Introduction to Petroleum Geochemistry Unit ONE Introduction to Petroleum Geochemistry Dr. Tabari

Preservation or Productivity ? The quantity and quality of organic matter preserved during digenesis is controlled by a number of factors, notably the oxygen content of the water column and sediments, water circulation, organic matter productivity, and sedimentation. Preservation: anoxia (reducing environment) is the main cause for enhanced preservation of organic matter leading to the formation of organic rich source rocks. Under oxic conditions, organic matter is rapidly degraded even though the rate of productivity may be high, leading to sediments with low organic carbon contents. Productivity: it is the driving force behind the accumulation of organic rich rocks rather than preservation. Their argument is based on laboratory experiments that show rate of destruction is similar under oxic and anoxic conditions, and the fact that anoxic Black Sea sediments show not high organic matter content.

Depositional Environment of Organic Rich Source Rocks Large anoxic lakes. Anoxic silled basins. Areas of coastal upwelling. Anoxic zones of the open oceans.

Evolution of Organic Matter in Sedimentary Rocks Origin of Fossil Fuels Fossil fuels result from the accumulation, preservation, and burial of organic matter in various types of sedimentary environment. Over a long period of time and with increasing temperature, this organic matter will be subsequently degraded by diagenetic reactions and later by thermal reactions, which ultimately lead to the production of liquid hydrocarbons. ~ Land Plants Aquatic Plants Oxic Anoxic Source Bed Potential Burial and Heat Oil Trap Oil and Gas Migration Oil Source Bed Heat - ~ 1 2 3 4 5 Depth (km) Products Biogenic Gas Thermogenic Gas Oil Biomarkers IMMATURE OIL WET GAS DRY GAS DIAGENESIS CATAGENESIS METAGENESIS Evolution of Organic Matter in Sedimentary Rocks

Origin of Fossil Fuels The starting point for the production of organic matter is the photosynthetic cycle. Higher plants on land and phytoplankton in the oceans are the major primary producers. They assimilate CO2 from the atmosphere incorporating carbon into compounds that become part of these living systems. Carbohydrate is the main form in which organic matter is stored in living cells. The reverse process of photosynthesis is respiration, which typically occurs at night in the absence of sunlight. The majority of the photosynthetically fixed organic carbon (99.9%) is relatively rapidly recycled back into the atmosphere with only a small amount (0.1%) escaping from the photosynthetic cycle and being incorporated into the sedimentary record.

Origin of Fossil Fuels An even smaller fraction of this trapped organic carbon may ultimately become part of some type of fossil fuel. MAJOR CYCLE MINOR CYCLE Atmospheric CO2 CO2 Combustion of Fossil Fuels Oil, Gas Incorporation Oxidation Photosynthesis Plants-Bacteria Sedimentary Organic Material Bitumen Leakage (0.01-0.1% Total Organic Carbon) Dead & Decaying Plants, Animals, Bacteria Shale, Coal Kerogen DAYS-YEARS ~3.0X1012 Tone Organic Carbon MILLION YEARS 6.5X1015 Tone Organic Carbon

Composition of Organic Matter Naturally occurring organic matter is composed of varying proportions of six major classes of organic compounds namely proteins, carbohydrates, essential oils, resins, lipids and pigments. Lipids constitute the precursors of many classes of compounds found in crude oils and source rock extracts. Substance Elemental Composition in Weight Percent C H S N O Carbohydrates 44 6 - 50 Lignin 63 5 0.1 0.3 31.6 Proteins 53 7 1 17 22 Lipids 76 12 Petroleum 85 13 0.5

Relative hydrogen contents [ example: 6-carbon compounds ] DECREASING HYDROGEN

Average Organic Matter in Rocks MINERAL MATTER ~99% ORGANIC MATTER ~1% KEROGEN ~90% BITUMEN ~10%

Evaluation of source Rock: TOC Quality of organic matter Thermal maturity

Methods of quantifying maturity in source Rock: Vitrinite reflection (Ro) Thermal alteration Index (TAI) C.P.I Ratio of hydrogen to carbon (H/C) Elemental composition Tmax Time temptation index (TTI) Spore & pollen Rock Eval pyrolysis Biomarker Guide

