Quality of Organic Matter


The organic matter in a potential source rock must be capable of generating petroleum. Recycled organic material, which has already been buried in sediments and can no longer generate oil or gas, must be excluded from the total when defining the TOC in a rock. Kerogens capable of generating hydrocarbons are derived from both marine and terrestrial sources. Geochemists recognize Type I, II, III, and IV kerogens, and classify these four types by optical and elemental criteria. Relatively high hydrogen contents in kerogen correspond to greater oil-generating potential. The amount of petroleum generated and expelled from a source rock increases as the atomic hydrogen-to-carbon (H/C) ratio of the organic matter increases (Hunt, 1996). Thus, the most useful classifications of kerogen types are based on hydrogen, carbon, and oxygen compositions of the organic matter.

 

Quality of Organic Matter*

Kerogen Type

Kerogen Composition
 

Hydrogen Index (HI)(mg HC/g TOC)

S2/S3
 

Main Product Expelled at Peak Maturity

I

amorphous/alginite

>600

>15

oil

II

exinite

300-600

10-15

oil

II/III

exinite/vitrinite

200-300

5-10

mixed oil and gas

III

vitrinite

50-200

1-5

gas

IV

inertinite

<50

<1

none

* Approximate ranges based on thermally immature source rocks

From Peters and Cassa, 1994

Kerogen Type

Kerogen represents about 90% of the organic carbon in sediments and is derived from the breakdown and diagenesis of plant and animal matter.  Kerogen can be classified based on chemical composition and visual properties.  Macerals are the individual organic components making up kerogen and they are classified by optical properties determined by organic petrology. The four maceral groups are liptinite or exinite, vitrinite, and inertinite.  The groups are determined by the type of organic material that the macerals are derived from. Organic geochemical techniques are used to classify kerogen into four types, I, II, III, and IV.  These four types are based on chemical composition and the relative amounts of carbon, Hydrogen, and oxygen present in the sample.  The higher the hydrogen content in kerogen the higher the oil-generative potential of the source rock.  During burial and resulting thermal maturation as oil and gas form/crack from the source rock the kerogen becomes depleted in hydrogen and oxygen relative to carbon.

  • Type I kerogen has high atomic hydrogen-to-carbon atomic (H/C) ratio (~1.5) and low atomic oxygen-to-carbon (O/C) ratio (<0.1) (Peters and Moldowan, 1993). It is predominately composed of the most hydrogen-rich organic matter known in the geologic record. The organic matter is often structureless (amorphous) alginate of algal or bacterial origin. In other instances, some Type I kerogens are morphologically distinct and can be assigned to specific genera. Examples include the lacustrine alga Botryococcus braunii , the marine algal phytoplankton Tasmanites sp. (an important type of organic matter in the Devonian black shales of the Appalachian basin), and the geochemically unique Ordovician microfossil, Gloeocapsamorpha prisca (important in Ordovician source rocks in the Appalachian basin). Type I kerogens appear to be derived from extensive bacterial reworking of lipid-rich algal debris from sources such as these (Peters and Moldowan, 1993, p.135). Petrographically Type I kerogens consist mostly of liptinite macerals, with trace to minor amounts of vitrinite and inertinite sometimes present.
     
  • Type II kerogen has high atomic H/C (1.2 – 1.5) and low O/C ratios compared to Types III and IV (Peters and Moldowan, 1993). It originates from mixtures of zooplankton, phytoplankton, and bacterial debris in marine sediments. Type II kerogens are dominated by liptinite macerals with lesser amounts of vitrinite and inertinite. Type II kerogens account for most petroleum source rocks (Peters and Moldowan, 1993).
     
  • Type III kerogen has low H/C (<1.0) and high O/C (up to ~0.3) (Peters and Moldowan, 1993). Such low hydrogen organic matter is polyaromatic and derived mostly from higher plants. Type III kerogen is the chemical equivalent of vitrinite, telinite, collinite, huminite, and so-called humic or woody kerogen (Miles, p. 131). It produces natural gas and occasionally associated condensate if the thermal maturation is adequate.
     
  • Type IV kerogen has low H/C (=0.5) and relatively high O/C (0.2 – 0.3). Type IV kerogen is oxidized and hydrogen-poor.
     

Kerogen Type

Maceral group

Origin

Hydrogen content

I and II

Exinite

Spores, planktonic debris

Hydrogen-rich

III

Vitrinite

Land plants

Hydrogen-poor

IV

Inertinite

Fossil charcoal, fungal remains

Hydrogen-poor

van Krevelen diagram

Van Krevelen plots of atomic H/C-O/C are the best method for evaluating the quality and maturation state of the source rock in the subsurface (Hunt, p. 340). This technique is time consuming and expensive making it unsuitable onsite drilling operations or screening large sample populations. A quicker and less expensive method utilizes a modified van Krevelen diagram that utilizes Rock-Eval parameters and TOC analysis. Modified van Krevelen diagrams plot atomic HI versus OI. The four kerogen types mature along different evolutionary paths with the arrows pointing towards higher maturation states.

vankrevelen_modified

Rock-Eval Parameters

Hydrogen Index

The hydrogen index is a geochemical parameter measured from Rock-Eval data. It is the ratio of S2 hydrogen (in mg HC/g dry rock) to TOC (in grams). The hydrogen index is a measure of the hydrogen richness of the source rock and when the kerogen type is known it can be used to estimate the thermal maturity of the rock. When plotted against the oxygen index (OI) on a modified van Krevelen diagram the HI can be used to provide a crude assessment of the petroleum generative potential in a source rock (Peters and Moldowan, 1993).

S2/S3

Comparing the ratio of S2 (hydrocarbons generated by pyrolysis of the kerogens) to S3 (trapped CO2) also provides information about the type of organic matter present in the source rock. If the type of kerogen type is known the S2/S3 ratio can be used to determine what product is likely to be expelled from the rock during peak maturity.