Introduction to PETROLEUM Geoscience.pptx

Introduction to petroleum science

1. PETROLEUM GEOLOGY

2. Chemical composition of Oil Hydrocarbons Gas – C 1 -C 4 Liquid – C 5 -C 15 Solid – above C 15

3. The hydrocarbons in crude oil are mostly alkanes (paraffins), cycloalkanes ( naphthenes ) and various aromatic hydrocarbons while the other organic compounds contain nitrogen, oxygen and sulfur, and trace amounts of metals such as iron, nickel, copper and vanadium. The exact molecular composition varies widely from formation to formation but the proportion of chemical elements vary over fairly narrow limits as follows:

4. Composition by weight Element Percent range Carbon, : 83 to 87% Hydrogen: 10 to 14% Nitrogen: 0.1 to 2% Oxygen: 0.05 to 1.5% Sulfur: 0.05 to 6.0% Metals: < 0.1%

5. Composition by weight Hydrocarbon Average Range Paraffins 30% to 60% Naphthenes 49% to 60% Aromatics 15% to 30% Asphaltics 6% remainder Paraffins are a large group of hydrocarbons found in petroleum, and are a key component of crude oil. They are alkanes with a chemical composition of CnH2n+2, and can be straight or branched polymeric chains. Paraffins can be gaseous, liquid, or solid, depending on their weight. Paraffins containing fewer than 5 carbon atoms per molecule are usually gaseous at room temperature, those having 5 to 15 carbon atoms are usually liquids, and the straight-chain paraffins having more than 15 carbon atoms per molecule are solids. Branched-chain paraffins have a much higher  octane number  rating than straight-chain paraffins and, therefore, are the more desirable constituents of gasoline.

6. The  naphthenic series   has the general formula C n H 2 n   and is a saturated closed-ring series. This series is an important part of all liquid refinery products, but it also forms most of the complex residues from the higher boiling-point ranges. For this reason, the series is generally heavier. The residue of the refining process is an  asphalt , and the crude oils in which this series predominates are called asphalt-base crudes. The  aromatic series  is an unsaturated closed-ring series. Its most common member,  benzene  (C 6 H 6 ), is present in all crude oils, but the aromatics as a series generally  constitute  only a small percentage of most crudes.

7.   I nfinite  mixtures of  hydrocarbon   compounds  that form  crude oil ,  sulfur ,  nitrogen , and  oxygen   are usually present in small but often important quantities. Sulfur is the third most abundant atomic  constituent  of crude oils. It is present in the medium and heavy fractions of crude oils. In the low and medium molecular ranges, sulfur is associated only with  carbon  and  hydrogen , while in the heavier fractions it is frequently incorporated in the large polycyclic molecules that also contain nitrogen and oxygen. The total sulfur in crude oil varies from below 0.05 percent (by weight), as in some Venezuelan oils, to about 2 percent for average Middle Eastern crudes and up to 5 percent or more in heavy Mexican or Mississippi oils.

8. H igher the  specific gravity  of the crude oil (which determines whether crude is heavy, medium, or light), the greater its sulfur content. The excess sulfur is removed from crude oil prior to refining, because sulfur oxides released into the atmosphere during the combustion of oil would  constitute  a major  pollutant , and they also act as a significant corrosive agent in and on oil processing equipment. Many  metallic  elements are found in crude oils, including most of those that occur in  seawater . This is probably due to the close association between seawater and the organic forms from which oil is generated. Among the most common metallic elements in oil are  vanadium  and  nickel , which apparently occur in organic combinations as they do in living plants and animals.

9. Crude oil also may contain a small amount of decay-resistant organic remains, such as siliceous skeletal fragments, wood, spores, resins,  coal , and various other remnants of former life. https://ucmp.berkeley.edu/fosrec/Authors.html#O'Neill

10. Natural “traps” keep oil and gas from oozing out at the surface

11. How to find oil: Source rock, reservoir rock, structural traps

12. Source Rock Organic rich shale Late Cretaceous (90 Ma) Late Jurassic (150 Ma) Oil window Thermal history 60-120 o C 2000-4500 meter

13. Reservoir Rock Porosity and permeability Sandstone and limestone

14. Effective porosity is that portion of the total void space of a porous material that is capable of transmitting a fluid . Total porosity is the ratio of the total void volume to the total bulk volume

15. Oil Traps

16. Preservation of Organic Matter Demaison and Moore, 1980

17. Conversion of Organic Matter Barker, 1996 biopolymers bitumen biomarkers A complex fossilized organic material, found in oil shale and other sedimentary rock, which is insoluble in common organic solvents and yields petroleum products on distillation.

18. Conversion of Kerogen Barker, 1996 Organic matter: 1% Kerogen 90% Bitumen 10%

19. General Scheme for Hydrocarbon Formation Tissot et al., 1974

20. Catagenesis 50 to 200 o C Oil Window 60 to 120-160 o C Gas Window 120 to 225 o C Thermal maturation of kerogen H:C decreases

21. Diagenesis refers to the sum of all the processes that bring about changes ( e.g., composition and texture) in a sediment or sedimentary rock subsequent to deposition in water. The processes may be physical, chemical, and/or biological in nature and may occur at any time subsequent to the arrival of a particle at the sediment‐water interface. The range of physical and chemical conditions included in diagenesis is 0 to 200 o C, 1 to 2000 bars and water salinities from fresh water to concentrated brines. In fact, the range of diagenetic environments is potentially large and diagenesis can occur in any depositional or post‐depositional setting in which a sediment or rock may be placed by sedimentary or tectonic processes. This includes deep burial processes but excludes more extensive high temperature or pressure metamorphic processes. Early diagenesis refers to changes occurring during burial up to a few hundred meters where elevated temperatures are not encountered (< 140oC) and where uplift above sea level does not occur, so that pore spaces of the sediment are continually filled with water.