PETROLEUM

Petroleum is a naturally occurring, yellow-to-black liquid found in geologic formations beneath the Earth's surface, which is commonly refined into various types of fuels. It consists of hydrocarbons of various molecular weights and other liquid organic compounds.

COMPOSITION

Composition by weight Element Percent range Carbon 83 to 85% Hydrogen 10 to 14% Nitrogen 0.1 to 2% Oxygen 0.05 to 1.5% Sulfur 0.05 to 6.0% Metals < 0.1%.

Hydrocarbon Average Range Alkanes (paraffins) 30% 15 to 60% Naphthenes 49% 30 to 60% Aromatics 15% 3 to 30% Asphaltics 6% remainder.

lighter hydrocarbons methane, ethane, propane and butane occur as gases.

pentane and heavier ones are in the form of liquids or solids

CHEMISTRY

paraffins, are saturated hydrocarbons, general formula CnH2n+2, from 5 to 40 carbon atoms per molecule,

The alkanes from pentane (C5H12) to octane (C8H18) are refined into petrol

the ones from nonane (C9H20) to hexadecane (C16H34) into diesel fuel, kerosene and jetfuel.

Alkanes with more than 16 carbon atoms can be refined into fuel oil and lubricating oil.

paraffin wax is an alkane with approximately 25 carbon atoms.

asphalt has 35 and up carbon atoms. those with four or fewer carbon atoms, are in a gaseous

state at room temperature.

The cycloalkanes, known as naphthenes, saturated hydrocarbons, have one or more carbon rings to which hydrogen atoms are attached, the formula CnH2n,

Cycloalkanes have similar properties to alkanes but have higher boiling points.

The aromatic hydrocarbons, unsaturated hydrocarbons, have one or more planar six-carbon rings called benzene rings, to which hydrogen atoms are attached, formula CnHn. They tend to burn with a sooty flame, and many have a sweet aroma

The cycloalkanes, known as naphthenes, saturated hydrocarbons, have one or more carbon rings to which hydrogen atoms are attached, the formula CnH2n,

Cycloalkanes have similar properties to alkanes but have higher boiling points.

The aromatic hydrocarbons, unsaturated hydrocarbons, have one or more planar six-carbon rings called benzene rings, to which hydrogen atoms are attached, formula CnHn. They tend to burn with a sooty flame, and many have a sweet aroma

The maturation and degradation follows two pathways (1) One where the H/C ratio decreases- i.e. hydrogen stripped

from compounds and number of carbon atoms in compounds increase. Ultimately reaches H/C ratio of 0, ie. graphite (coal)

(2) one where the H/C ratio increase- this ultimately reaches a H/C ratio of 4- the compund is methane.

METHANE

Marsh gas=if mathane present at the surface Colorless,flammmable gas Produced by distillation of coal Chemically non reactive,sparingly soluble in water Lighter than air(0.554 relative density) FORMATION 1)derived from mantle 2)thermal maturation of burrried organic matter 3)bacterial degradation of organic matter at shallow

depth Biogenic methane=the by product of bacterial decay

of organic matter 20% of natural gas is of biogenic source

VARIOUS DESCRIPTIVE TERMS FOR NATURAL GAS:

Dissolved gas- That portion of natural gas that is dissolved in liquid phase in the sub- surface. It can be (and usually is) physically separated from the liquid when the fluids are produced.

Associated gas- Also known as the "gas cap" is free gas (not dissolved) that sits on top of, and in contact with, crude oil in the reservoir.

Non-associated gas- Free gas that is trapped without a significant amount of crude oil.

Natural gas liquids- The liquids that an be, and are liquified, in the field and at gas processing plants. Include the wet gases, natural gasoline, and condensate.

Sweet Gas (and Oil)- Contains no H2S Sour Gas (and Oil)- Contains H2S.

THE ORIGIN OF HYDROCARBONS: (1) Inorganic- Hydrocarons form from reduction of

primordial carbon or oxidized forms at high temperatures in the earth

(2) Organic- accumulation of hydrocarbons produced directly by living organisms, as well as the thermal alteration of biologically formed organic matter.

