8. Maturation Analysis
8.2. Interpretation
Data sets based on the integrated (optical and geochemical) maturation analysis show good consistency between the different data sets (Figs. 42–50). Overall, the VR, SCI and Tmax values conformingly indicate immature conditions in the Upper Tertiary II, and immature to mid oil-‐
window settings in the Upper Tertiary I to Pechelbronn Group. PI suggests that some hydrocarbon generation occurred in the Eocene to early Miocene formations.
Well W1
According to the results from VR and SCI analysis a downward slightly increasing maturation trend is observed in well W1, from the immature Upper Tertiary II interval at the top of the section to the basal oil-‐window maturation stage near the base (Fig. 42). Minor differences are detected between these two data sets down to the Corbicula Group. Elevated SCI values, such as in a sample from 1250 m depth, are probably related to higher coalified, recycled sporomorphs and the missing in-‐situ population. As vitrinite reflectance seems to be a more accurate reference parameter, a basal-‐oil window level of organic maturation is suggested for almost the entire Cenozoic succession. Tmax data suggest, that the stage of potential oil generation was barely reached. In the Cerithium Group and below this unit, some samples show extremely low Tmax. Unclear S2 peaks in the affected samples are the reason for this anomaly (Tmax= Temperature at maximum S2 peak) (Fig. 52). Unusually low Tmax values may represent pollution by drilling fluids or natural impregnation by migrating hydrocarbons (Bordenave et al. 1993); in consequence, these samples were applied to a special treatment to remove (eventually remaining) drilling mud. Afterwards, Rock-‐Eval pyrolysis has been repeated on the affected samples but the new measurements also provided results with an unclear S2 peak. One possible reason may be the occurrence of pyrobitumen in the affected samples.
Based on the PI data set, HC generation occurred in the (Lower) Hydrobia Formation and underlying lithostratigraphic units. As PI is derived from S1 and S2, and the S2 is questionable in samples with low Tmax (<360°C), it is better to not over-‐interpret the PI data. Rupel Clay Group samples, on the other hand, show clear S2 peaks. This indicates that oil generation has taken place, as also suggested by VR data. Oil generation may have occurred in the Corbicula Group, based on the PI, VR and SCI data. Maturation stage in well W1 is thus elevated as compared to the other wells discussed here. This may been caused by the exceptional situation of the well in a small pull-‐apart basin on the western border of the URG, which has undergone a slightly different subsidence history than other wells in the area.
Figure 52: Rock-‐Eval pyrograms from well W1 indicating (a) a clean and reliable S2 peak; (b) an unclean S2 peak that is possibly the result of pyrobitumen, affecting the sample. As a result, Tmax and PI are unreliable and must be interpreted with caution.
Well W2
Based on the VR, SCI and PI data obtained from well W2, maturation increases with depth (Fig.
43). The Upper Hydrobia Formation and overlying units are still immature, while samples from the underlying Lower Hydrobia Formation reached the mid oil-‐window. In contrast, immature conditions are suggested for this interval by SCI and Tmax data. The latter Tmax values from the Lower Hydrobia Formation are likely decreased slightly due to the high sulphur content (3.04–
3.98 %) as previously described by Isaksen et al. (2000). Therefore, a basal oil-‐window level of organic maturation is inferred, as also suggested by the general maturation trend. This suggests minor HC generation.
Well W7
Maturation data from well W7 indicates immature to basal oil-‐window conditions throughout the Cenozoic succession (Fig. 44). A vertically almost uniform maturation trend is observed, based on VR, SCI, and Tmax data, beginning in the Upper Hydrobia Formation. Only the overlying Upper Tertiary I and II samples are strongly immature based on VR data. Elevated SCI values for these samples are probably the result of recycled and higher mature sporomorphs, as also interpreted for one sample from the Cerithium Group. PI data suggest that hydrocarbon generation occurred within the Cyrena Marls Group and the Meletta Group. Yet this is very questionable, because silt lithologies with high carbonate content dominate lithologies. The potential source rocks of the Rupel Clay Group and Pechelbronn Group suggest no HC generation within these units.
Well W8:
Based on VR and SCI data, immature conditions are inferred for sediments of the Upper Tertiary I and II Groups of well W8, and a basal oil-‐window maturation stage in the underlying sediments (Fig. 45). Geochemical maturation analysis reveals similar but slightly less mature conditions.
Decreased Tmax values in the Lower Hydrobia Formation to Corbicula Group are likely caused by
elevated sulphur contents, as shown from another area by (Isaksen et al. 2000). Based on maturation analysis the generation of HCs is unlikely for these units, as also indicated by PI.
Elevated PI values (indicating basal oil-‐window) in the Lower Hydrobia Formation and Corbicula Group may indicate oil generation within these units. In the Cerithium Group, where elevated PI were also measured, HC generation is unlikely due to unfavorable lithologies as confirmed by well logs and literature (Schwarz 1997).
