Chemistry of Petroleum

the major classes of hydrocarbons in reserviors are:

  1. paraffins or alkanes
  2. olefins or alkenes
  3. naphthenes or cycloparaffins
  4. aromatics in addition to these hydrocarbon classes petroleum reservoir fluids also contain nitrogen, oxygen, sulfur, and metals such as nickel and vanadium.

High molecular weight constituent of oils usually contain nitrogen, sulfur, and oxygen compounds, which are referred to as resins and asphaltenes.

Reservoir gases or oils that contain hydrogen sulfide are called sour gases or sour crudes. Those devoid of are normally termed *sweet gases *or sweet oils.


The paraffin series is composed of straight-chain saturated hydrocarbons general formula of Paraffins are subdivided into two groups of normal and iso paraffins: the former written by convention as n-paraffins or n-alkanes and the latter as i-paraffins

from C1 to C4 exist in gaseous form, C5 to C16 liquid form and C17+ are solid and also known as waxlike solids

Naphthenes or Cycloparaffins

These are represented by ξ = 0 or by a general formula As the name suggests, carbon atoms are arranged in rings instead of chains


This class of hydrocarbons generally possess pleasant, sweet odor, and hence, they are called as aromatics. They are represented by a general formula that begin with benzene molecule () Other common examples include toluene (n = 7 or ) and xylene (n = 8 or ).

Solid components of petroleum

  1. Gas hydrates
  2. Waxes
  3. Asphaltenes
  4. Diamondoids

Classification of Reservoir gases and oils

  • Reservoir gases
    • Reservoir gases and oils can generally be classified according to their chemical characteristics or constituents and their physical properties.
    • Reservoir gases mostly contain lighter paraffin hydrocarbon components, dominated by methane in amount, and followed typically byethane through hexane
    • occasionally a rather small fraction of heavier hydrocarbonsand some nonhydrocarbons such as carbon dioxide, hydrogen sulfide, and nitrogen.
    • Therefore, considering the limited range of components, compositional analysis of reservoir gases is relatively easy and thus readily obtained by techniques such as gas chromatography. In terms of physical properties, gas gravity (ratio of gas and air density) is commonly used as a characterization parameter.
  • reservoir oils.
    • On the other hand, chemical classification of reservoir oils or crude oils (flashed, degassed, or dead oil) is not trivial given the complexity introduced due to large range of components and isomers that continually increase with the carbon number.
    • However, similar to gas gravity, oil gravity (ratio of oil and water density) also is simple to
    • define and is frequently used as a broad physical characterization parameter of oils
    • Chemical classification of crude oils Given the chemical complexity of oils, the determination of the exact discrete com- position in terms of individual components is nearly an impossible task. Therefore, broadly, the average chemical analysis of oils may include paraffins–isoparaffins– aromatics–naphthenes–olefins (PIANO). However, if all the paraffins are lumped and the rarity of olefins is considered, then the analysis merely reduces to PNA. Based on this broad analysis, the oil is then termed as paraffinic, naphthenic, or aromatic, depending on the domination of a given group
  • physical classification of crude oils Along with the oil gravity, viscosity, color, sulfur content, and refractive index are also some of the other properties that are used in physical classification of oils. However, the oil gravity or specific gravity is perhaps the most important because it is a good indicator of the commercial value or price of a given crude oil when it is sold to refiners. The specific gravity (dimensionless) of a crude oil (or any liquid for that matter), , is expressed by the ratio of oil density, , and water density, , usually at standard conditions of 60°F and 14.7 psia: The petroleum industry also uses another gravity scale known as API (American Petroleum Institute) gravity, or °API, defined as

Five Reservoir fluids

Although the classification of petroleum reservoir fluids as natural gas or crude oil may be satisfactory from a very broad perspective, it is certainly insufficient for proper characterization of various properties and phase behaviors from a reservoir engineering standpoint. Therefore, petroleum reservoir fluids are conventionally classified into the following types:

  1. Black oils API Gravity (°): 15–40
  2. Volatile oils API Gravity (°): 45-55
  3. Gas condensates or retrograde gases API Gravity (°): Greater than 50
  4. Wet gases API Gravity (°): Greater than 60
  5. Dry gases No liquid is formed, hence the name “dry”

Other Hydrocarbon fluids of interest

The other fluids of interest that are described here are those that are not naturally occurring or directly producible from reservoirs but are tailored primarily from natural gases or hydrocarbon gas streams by altering the temperature and pressure conditions. From a practical standpoint, natural gas is considered as a finished product resulting from the processing of raw hydrocarbon gas streams that basically originate as such directly from gas reservoirs or those that represent the evolved gas from oil reservoirs.

  1. Compressed Natural Gas (CNG) As the name suggests, compressed natural gas (CNG) means natural gas that is com pressed by pressurizing it to sufficiently high pressures. A substantial reduction in volume is achieved when compared with the volume of gas at standard conditions. For example considering natural gas to be principally methane, 1 ft3 of gas at standard conditions reduces to merely ∼0.005 ft 3 if pressurized to 2500 psia at the same temperature.

  2. Liquified Natural Gas (LNG) Similar to CNG, the primary objective in liquefying natural gas is to obtain a substantial reduction in volume for storage and transport of vast quantities of natural gas. One major difference between CNG and LNG though is, in the former, the gas remains as a gas phase even after pressurizing, whereas in the latter the gas is converted to a liquid phase by cooling it down to cryogenic conditions of approximately −250°F, close to atmospheric pressure. A volume reduction of the order of 600 is generally achieved in LNG. Prior to using the natural gas as fuel, the LNG is revaporized to a gas phase

  3. Liquified Petroleum Gas (LPG)

    • Liquefied petroleum gas (LPG) can be propane, butane, or a 50–50 mixture of pro pane and butane, which is more common.
    • For the most part, LPG can be extracted from raw hydrocarbon gas streams by fractionation.
    • LPG when stored in containers is approximately 80% liquid that is in equilibrium with 20% vapor phase under normal ambient temperature and moderate pressures of the order of 100 psi, which is basically the vapor pressure of the LPG at that temperature.
  4. Natural Gas Liquids (NGL) Natural gas liquids (NGLs) are stripped from hydrocarbon gas streams, generally by cooling, that are primarily composed of ethane through hexanes and some small fraction of high molecular weight hydrocarbon components. Heavier NGLs that typically consist of C5+ are usually in a liquid state at ambient temperature and pressure conditions. An important application of heavier NGLs is their use as a diluent or viscosity reducer for transport of heavy oils through long-distance pipelines