Geol Supplementary Notes 463-RWR-1,2 GEOL RWR-1 GENERAL INTRODUCTION TO PETROLEUM GEOLOGY: OUTLINE OF MATERIAL TO BE COVERED

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1 GEOL RWR-1 GENERAL INTRODUCTION TO PETROLEUM GEOLOGY: OUTLINE OF MATERIAL TO BE COVERED Recommended sections to read in the textbook: Chapters 1 and 2 (p. 2-22): Background to development of petroleum geology and geochemistry; life and the carbon cycle. (Not covered specifically in class, but useful general background reading). TWO DEFINITIONS OF PETROLEUM (LATIN: Petra: ROCK; Oleum: OIL): A naturally occurring liquid, gas, semi-solid or solid mixture of hydrocarbon and non-hydrocarbon molecules. A form of bitumen composed principally of hydrocarbons and existing in the gaseous or liquid state in its natural reservoir. Note that the second example (from Hunt 1996) does not mention solids and semisolid hydrocarbons in the definition. OTHER DEFINITIONS Crude oil unrefined, liquid part of petroleum. Natural gas mostly methane (CH 4 ) and ethane (C 2 H 6 ) does not condense at STP. Condensate gaseous in the reservoir but condenses to liquid at the surface. Solid and semi-solid petroleum: Asphalt blackish brown semi-solid and solid bitumens that liquefy when heated. Bitumen commonly used as a generic term for petroleum. Waxes variety of bitumen with waxy feel and lustre. Tar black-brown viscous liquid obtained by distillation not a natural product. 1

2 Hydrocarbons can dissolve intimately in each other: Separation into different phases LIQUID GASES SOLIDS Surface T and P High T and P All liquid mixture ( incl. gas, dissolved solids) But, mutual solution is not unlimited: Gas Light oils Asphatics + H 2 O Gas Light oils (Asphaltics) Asphatics = + + Asphalts liquid liquid liquid solid Gas Light oils Asphatics + CH 4 Gas Light oils (Asphaltics) = + Asphalts liquid gas liquid solid *Asphaltics: generally large molecules in semi-solid and solid fractions; often with N, S and O in H-C molecule. 2

3 Petroleum originates from heating and alteration of organic matter: Diagenesis: Normal T (up to ~50 C) and P. Bacterial decay, oxidation, dehydration, etc. H 2 O, CO 2 and CH 4 are expelled. Produces kerogen or its partly soluble precursor (humin) Catagenesis: T up to 250 C. Oil generation begins at about C, peaks at about C, and is replaced by gas at about C. Metagenesis: T > 250 C. Threshold of metamorphism. Graphite forms. Once formed, petroleum migrates from the source rock through the carrier bed(s) to the reservoir, where further migration is prevented by a caprock. Primary migration is expulsion of petroleum from the source rock into a permable carrier bed; secondary migration is the flow of petroleum within the carrier beds to, and in. the reservoir. Tertiary migration is any subsequent movement after initial trapping. P: Primary migration S: Secondary migration CAPROCK CARRIER BED (SANDSTONE) P S SOURCE ROCK (SHALE) RESERVOIR FAULT (IMPERMEABLE) The fluids may then segregate into different types according to their fluid densities: gas water oil water 3

4 GEOL RWR-2 PETROLEUM CHEMISTRY Recommended sections to read in the textbook: Chapter 3 (all of it!). This chapter contains details of the main hydrocarbon groups and the composition of the main fractions of petroleum. Essential reading. Molecular groups of hydrocarbons: Paraffins = alkanes (aliphatics) Napthenes (cycloparaffins) = cycloalkanes Aromatics = arenes Naphthenoaromatics (complex polycyclic molecules) Olefins = = alkenes The size range of molecules in petroleum is huge. The smallest molecule is methane (CH 4 ) with a molecular weight of 16 [C (12) + 4 x H (1)]. The largest molecules are molecular weights of Normal (n-) Paraffins (alkanes) Homologous series (families of molecules whose members have similar properties and differ in size by a (CH 2 ) group). They are saturated hydrocarbons: all C bonds are saturated with hydrogen. General formula: C n H 2n + 2, where n = 1 >60 CH4 C2H6 C3H8 Methane Ethane Propane Main components of natural gas N = 1 4 are gases; N = 5 17 are mainly liquids (dominate gasoline fraction of petroleum) N = > 17 are waxes, semi-solids, asphalts, etc. 4