Well W9
Based on VR and SCI data, an almost vertically uniform trend is inferred for the Cenozoic succession covering the immature to basal oil-‐window fields (Fig. 46). Tmax data indicate that no major changes occur in maturation level. Slightly lower Tmax values in the Lower Hydrobia Formation may again be related to the geochemical composition of the samples. The PI data suggest that oil generation took place in the late Eocene to early Oligocene sediments. This interpretation does, however, not fit optical maturation parameters. Elevated PI values correspond to elevated S1 peaks in the Rock-‐Eval pyrolysis. However, well W9 was drilled into a footwall structure, which produced HC until the 1960ies. The existence of small amounts of migrated, free hydrocarbons (S1) in the sediments is therefore likely, even though the well was classified as dry. No oil based drilling mud was used. To sum up, no hydrocarbon generation has occurred in well W9 based on integrated maturation analysis.
Well W10
Based on VR and SCI data the Upper Tertiary I and II Groups show slightly increasing values of organic maturation within the immature field (Fig. 47). Below, from the Hydrobia Group to the top of the Pechelbronn Group, an almost vertically uniform maturation trend is recognized. Low maturities within the Pechelbronn Group are most probably related to degraded dark vitrinite, which causes therefore low reflection values. The Lower Pechelbronn Formation was deposited under terrestrial-‐fluvial conditions; in consequence, degradation of organic material occurred during sediment transport from the graben shoulders to the depositional area in the basin. Tmax corresponds well with the maturation values obtained by optical analysis. Except for the Lower Hydrobia Formation to Corbicula Group in which Tmax values are slightly lower than those obtained by optical analysis. This difference is explained by high sulphur contents of this interval (2.55–3.73 wt.%) (Isaksen et al. 2000). Extremely low Tmax values in the Pechelbronn Group are related with unclear S2 peaks (Fig. 52). PI values indicate that no HC generation took place.
Elevated PI values in the Pechelbronn Group (and Meletta Group) are unreliable because these values are related to samples with an unclear S2 peak.
Wells W12 & W14
An almost vertically uniform, downwards only slightly increasing maturation trend is identified in these wells. Immature conditions are inferred for the Upper Tertiary I (Figs. 48 & 49). Based on optical (VR and SCI) analysis, maturation reaches the mid oil-‐window in the Pechelbronn Group in well W14 (2196.6 m) and in the Cerithium Group in well W12 (1634.3 m).
Elevated SCI values, as observed in the Cerithium Group in well W14, are caused by recycled sporomorphs. Based on geochemical analysis, a trend from an uppermost immature stage to a basal oil-‐window stage is observed in both wells.
Apparently low Tmax and high PI values in well W14 are due to unreliable pyrograms. When maturation data from optical and geochemical analysis are combined it appears that oil generation has not been reached in the Hydrobia Group and the Corbicula Group. Yet, based on VR and SCI data, a maturation stage sufficient for oil generation (0.60–0.66 %Ro) is indicated for the underlying source rocks of the Rupel Clay Group (and Middle Pechelbronn Formation) in well W14. This example nicely shows that only the combination of different methods and the discussion of the data result in a conceivable interpretation.
Well W16
Well W16 well shows a downward increasing maturation trend ranging from immature conditions in the Upper Tertiary II/I to a mid-‐oil-‐window stage in the Pechelbronn Group (Fig.
50). This is indicated by VR and SCI data. Geochemical Tmax data suggest, however, that no depth-‐
related change occurred in organic maturation. Slightly lower Tmax values in the Corbicula Group are again referred to elevated sulphur contents (2.92–3.14 %) as discussed earlier. Rock-‐Eval results from one sample of the Pechelbronn Group (2435.6 m) are apparently unreliable, most probably due to the presence of pyrobitumen (Fig. 52). Based on PI data, the source rocks of the Rupel Clay Group as well as the Corbicula -‐ and Hydrobia Groups indicate maturation levels sufficiently high for oil generation, which fits well with the results from optical analysis.
The maturity level based on VR data for selected potential source rock intervals is shown in Fig.
52. For the Pechelbronn Group and Rupel Clay Group, the VR map shows a somewhat subsidence-‐controlled maturation trend from North (low-‐shallow), where VR values range around 0.53–0.65 towards the South, where VR values reach up to 0.88 %Ro for the Pechelbronn Group and 0.8 %Ro for the Rupel Clay Group (high-‐deep). Within the Hydrobia Group, similar VR maturation data in the range of 0.48–0.66 %Ro are measured for the Lower and Upper Hydrobia Formation. For these units, no clear trend can be observed within the study area. When comparing the different stratigraphic units, it becomes apparent that even though several 100s of meters of sediments were deposited in between the Rupel Clay Group and the Hydrobia Group, maturities expressed by VR do not correspond to a solely burial-‐controlled subsidence trend, with would we characterized by increasing maturities with depth.