Origins of Oil and Gas (Part 3) : Transformation

TRANSFORMATION

The organic fraction of the sediments goes through three phases of conversion:

Diagenesis

Catagenesis

Metagenesis.

The first two phases are important in petroleum formation. The process is shown graphically in (Fig HYDROCARBON EVOLUTUION)

HYDROCARBON EVOLUTUION

Diagenesis

When first deposited, marine clays contain as much as 80% water by volume.

Diagenesis describes the chemical changes that occur as the sediments become consolidated and water volume and circulation decrease. Diagenesis affects the organic content of the rock, as well as the rock itself. During diagenesis, the oxidising environment at shallow burial depth (<40 m) gradually changes to a reducing environment. In the reducing environment, anaerobic organisms obtain oxygen from carbohydrates in buried organic material, and by breaking down sulphate compounds available in the sediments. The organisms obtain energy by consuming proteins from the buried organic matter.

The chemical processes involved in liberating oxygen from carbohydrates also produce methane (CH4), the most common of the hydrocarbon gasses. Reduction of sulphate compounds produces hydrogen sulphide (H2S), which is normally destroyed by aerobic sulphur-reducing bacteria

(Fig FORMATION OF H2S IN SEDIMENTS) In an oxygen-poor environment, however, some of the H2S produced will remain in place.

FORMATION OF H2S IN SEDIMENTS

Proteins from the buried organic matter degrade into amino acids and sugars that serve as food for various kinds of anaerobic life. The residue, by polymerization and condensation, eventually forms various geopolymers.

Depending on their composition, geopolymers may be further condensed into kerogen

considered the direct source material for hydrocarbons; or condensed to form bitumen.

Kerogen is broadly defined as a family of compounds that are insoluble by organic solvents.

Bitumen, however, may be dissolved by solvent action. The average proportions in rock between the original organic matter, kerogen, and bitumen are shown in (Fig Relative Proportions of Organic Matter in Sediments).
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Diagenesis usually occurs in the first few hundred meters of burial, but in rare cases may continue to as much as 2000 m. At such depths, temperature and pressure will usually have little to do with the process.

Relative Proportions of Organic Matter in Sediments