Composition of Organic Matter H 24-Ethytcholeatane 10.4 ppm 49 ppm C28TA-Steroid 145 ppm 17a(H)-Hopane 3.8 ppm C29MA-Steroid Triaromatic Steroids Monoaromatic Steroids 17a(H)-Hopanes Steranes The compounds derived from the lipid precursors and occurring in crude oils, typically known as biomarkers. Saturates Aromatics 29% Whole Oil 24%

Chemical Structures and Nomenclature It is the building block of biomarkers. The end closer to the methyl branch (-CH3) is called the “head”, and the other end is the “tail”. Monoterpane: two isoprene units joined head- to- tail. Diterpane: four isoprene units or two monoterpanes. Triterpane: six isoprene units or three monoterpanes. Monoterpane Isoprene

Biomarkers Complex organic compounds composed of carbon, hydrogen, and other elements which are found in oil, bitumen, rocks, and show little or no change in structure from their parent organic molecules in living organisms. H 24-Ethytcholeatane 10.4 ppm 49 ppm C28TA-Steroid 145 ppm 17a(H)-Hopane 3.8 ppm C29MA-Steroid Triaromatic Steroids Monoaromatic Steroids 17a(H)-Hopanes Steranes Saturates Aromatics Whole Oil They are very useful in determining the source and diagenesis, and the maturity of organic matter as well as the reservoir transformations.

Examples of Biomarkers Steranes Four rings A, B, C and D. D-ring contains only “5C” atoms. Triterpanes 3-6 rings. 5-rings are the most common (hopanes) in which E-ring contains only “5C atoms”. Gammacerane: 5-rings with “6 C atoms” in E-ring. A B C D Cholestane Diacholestane STERANES A B C D E A-Tricyclic Terpane De-A-Lupane Hopane Gammacerane TRITERPANE

Numbering System The precise number of carbon atoms depends on: 1 STERANES 2 3 4 5 19 9 10 8 6 7 12 11 17 13 16 15 14 18 21 20 22 23 24 25 26 29 28 The precise number of carbon atoms depends on: Source material. Diagenesis. Thermal maturity. Biodegradation. The numbering system indicates where side chains are attached to the ring system. TRITERPANES 1 2 3 4 5 25 9 10 8 6 7 12 11 28 13 16 15 14 18 26 19 20 21 34 35 22 27 30 32 33 29 31 23 24

Nomenclature of Biomarkers Nor- Prefix meaning without, indicating that a methyl group has been lost from the base compound (parent molecule) of the biomarker at the position indicated by the preceding number. Thus, 28,30-bisnorhopanes lack the C28 and C30 methyl groups found in hopanes. Homo- Prefix referring to additional carbon atom on the structure of the parent molecule.

Stereochemistry It refers to the spatial relationship of atoms in a molecule. Whenever two rings are joined, each of the atoms at the junction is attached to three other carbon atoms in the ring structure. Its fourth bond (H2 or CH3-) can point either up or down with respect to the plane of the ring.

Stereochemistry Alpha (α) position: refers to substituents that attached below the plane of the ring structure. Beta (ß) position: above the plane of the ring structure. Two Systems of Presentation Wedges Solid wedges indicate Beta (ß) position. Dashed or dotted wedges indicate Alpha (α) position. Circles Solid (dark) circles indicate Beta (ß) position. Open circles indicate Alpha (α) position. A

Stereochemistry Chirality or Asym.carbon atoms: If four different substituents are attached to a particular carbon atom (e.g. C22 in hopanes or C20 in steranes), the atom is called an asymmetric or chiral carbon atom. In a molecule containing more than one asymmetric center, inversion of all the centers leads to the enantiomer (mirror image). Inversion of only one center yields an epimer. Inversion of more than one or less than all yields a diastromer. d c a b d c b a

Stereochemistry R (right or rectus) epimer It indicates that the rank of the different four groups bonded to an asymmetric carbon atom decreases in the clockwise direction. S (left or sinister) epimer It indicates that the rank of the different four groups bonded to an asymmetric carbon atom decreases in the counterclockwise direction. H C I-C3H7 C2H5 n-C4H9 C20H35 X H 22 22R 22S