Hydrocarbons formed organically be two pathways: (1) Through generation of hydrocarbons directly by

organisms, 10 to 20% of the hdyrocarbons in the crust, contain more than 15 C atoms

(2)Through conversion of organic matter (lipids, proteins, and carbohydrates) into kerogen, then to bitumen, and finally to petroleum as it gets buried to higher temperatures.

II. NATURAL GASES- THEIR COMPONENTS

A. Hydrocarbon gases Hydrocarbon gases are largely composed of the paraffin series-

straight and branched, single bonded changes of hydrocarbons (1) Methane is the largest constintuent of natural gas. It can

form in three ways: (a) Mantle methane. Derived from the mantle (presumably

primordial methane). Commonly assumed to form by the other two processes: (b) Microbial methane As a reaction product of the bacterial

decay of organic matter. Large caused by the reduction of CO2 during oxidation of the organic matter.

(c)Thermogenic methane Thermal breakdown of heavier hydrocarbons. Appears that the thermal degradation has to be catalyzed for it to occur in nature.

(2) HEAVIER HYDROCARBONS:

Rarely (perhaps never) formed by bacterial processes

Gases are important during drilling of wells- (1) commonly overpressured, can cause

blowouts 2) Useful way to identify producing (or

hydrocarbon bearing) horizons. Extract gas from drilling mud, run through chromatograph, identify the amount and type of gases entering the well bore.

B. NON-HYDROCARBON GASES

(1) Noble Gases- Helium, Argon, and Radon

These gases are inert- do not take part in chemical reactions. They originate from decay of radioactive isotopes of various elements, predominately the U series elements.

They can be quite concentrated in natural gas (>1% of the gas present)

A) HELIUM-

Origin is from alpha decay of radioactive elements: Uranium decay series

238U --> 234Th --> 230Th --> 226Ra --> 222Rn --> (218 Po, 218At, 218Rn) --> (214Pb, 214Bi, 214 Po) --> (210Tl, 210Pb, 210Bi, 210Po) --> (206Hg, 206Tl, 206Pb)

He present in subsurface reservoir The He atom is light, energetic and is difficult to trap in

the subsurface. In atmosphere at 5ppm,in hotsprings,mines etc In oil field gas up to 8% Found especially in granite Lighter than air

B) ARGON AND RADON.

Argon is produced by the radioactive decay of K (beta decay)

No economic significance. Very big scientific significance (K-Ar

dating, e.g. Foster). Radon is part of U decay scheme Also no economic significance, but might

be a major health problem Radon inhelation cause laung cancer

(2) NITROGEN

Three potenetial origins of nitrogen: (1) through oxidation of NH4, which is derived from

thermal breakdown of organic matter. (2) Atomospheric origin (atmosphere is 70% nitrogen) (3) Mantle outgassing- continuation of the outgassing

that started soon after the formation or the earth. Occur in earth crust Form by the bacterial degradation of nitrates Thermal metamorphism of bitumen produce N and co2

Main source of nitrogen is igneous rock

(3) HYDROGEN

Hydrogen is so mobil and reactive, it cannot be permenantly retained in the subsurface

It must be actively produced with in reservoir, adjacent source beds, or diffusing upward from depth.

Possible origins include: (1) from reactions in the crust that involve Ferrous

iron reduction, (2) during thermal maturation of organic matter. Occur in petroleum as traces Found in subsurface water

(4) CARBON DIOXIDE

Found in hydrocarbon natural gas Carbon dioxide is the normal product of thermal maturation of

kerogen Permeable limestone and dolomite when leached by acid water

produce co2

also produced during volcanic eruption and earth quakes has erratic distribution in the subsurface because of numerous

sources and variations in solubility (1) Solubility of CO2 @ STP, about 1/1 in water (volume ratio) @ 7,000 psi (~15,000 ft depth), about 30/1 in water @ 7,000 psi, about 170/1 in 40° API oil @ T > 31°C, any undissolved CO2 will exist as a gas regardless of

the pressure

(2) Sources: two organic and one inorganic a) Thermal degradation of organic matter. Largely from

the decomposition of oxygen bearing groups in organic matter. Usually derived from continentally derived organic matter

b) Inorganic clay reactions- largely from reaction between carbonates and kaolinite to form chlorite:

5FeCO3 + SiO2 + Al2Si2O5(OH)4 +2H2 <---> Fe5Al2Si3O10(OH)8 + 5CO2

Occurs between temperature of 100 and 160°C c) Volcanic activity Decomposition of carbonate rocks

during injection of high temperature magma CO2 now used for enhanced oil recovery so it can be

economic.

