Source Rocks: The Origin of Petroleum

Diagenesis of Organic Matter

We recognize three principal stages of evolution of organic matter into petroleum: 1) diagenesis; 2) catagenesis; and 3) metagenesis. Diagenesis refers to all changes and processes affecting organic matter from the time that the organics are buried until the formation of kerogen. Catagenesis is the stage during which oil and natural gas are generated from kerogen. During metagenesis, dry gas forms.

As muds compact to form shales, organic matter contained within the muds undergoes complex changes. Four groups of organic compounds, known as bio-polymers, are biologically synthesized by plants and animals: 1) carbohydrates; 2) proteins; 3) lignins; and 4) lipids. Lignins consist of high molecular-weight aromatic carbon rings and are found only in higher-order land plants. Lipids, which occur in both plants and animals, are insoluble compounds of fats, oils and waxes, and constitute the primary source materials for the formation of liquid hydrocarbons.

During early diagenesis, complex bio-polymers break down into smaller, simple molecules known as geo-monomers. Of the bio-polymers, proteins are least stable, followed by carbohydrates, lipids and lignins in increasing stability. Geo-monomers react spontaneously with each other and polymerize to yield stable, complex geo-polymers, which have random structures and resist biodegradation.

The change from bio-polymers to geo-polymers is initiated by bacterial and non-geological, chemical processes. Geo-polymers are subsequently affected by thermal cracking. The formation of geo-polymers occurs over a geologically short time of hundreds to a few thousands of years. With increasing burial, organic matter loses almost all of its nitrogen, much of its oxygen and sulfur, and some of its hydrogen and carbon. At this stage, the only measurable hydrocarbon produced is biogenic methane, as a result of anaerobic decay.

Kerogens are the end product of the diagenesis of organic matter. Due to their large molecular sizes, kerogen molecules are insoluble in organic solvents. Insolubility, rather than differences in chemical structure, distinguishes kerogen from bitumen, which is the soluble portion of organic matter. Kerogens are fragments of plant tissue, spores, algae and other fragments with definite biological structure. We group these plant-derived fragments into biological units known as macerals, which are equivalent to minerals in rocks.

There are three important groups of macerals: 1) vitrinite; 2) exinite; and 3) inertinite. Vitrinite is the dominant maceral and the major component of coal. It is derived primarily from woody tissues of higher-order land plants, is found in most depositional environments, and is difficult to decompose. Exinite macerals are derived principally from algae, spores, pollen and leaf-cuticle waxes. They are uncommon and indicate lacustrine or shallow-marine environments. Inertinite macerals have been extensively oxidized before deposition. Charcoal is the predominant type and is abundant only where the organic matter has been recycled.

Some components of kerogen are amorphous. These particles have been extensively altered by bacteria and fungi or mechanically disintegrated so that maceral types and cell structures have been obliterated. Amorphous kerogens are the best source materials for hydrocarbons because they are so chemically altered they can mature to oil at significantly lower temperatures than other macerals. Because its density is greater, a given volume of amorphous kerogen will yield greater amounts of hydrocarbons than other macerals. Amorphous kerogens contain an abundance of particles derived from exinites, because these macerals break down more readily and earlier than do vitrinites.