21 History of maturation and petroleum generation
21.2 Maturation and petroleum generation in the Jurassic source rocks
The original distribution of the oil-prone Posidonia Shale Formation along the cross-section is restricted to the central and southern part of the Broad Fourteens Basin (Appendix 4). After the Mid and Late Kimmerian I tectonic phase the Posidonia Shale Formation was preserved in two separate areas along the cross-section: one in the southern part between 0 and 28 km and one in the central part of the basin between 37 and 56 km. The burial and temperature histories of the Posidonia Shale Formation
240 280
320 200 160 120 80 40 0
Age (Ma) P9
Variscan Pre- and early rift Main syn-riftPost-riftSyn-inversionPost-inversion
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320 200 160 120 80 40 0
Age (Ma) 50 km
Variscan Pre- and early rift Main syn-riftPost-riftSyn-inversionPost-inversion
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320 200 160 120 80 40 0
Age (Ma) Q1
Variscan Pre- and early rift Main syn-riftPost-riftSyn-inversionPost-inversion
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320 200 160 120 80 40 0
Age (Ma) Northern platform
Variscan Pre- and early rift Main syn-riftPost-riftSyn-inversionPost-inversion 0
Figure 65 Predicted history of the transformation ratio of the Limburg Group source rock (Ruurlo Formation) at P9, 50 km and Q1 in the Broad Fourteens Basin and in the northern platform area
in these two parts of the basin are different. The associated distinct maturity histories of the source rock are given in Figure 68.
During the main syn-rift phase, rapid subsidence of the Posidonia Shale Formation in the central part of the basin, which is related to deposition of Schieland Group sediments, started the maturation process in the source rock. This is reflected in the increasing values of vitrinite reflectance (Figure 68, location 50 km and Q1). The southern part of the basin south of 22 km was outside the area of deposition of the Schieland Group.
Subsidence of the basin resumed here with the deposition of the Lower Cretaceous Rijnland Group sediments. Figure 68 shows that the predicted vitrinite reflectance values of the Posidonia Shale Formation in the southern part of the basin started to increase approximately 20 - 25 My later than the values predicted for the central part.
Total generated hydrocarbons (kg/t)
Depth (m) 4000
Figure 66 Predicted total amount of hydrocarbons generated in the Limburg Group source rocks along the cross-section through the Broad Fourteens Basin
Variscan Pre- and early rift Main syn-riftPost-riftSyn-inversionPost-inversion
Age (Ma)
Total generated hydrocarbons (kg/t)
0
Ruurlo Formation (45 km) Ruurlo Formation (50 km) Maurits Formation (18 km)
Figure 67 Predicted history of hydrocarbon generation in the Ruurlo Formation in the central part of the Broad Fourteens Basin and in the Maurits Formation in the southern part of the basin
Ongoing regional sedimentation in the later stage of the main syn-rift period and in the post-rift period resulted in continued maturation of the Posidonia Shale Formation in both the southern and the central part of the basin. Late Cretaceous inversion stopped the maturation process at the strongly inverted Q1 location and slowed down the maturation process at the other locations in the basin. The predicted vitrinite reflectance values and transformation ratios just prior to inversion are highest for the deepest part of the pre-inversion
central part of the basin (at 50 km: 0.9%Ro, transformation ratio 80%; Figure 68 and 69).
The pre-inversion vitrinite reflectances in the southern part of the cross-section reached maximum values of 0.68%Ro(at 15 km).
Post-inversion reburial of the Posidonia Shale Formation in Early Tertiary times continued the maturation process of the source rock, especially in the southern part of the basin.
The calculated vitrinite reflectance values and transformation ratios just prior to the Eocene-Oligocene tectonic phase are: %Ro = 0.78, transformation ratio = 60% (at 3 km and 15 km, Figures 68 and 69). The model results indicate that after the Eocene-Oligocene tectonic phase the maturation process levelled out at all locations along the cross-section in accordance with the temperature evolution in the basin (Chapter 20).
