I. Introduction

A. Some Definitions:

Hydrocarbons- substance made of hydrogen and carbon (among other elements)

Grade through three states of matter: gases, liquids and solids. Petroleum exploration largely concerned with the fluids (gases and liquids).

Several specific forms of hydrocarbons-

• Dry gas- contains largely methane, specifically contains less than 0.1 gal/1000ft3 of condensible (at surface T and P) material.
• Wet gas- contains ethane propane, butane. Up to the molecular weight where the fluids are always condenced to liquids
• Condesates- Hydrocarbon with a molecular weight such that they are gas inthe subsurface where temperatures are high, but condence to liquid when reach cooler, surface temperatures.
• Liquid hydrocarbons- commonly known as oil, or crude oil, to distinguish it from refined hydrocarbon products.
• Plastic hydrocarbons- asphalt
• Solid hydrocarbons- coal and kerogen- (kerogen strictly defined is dessimenated organic matter in sediments that is insoluble in normal petroleum solvents.
• Gas hydrates- Solids composed of water molecules surrounding gas molecules, usually methane, but also H2S, CO2, and other less common gases.
  
  

Natural Gas:

Given a strict definition by the petroleum industry- "a mixture of hydrocarbons and varying quantities of nonhydrocarbons that exists either in the gaseous phase or in solution with crude oil in natural underground reservoirs".

The common gasses in reservoirs can be divided based on their origins:

(1) Inorganic- Helium, Argon, Krypton, Radon

(2) Mixed inorganic and organic- CO2, H2S

(3) Organic- Hydrogen (???), Methane, Ethane, Propane, Butane

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.
  
  

B. Generalities of the origin of hydrocarbons:

Two possible sources: Inorganic and organic

(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.

It is generally recognized that most hydrocarbons are produced by the organic method. A few hydrocarbons in the crust may be from inorganic sources, but the majority of them are from organic.

See Handout (Hunt, Fig. 4-1)

Hydrocarbons formed organically be two pathways:

(1) Through generation of hydrocarbons directly by organisms. This constitutes perhaps 10 to 20% of the hdyrocarbons in the crust. They generally contain more than 15 C atoms, and are ealisy recongized structures (biomarkers).


(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.

The thermal alteration of formed hydrocarbons continues with continued burial depth. The maturation and degradation follows two pathways again

(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.

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.

Look at Table 2.2 for names of the compounds

The gases decrease in abundance up through pentane (C5).

(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.



Microbial methane can be distinguished from thermogenic methane on the basis of its isotopic composition. Microbial is lighter in both d13C (range from -80 to -60ä) and dD values (range from xxxx to xxxx ä) than thermogenic methane (range -40 to -30ä) and dD value (range from xxxx to xxxxä).


Isotopic composition controlled by various fractionation processes-


Microbes create large fractionation- there is a vital effect and the reaction occurs at lower temperature with larger fractionation factor


Thermogenic breakdown has little fractionation- breakdown occurs at high temperature and there is no vital effect.

(2) Heavier hydrocarbons:

Rarely (perhaps never) formed by bacterial processes. Thus the presence of heavier hydrocarbons in natural gas probably reflects proximity to liquid hydrocarbon reservoir.

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. Not all steps shown, only those with alpha decay:


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


Similar decay scheme occures for 235U and 232Th. These elements also have similar decay schemes and will produce He for each alpha decay. Note that 234U is part of the 238U series (beta decay), and not listed as a separate decay series.



Each step of the three decay schemes produce one He atom. May be surprising that there is not more He present in subsurface reservoir. The He atom is light, energetic and is difficult to trap in the subsurface.


He is economic when in high enough concentrations. Produced from the Hugoton field in the Panhandle of Texas.


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.


(2) Nitrogen


Three potenetial origins of nitrogen:


(1) through oxidation of NH4, which is derived from thermal breakdown of organic matter. The oxidation-reduction reaction includes Ferric Fe redbeds- Nitrogen-bearing deposits are commonly associated with redbeds.


(2) Atomospheric origin (atmosphere is 70% nitrogen)- How does it get into the subsurface?


(3) Mantle outgassing- continuation of the outgassing that started soon after the formation or the earth.


There is no economic significance to the nitrogen (other than is reduces the concentrations of economic hydrocarbons).

(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.

Nonetheless, it can be concentrated- 35% of the gas in the Mid-continent rift system is hydrogen.

Possible origins include: (1) from reactions in the crust that involve Ferrous iron reduction, and (2) during thermal maturation of organic matter.

(4) Carbon dioxide

CO2 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 magmas and by acidic water.



