1. Discuss how the type of formation can influence the composition of hydrocarbons stating clearly

your facts:

Hydrocarbon generation is the result of the maturation of buried organic matter. The processes

which allow for hydrocarbon generation take a long period of time and occurs within formations

containing sedimentary rocks. Here, the deposition of organically rich material has been followed by

clean sandstones that form high porosity, well connected pore spaces and are subsequently capped

by shale with very low permeabilities. The burial of the initial deposition provides the pressures and

temperatures to produce hydrocarbons. The hydrocarbons are less dense than water and as such will

migrate upwards into the sandstones, replacing the water that originally occupied the reservoir

sandstone, where the hydrocarbons are constrained from rising further by the shale cap. The

depositional and post-depositional history of the reservoir rock, and particularly its diagenetic history

(compaction, cementation and dissolution), all contribute to the mineralogical composition of the

rock, and hence its grain size distribution, porosity, pore size distribution and the connectivity of its

pores. Organic matter undergoes changes in composition with increasing burial depth and

temperature. Petroleum hydrocarbons can exist as gaseous, liquid and solid phases depending on

temperature, pressure, burial timeand composition of the system. The three steps in the

transformation of organic matter to petroleum hydrocarbons are termed diagenesis, catagenesis and

metagenesis. Diagenesis acts during shallow burial, down to approximately 500m and at

temperatures not more than 50C. The initial processes of diagenesis consists of activity of living

microbes of various kinds, buried along with the sediment. These microbes are the main agents of

chemical transformation of organic matter. Diagenesis acts during shallow burial, down to

approximately 500m and at temperatures not more than 50C. The initial processes of diagenesis

consists of activity of living microbes of various kinds, buried along with the sediment. These

microbes are the main agents of chemical transformation of organic matter. If the sediments are

deposited in an oxygenated environment, then these microbes will use up the free oxygen. As the

depth increases, anaerobic sulfate reducing bacteria extract oxygen from sulfates and reduce the

sulphur to sulphide ions. The sulphide ions will then combine with ferrous iron to form various iron

sulphides, which are slowly converted to pyrite after further burial. Finally, by fermentation, certain

other anaerobic bacteria feed on oxidized forms of organic matter generating methane. In the later

stages of diagenesis, which is the point at which the sediments are buried to greater depths, the

organic matter remaining after microbial activity is progressively converted or synthesized into

higher molecular weight organic compounds that are more stable and less water soluble. The

compounds which undergo diagenetic change are termed kerogens. Kerogen is the raw material

which is utilized for the synthesis of mobile liquid hydrocarbons during further burial. At this stage,

certain organic compounds would have been produced that are soluble in organic solvents and can

be extracted commercially. One of these compounds is bitumen which is the material for which oil

shales can be mined. Catagenesis then occurs which leads to the formation of fluid hydrocarbons

through various organic chemical reactions from some of the kerogen that was produced at earlier

stages of diagenesis. The amount and composition of these hydrocarbons changes progressively

toward lower molecular weight hydrocarbons during this process until only light hydrocarbons are

produced during the metagenesis process.

2. Describe the typical properties and composition of the different HCs and discuss how these are

used in the oil and gas industry.

Hydrocarbons of relatively high molecular weight are in liquid form both in the subsurface and when

they are pumped (or flow naturally) to the surface. In a general way, the higher the molecular weight

of the hydrocarbon mix in the petroleum, the more viscous the liquid. The hydrocarbon with lowest

molecular weight, methane, CH4, is a gas even at the high pressures of petroleum-forming depths.

The hydrocarbons with slightly greater molecular weights (ethane, C2H6; propane, C3H8; and

butane, C4H10), are liquid under the great pressures of petroleum-generating depths but undergo a

change of state from liquid to gas on the way to the surface, as the pressure decreases. Liquid

petroleum consists mainly of a mixture of alkanes of various molecular weights (alkanes are organic

compounds with chains of single-bonded carbon, with hydrogens bonded to the carbons along the

chains) and aromatics (aromatic compounds are those with benzene rings in their structures).

Natural gas is composed predominantly of methane, with much smaller percentages of ethane,

propane, and butane. Natural gas also contains variable percentages of carbon dioxide, hydrogen

sulfide, nitrogen, hydrogen, helium, and argon; these are largely of inorganic origin, and are included

in the natural gas basically because they were there.

Petroleums are frequently characterized by the relative amounts of four series of compounds. The

members of each series are similar in chemical structure and properties. The four series (or classes of

compounds) that are found in petroleums are:

(1) the normal and branched alkane series (paraffins),

(2) cycloalkanes (naphthenes),

(3) the aromatic series, and

(4) asphalts, asphaltenes, and resins (complex, high-molecular-weight polycyclic compounds

containing nitrogen, sulfur, and oxygen atoms in their structures)

The petroleums are generally classified as paraffinic, naphthenic, aromatic and asphaltic according to

the relative amounts of any of the series.

The alkanes with twenty-five or more carbon atoms are solids at room temperature and are

extracted from the crude oils to make industrial paraffin waxes. Crude oils containing these alkanes

become cloudy when cooled. The temperature at which this occurs is called the cloud point and is

used in refineries as a general indication of the abundance of paraffin waxes. The formation around

the wellbore and production tubing must be cleaned periodically to remove precipitated high-

molecular-weight alkanes which reduce the rate of production.

Crude oils derived principally from terrestrial plant organic material contain high amounts of alkanes,

whereas the oils generated from marine organic materials generally contain greater amounts of

cyclic saturated and unsaturated compounds. If, after it has migrated from the source rock to an oil

trap, a paraffinic oil is exposed to the percolation of meteoric water, aerobic bacteria will remove the

paraffins by gradual degradation to carboxylic acids and carbon dioxide. A crude oil that has been

exposed to aerobic bacterial degradation will be chiefly composed of aromatics, asphalts, and resins.

The cycloalkanes (naphthenes) are composed of carbon atoms bonded in a cyclic chain with the

remaining valence satisfied by hydrogen atoms. The structure of cyclohexane and decalin, which,

along with the methyl derivatives, are important constituents of petroleum. Tri-, tetra, and

pentacycloalkanes are present in crude oils in smaller quantities than the mono- and dicycloalkanes.

The naphthenes are important constituents of petroleum-derived commercial solvents.

The series of compounds known as aromatics are composed of multiples of benzene. Aromatic

compounds occurring in petroleum contain side chains of various lengths.

The asphalts and resins are composed of high-molecular-weight condensed ring structures containing

aromatics, saturated ring compounds, and alkane side chains and are interspersed with nitrogen,

sulfur and oxygen compounds.

3. Explain how the presence of impurities in the HC can affect its production and value.

Nitrogen, Carbon dioxide, and hydrogen are common non-hydrocarbon constituents of petroleum.

All three are light molecules and mainly are part of the gas at the surface. Also, hydrogen and helium

are found in some natural gases. Petroleum also contains compounds in which sulfur, oxygen and

nitrogen atoms are combined with carbon and hydrogen. These elements usually are combined with

the complex ring structures that make up the larges molecules of petroleums. These larger non-

hydrocarbon, compounds form a class of chemicals generally called resins and asphaltenes. The

quantity of these compound in petroleum is very small.