HYDROGEN SULFIDE In subsurface occur as free gas Poisonous,evil smelling gas,corrosive to steel presence of H2S in hydrocarbon reserves VERY UNDESIRABLE-

destroys the well equipments Reduces the value of the hydrocarbon deposit ORIGION a) Inorganic cracking of sulfur bearing organic compounds-

generally found at temperatures > 120°C b) Reduction of sulfate- perhaps the greatest source of sulfate,

particularly in oceanic sediments. One way to write such a reaction:

SO4 + 1.33(CH2) + 0.66H2O ----> H2S + 1.33CO2 + 2OH Also can beformed from dissolution of sulfate minerals (gypsum

and anhydrite): CaSO4 + 2CH2O <----> CaCO3 + H2O + CO2 + H2S H2S is highly reactive, and will convert to metal-sulfides if

sufficient metals (particularly Fe) is present

GAS HYDRATES

Composed of frozen water that contain gas molecule Ice molecule=clathrates Color white powdery snow Contain two types of unit structur 1)small structure with lattice structure of 12A(angstrom)

hold up to 8 methane molecule with 46 water molecule,it also contain ethane,hydrogen sulfide,carbon dioxide

2)large structure with lattice structure of 17.4A with in 136 water molecule,it contain n-butanes

Stable at high pressure and low temprature Occur in arctic sediment in deep ocean

CRUDE OIL *a mixture of hydrocarbons that existed in liquid phase in

natural underground reservoir and remain liquid at atm pressure after passing through the surface*

Appearance- color- yellow, green, brown to black Texture- Oily Viscosity- generaly decreases with temperature, so that oil

at surface is less viscous than oil in subsurface. API = (141.5/specific gravity 60/60_F) - 131.5 Light oils are described as being > 40_API (these would

have a specific gravity of 0.83) heavy oils are < 10_API (with a specific gravity of 1) Most oil is > 10_API, and thus will float on water. eneral, viscosity and API gravity are inversely related. Heavy oil denser than water

CHEMICAL COMPOSITION

Crude oil have two major groups 1)hydrocarbons=which contain only

hydrogen and carbon 2)heterocompounds=which contain

elements in addition to H and C. Oil is largely carbon and hydrogen (>

99.9% by weight) Other components include sulfur,

oxygen, hydrogen and other elements.

Saturated hydrocarbons- that is all carbon are bonded by single bonds so that they are saturated with hydrogen.

general formula is CnH2n + 2 Alkanes with < 5 carbons are gas Alkanes with 5 to 15 carbon atoms are liquids Alkanes with > 15 carbon atoms are viscous liquids and

solids Two types of alkane isomers Straight chain, called "normal alkanes" e.g. normal

butane, these have higher boiling points than the branched alkanes

Branched chain, called "isoalkanes" e.g. Isobutane Largest molecule recorded from crude oil contains 78

carbons.

A) ALKANES (OR PARAFFINS)

B) NAPHTHENES (CYCLOALKANES) Composed of 5 and 6 member rings General formula is CnH2n All are liquid at surface temperature

and pressure They compose ~40% of oil.

C) AROMATICS

benzene ring- Six carbon ring with general formula C6H6

have a sweet smell- thus named aromatics. It contain asphaltic compounds *resin=soluble in n-pentane *asphaltenes=not soluble in n-pentane Liquid at normal temprature and pressure In light oil 10% In heavy oil 30% Toulene is the most common aromatic compound in

oil

(2) HETEROCOMPOUNDS

Contain element other then H and C,such as Oxygen, Nitrogen, and sulfur

Oxygen can range between 0.06 and 0.4 wt. %, Nitrogen between 0.01 and 0.9 wt %, and sulfur between 0.1 and 7 wt%

Other element nickle and vandeum

ANOTHER CLASSIFICATION

Young shallow=heavy and viscous,low in paraffin,rich in aromatics

Young deep=less viscous,high API,rich in paraffin

Old shallow=sulferous Old deep=less in viscosity

SUBSURFACE ENVIRONMENT