Kerogen type and maturity

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Critical elements of the petroleum system
Chapter Evaluating source rocks
Author Carol A. Law
Link Web page
Store AAPG Store

Each kerogen type has its own relationship to maturity parameters. Maturity measurements are made on materials other than kerogen; therefore, they are not a direct measure of the hydrocarbon generation stage of the source intervals. A relationship must be established between maturity of the geologic section and hydrocarbon generation for each kerogen type in a basin. This relationship can be derived using 1-D basin modeling techniques.

Hydrocarbon generation and maturity

Figure 1 Hydrocarbon generation-depth curve. Copyright: results of Genex 1-D basin modeling software, courtesy Institute Français du Petrole.

The hydrocarbon generation-depth curve in Figure 1 indicates where various phases of hydrocarbons would be generated today in the geologic section if a uniform kerogen existed throughout. We would use the following steps to compare the relationships.

  1. Define the hydrocarbon generation stage for a calibrated well based on the depth vs. hydrocarbon yield plot (left figure). For example, the onset of oil generation occurs at approximately length::2.1 km. Then transfer to the depth versus maturity plot (in this case vitrinite reflectance, right figure) and follow across at length::2.1 km until you reach the maturity profile. This vitrinite reflectance value (0.55 %Ro ) would indicate the onset of hydrocarbon generation in this well.
  2. Apply this relationship to predict the generation zones for this specific kerogen in wells with similar thermal and burial histories in the basin for which maturity data are available.

Comparison and maturity measures

The table below shows examples of the relationships between hydrocarbon generation zones, maturity, and transformation ratio for standard types II and III kerogens, based on a specific burial and thermal history model. The most significant difference is in the depth to the onset of oil generation, where depth::1000 m separates the top of the oil windows of these two kerogen types.

Hydrocarbon generation zone Vitrinite refl., % Ro, Type II Vitrinite refl., % Ro, Type III Transformation ratio, %, Type II Transformation ratio, %, Type III Present-day depth, m, Type II Present-day depth, m, Type III
Onset oil 0.55 0.85 5 12 2200 3200
Onset peak rate generation 0.65 1.00 17 31 2600 3500
Onset gas/cracking liquids 0.95 1.35 88 64 3400 4050

See also

External links

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Kerogen type and maturity
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