratio is

4, which is found in the hydrocarbon gas methane The O/C ratio helps define the kerogen origin, but most of the oxygen is lost in diagenesis as CO2 and H2O and very

little survives to affect the petroleum generation process

• Of the four kerogen types, the Type I algal kerogens

have the highest atomic H/C ratios during diagenesis,

initially about 1.65 Type II, III and IV start out with

progressively lower H/C ratios

Figure 08

• As any of these kerogens are heated, they may reach

the second stage in the evolution of organic matter, the stage of catagenesis (Figure 08) Catagenesis is defined

as the stage at which oil and natural gas is generated

from kerogen

• Since oil and gas molecules have very high H/C ratios, generation of petroleum will cause the H/C of the

residual kerogen to decrease Ultimately, all kerogen

types will converge along a common path during the

final stage in the evolution of organic matter, the stage

of metagenesis

• During metagenesis, oil and gas generation directly

from kerogen ceases, but considerable methane gas can still be generated from the thermal alteration of

previously generated crude The kerogen residue of this stage approaches the pure carbon state, that is, graphite

• Since it starts out with a lower H/C ratio (Figure 07 &

Figure 08), Type II kerogen can generate less

hydrocarbons than Type I, even though both are

oil-prone Similarly, Type III is less significant in the total quantity of hydrocarbons it can generate, and Type IV

is almost barren

1.3-Depth, Temperature and Time in

Petroleum Formation

• The generation of hydrocarbons can be related to burial depths of source rocks, since temperature increases with increased depth The actual generation depths for

particular source rocks will depend on the local

geothermal gradient, as well as kerogen type and burial history The depths given in Figure 09 are average,

maximum and minimum generation depths

• During diagenesis and at very shallow depths, only

biogenic methane, or marsh gas, is generated by the

action of anaerobic bacteria

•

1.3-Depth, Temperature and Time in

Petroleum Formation (Cont.)

• At about a depth of 1 to 2 kilometers the catagenesis

stage begins The early stage of catagenesis, down to a depth of about 3 kilometers, corresponds to the principle zone of oil formation Source rocks buried within this

depth range are said to be within "the oil window"

• Late catagenesis typically begins at depths of about 3

kilometers to 3.5 kilometers This is the principle zone of gas formation, and both wet gas and methane are

produced But below depths of about 4 kilometers, the source rocks become overmature At this point,

metagenesis begins and only methane is produced

GENERATION OF PET RELATION TO AVERAGE, MAX.,

MIN OF BURIAL DEPTH

Figure 09

•The correlation of petr generation to depth is primarily a function of the increased Temp., and the graph in Fig 09

can also be constructed with Temp as the ordinate axis

(Figure 10)

•Major oil generation does not occur until source rocks are heated above approximately 60°C These low Temp oils which form at shallower depths tend to be heavy and rich in NSO-compounds With increasing temperature and greater depth, the oils become lighter Maximum oil generation occurs at temperatures of about 100°C Above this temperature oil generation gradually declines and condensates form

•The oil window closes, and the principal zone of gas generation begins, at temperatures of about 175°C Generation directly from kerogen stops at about 225°C, but methane is still generated from the cracking of previously formed oil at temperatures up to 315°C, the point at which source rocks begin to undergo regional metamorphism At those elevated temperatures, however, porosity may be so reduced that gas generated at this stage might not be economically recoverable.

GENERATION OF PET RELATION TO TEMPERATURE

Figure 10

An example of the maturation progression is

found in the western Canada basin

•Immature source rocks are present in the east within

shallow Upper Cretaceous sediments (Fig 11) These yield dry gas with a high N2 content Deeper burial has resulted in Cretaceous and Devonian rocks rich in oil and wet gas

Evans and Staplin (1971) have estimated that the wet gas

and liquid hydrocarbons in the western Canada Basin were formed in the temperature range of 60 to 170°C Near the basin's western margin, Paleozoic rocks are deeply buried and the dominant gases produced are methane and hydrogen sulfide

The maturation progression in the western Canada basin

Figure 11

• The laws of chemistry tell us that the rate of a reaction

is the function of both temperature and time Fig.12

shows the present formation temperature plotted

against the age of various source rocks

• This graph has been constructed using data from many actual case studies Formation temperatures are lowest, less than 60°C, for old Paleozoic source rocks, and

increase to more than 150°C for young Cenozoic ones

• Figure 13 compares the depth and temperature of the beginning of the oil window for several source rocks of different ages

Fig.12-The present formation temperature against the age

of various source rocks

Fig.13-DEPTH AND TEMPERATURE @ THE BIGINNING OF THE

PRINCIPAL ZONE OF OIL FORMATION

1.4.1-Kerogen Analysis

1.4.2-Rock Properties Analysis 1.4.3- Comparison of Methods

1.4-Paleothermometry

1.4.1-Paleothermometry: Kerogen Analysis

• Some paleothermometry methods are based on the physical and chemical properties of the kerogen

itself

• One method employs the Van Krevelen diagram (Fig.07)

• The color of some of the kerogen macerals can

also be used as a paleothermomete

•Another method is based on the vitrinite reflectivity of

the kerogen This is measured by means of a reflecting microscope equipped with a photo-multiplier

•A linear increase in temperature causes the reflectivity of vitrinite to increase approximately exponentially, and plots

as a straight line on semi-log paper (Figure 14) Actually, the percent mean reflectance, called Rm, or sometimes Ro, averaged from several measurements is reported, because individual reflectance will vary somewhat with plant tissue type and with grain orientation under the microscope

•Crude oil generation takes place for Rm values betw 0.6% and 1.2% Wet gas generation occurs mostly for Rm betw 1.2% and 2%, and the zone of dry gas generation lies betw

Rm values of 2% and 4%