CO2 now used for enhanced oil recovery so it can be economic.

(5) Hydrogen sulfide

The deadliest gas produced in large quantities. 1 ppt causes respiratory paralysis and sudden but agonizing death from asphyxiation. Can't rely on foul odor as warning- at concentrations below 0.1 ppt, H2S dulls sence of smell, increasing concentrations won't be noticed.

Combination of H2S, CO2, and water easily corrode metal. Thus presence of H2S in hydrocarbon reserves VERY UNDESIRABLE- destroys the well equipments (pumps, casing, rods etc.). Also must be disposed of safely. Reduces the value of the hydrocarbon deposit

Origin:

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:

Also can beformed from dissolution of sulfate minerals (gypsum and anhydrite):

Economics

H2S is highly reactive, and will convert to metal-sulfides if sufficient metals (particularly Fe) is present. It can also be converted to sulfur metal, which is sold.

III. Gas hydrates

Compounds of frozen water and gas. Called a "clathrate" structure. Two structures of hydrates:

Small structure- lattice is 12 Å, contains 1/8 mole ratio of gas to water molecules. Gases may be methane, ethane, H2S, and CO2

Large stucture, lattice is 17.4 Å, contains 1/136 mole ratio of gas to water molecule. Now larger void space accomidates larger gases up through pentanes and n-butanes.

Theremodynamic stability of hydrates are what make them important:

See fig. 2.5 in book, and handouts

The P and T conditions of stability allow hydrates to form in many continental shelf marine sediments, provided that there is sufficient organic matter present to make them.


They can be observed in seismic sections as a BSR "Bottom simulating reflector". BSR are generally believed to be the transition from solid hydrate to free gas at the point in the sediment where the hydrate dissociations.

See Fig. 2.6

This allows their distribution to be mapped, also allows for calculation of geothermal gradient on the basis of knowledge of the P and T conditions of stability. (Need to assume composition of water and gas).

Possibility that hydrates are the single largest reservoir of methane on the earth. This has implications:

(1) Their dissociation and formation may be a feedback mechanism of greenhouse gas (methane is 100 times more important greenhouse gas than CO2). Dissociation/Formation would be controlled by changes in sealevel (pressure in the shelf) and changes in bottom water temperature.


(2) They may have economic significance. (a) Hydrates are essentially "solid" methane. Thus contain more molecules of methane in small area than the gas form of methane. Rerquire techiques for removing hydrate from the sediments- change their stability


Also (b) they greately reduce the permeability of the sediment. May act as a seal for reservoirs.

IV. Crude Oil

A. Introduction- physical properties

(1) Appearance- color- yellow, green, brown to black.

(2) Texture- Oily

(3) Viscosity- generaly decreases with temperature, so that oil at surface is less viscous than oil in subsurface.

(4) Density- most commonly expressed according to a formula that uses the specific gravity of the oil, given units of _API (API stands for American Petroleum Institute)

Light oils are described as being > 40_API (these would have a specific gravity of 0.83) while heavy oils are < 10_API (with a specific gravity of 1).

Most oil is > 10_API, and thus will float on water.

In general, viscosity and API gravity are inversely related.

B. Chemical composition

Oil is largely carbon and hydrogen (> 99.9% by weight). Other components include sulfur, oxygen, hydrogen and other elements.

The limited chemical composition is misleading- there are hundreds of different compounds that can be genereated from C and H. These compounds divided into (1) Hydrocarbons, which contain only hydrogen and carbon and (2) Heterocompounds, which contain elements in addition to H and C.

(1) Hydrocarbons-

a) Alkanes (or paraffins)

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

Their 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. Largest molecule recorded from crude oil contains 78 carbons.

Two types of alkane isomers (ie molecules with identical compositions, but different structures:

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

See fig. 2.8 for two examples of these.

b) Naphthenes (cycloalkanes)

Composed of 5 and 6 member rings. They are saturated as well- only single bonds between carbons.

General formula is CnH2n

All are liquid at surface temperature and pressure

They compose ~40% of oil.

Fig. 2.9

c) Aromatics

Composition is based on the benzene ring- Six carbon ring with general formula C6H6. They have a sweet smell- thus named aromatics.

The can be modified by substituting a alkane for one of the hydrogens. e.g. toluene which substitutes a methyl group for one of the hydrogens

fig. 2.10

(2) Heterocompounds

They main "other" elements in crude oil include 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%.

These elements are not other compounds (e.g. H2S or free N) or contaminants

There are also some metals, but only Nickel and Vanadium have been shown to be part of the compounds and not contaminants.