5 Isoparaffins (branched paraffins; cycloalkanes) Isomers (compounds with the same composition or molecular formula, but with different molecular properties and structure) General formula: C n H 2n + 2 Butane C4H10 Physical properties change between different isomers: Isobutane C4H10 Boiling point (n-paraffin) Hexane (C 6 H 14 ) 69 C (isoparaffin) 2-Dimethylbutane (C 6 H 14 ) 50 C The number of possible isomers increases with the number of carbon atoms in the molecule: Carbon number Number of possible isomers C1, C2, C3 1 C4 2 C5 3 C6 9 C C x 10 6 C40 62 x Although the number of possible isomers can be extremely large, thermal cracking breaks C-C bonds in complex molecules to produce simpler molecular structures. The number of isomers is thus less than the theoretical possibility. 5

6 Naphthenes (cycloparaffins) General formula: C n H 2n Formed by joining C atoms in a ring. These saturated hydrocarbons are the most common molecular structures in petroleum (typically about 50%), mainly in the heavier fractions. Homologous to about C 30 H 60, but no rings larger than C7 are normally found in crude oil- ring structure becomes strained and unstable. C 3 H 6 Cyclopropane (unstable) C 5 H 10 Cyclopentane (stable) C 6 H 12 Cyclohexane (stable) Decalin (C 10 H 18 ) (polycyclic - shared atoms) C 7 H 14 [unstable - warped (strained) lattice] 6

7 Aromatics (arenes) General formula: C n H 2n -6r (where r = number of benzene rings). Fourth bond of each C atom is shared throughout the ring. Aromatics are unsaturated: they react to add H or other elements to the ring. Rarely more than 15% in crude oils, they are mainly found in heavy fractions. Toluene is most common form. Aromatics have highest octane rating valuable in gasoline blends. Benzene C 6 H 6 Toluene C 6 H 5 CH 3 Olefins (alkenes) Rare in natural crudes reduced to paraffins; highly reactive; unsaturated. Common in hydrocarbons produced by plants and animals (esp. fish and vegetable oils). Propylene (gas) C 3 H 6 7

8 Naphtheno-aromatics (cycloalkyloaromatics, alkyloaromatics, etc.) Complex, condensed polycyclic hydrocarbons produced by joining of rings, chains, etc. Common in heavy fractions of petroleum. Some are carcinogenic. Napthene Napthene Tetralin C 10 H 12 Benzene Paraffin NITROGEN, SULPHUR, AND OXYGEN (NSO-) COMPOUNDS (ASPHALTICS) NSO-compounds include N, S, or O in the molecular structure, bonded with H and C. They are mainly present in the heavy (large molecule) residual fractions of petroleum, but are present in most boiling fractions. Examples: Sulphur compounds 3-(1-methyl)propylindane C 13 H 18 Thiols (mercaptans) Thoiophenes Free S and H 2 S /ctd. Range in natural crude oils: < 0.5 5% S 8

9 Nitrogen Compounds Pyridines Quinolenes Indoles Range in natural crude oils: < % N Oxygen Compounds Organic acids Alcohols, phenols, esters Indoles Range in natural crude oils: < % O Other components include various metallic compounds, spores, altered microfossil remains, etc. In general, the higher the API Gravity, the higher the proportion of NSO-compounds. COMPOSITION OF S BASED ON MAIN HYDROCARBON GROUPS (In Weight Percent; from Hunt 1979) TYPE Paraffinic Naphthenic Aromatic Asphaltic Paraffinic Paraffinic Naphthenic Average crude oil Naphthenic Mixed Asphaltic Asphalt The chemical compositions of all these crude oils is very similar, but the proportions of the different HC-molecular groups is very different. Important: See Fig. 3.7 in Hunt (1996) for Chemical Composition of Crude Oil 9