In the central part of the basin, most oil was generated prior to the syn-inversion period (Figure 70). In the southern part of the basin the Early Tertiary post-inversion period was an important phase of oil generation (Figure 71).
Pre- and early rift Main syn-rift Post-rift Syn-inversion Post-inversion
180 160 140 120 100 80 60 40 20 0 Vitrinite reflectance (% Ro)
Figure 68 Predicted maturity history of the Posidonia Shale source rock along the cross-section through the Broad Fourteens Basin
Pre-and early rift Main syn-rift Post-rift Syn-inversion Post-inversion
180 160 140 120 100 80 60 40 20 0
Figure 69 Predicted history of the transformation ratio for the Posidonia Shale source rock in the southern and central parts of the Broad Fourteens Basin
The modelling results indicate that in the period between Eocene-Oligocene tectonic phase and present-day little or no oil was generated from the Posidonia Shale Formation.
Originally, the oil-prone Aalburg Formation was present along the entire cross-section.
After the Mid and Late Kimmerian I and II uplift and erosional phases, the shales are absent in the northern platform area. The pattern of the history of oil generation from these Aalburg shales is the same pattern as that of the Posidonia Shale Formation (Figure 70 and 71). However, at the same location, the total amount of oil generated (in kg/t) from the Aalburg Formation is less than the amount generated from the Posidonia Shale Formation.
Age (Ma)
Posidonia Shale Formation (50 km) Posidonia Shale Formation (Q1) Aalburg Formation (50 km)
Total generated hydrocarbons (kg/t)
Pre-and early rift Main syn-rift Post-rift Syn-inversion Post-inversion
0 20 40 60 80 100 120 140 160 180 8 12 16 20 24 28
4
0
Figure 70 Predicted history of hydrocarbon generation in the Posidonia Shale source rock and the Aalburg source rock in the central part of the Broad Fourteens Basin
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16
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8
Total generated hydrocarbons (kg/t) 4
Age (Ma)
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Post-inversion Pre- and early rift Main syn-rift Post-rift Syn-inversion
Posidonia Shale Formation (3 km) Aalburg Formation (3 km) Posidonia Shale Formation (P9)
Figure 71 Predicted history of hydrocarbon generation in the Posidonia Shale source rock and the Aalburg source rock in the southern part of the Broad Fourteens Basin
21.3 Conclusions
The modelling demonstrates that large differences in the burial histories of the gas-prone Carboniferous source rocks along the cross-section result in very different temperature and maturation histories for the source rocks depending on their structural position. The Limburg Group source rock in the central and southern part of the basin first entered the gas window at the end of early-rift times, except for the basinal area between P6 and 50 km along the cross-section. Here, the gas window was reached during the main syn-rift period. Most gas was generated prior to the syn-inversion period. In the northern platform area, maximum maturity was reached in the Early Tertiary post-inversion period, initiating a first phase of gas generation in the most deeply buried rocks. The predicted total amount of gas generated during the evolution of the Broad Fourteens Basin is largest in the southern part of the cross-section. Based on the modelling results it was deduced that in the basinal area CO2generation peaked during early syn-rift times, while in the northern platform area CO2was not generated until syn-inversion times.
The modelling revealed that there was major generation of oil from the Posidonia Shale Formation in the central part of the basin during post-rift times prior to inversion. The Early Tertiary post-inversion period was an important phase of oil generation in the southern part of the cross-section. Little or no oil generation was predicted along the entire cross-section after the Eocene-Oligocene tectonic phase.
The pattern of the simulated history of oil generation from the Aalburg Formation was the same pattern as the one for the Posidonia Shale Formation, but the total amount of oil generated (in kg/t) from the Aalburg Formation was smaller.
Based on the modelling results, it seems likely that today there is no to only minor generation of gas from the Limburg Group source rocks and no to only minor generation of oil from the Posidonia Shale Formation along the cross-section. This answers one of the research questions listed in Chapter 17, namely ‘Is it likely that oil and gas is generated in the basin today’.