|THE CHALK IN THE NETHERLANDS, A VERY POOR PLAY OR A VERY POORLY UNDERSTOOD PLAY?|
|Exploration Hot Spots|
The Chalk Group includes important hydrocarbon reservoirs in the
The Hanze oil field is located in the Dutch offshore, with a top reservoir depth at of 1340 mTVD. The strongly naturally fractured reservoir (Price et al, 2002) is located in the Ekofisk Formation and in the Maastrichtian part of the Ommelanden Formation of the Chalk Group which is situated directly above the salt of a major salt diapir. The Chalk Group is overlain by hydraulically sealing Lower Tertiary shales.
|A MULTIDISCIPLINARY APPROACH|
In this project we combine basin modeling, biostratigraphy and detailed seismic interpretation on the Hanze field, which significantly contributes in establishing key conditions that indicate areas of higher prospectivity within the Netherlands Chalk.
New boundary conditions have been derived and used in basin modelling. A detailed study, executed at TNO, on Cenozoic paleoclimate conditions including the reconstruction of paleo-sea surface and continental temperatures indicates the Cenozoic evolution of sediment water interface temperatures (SWIT) (Donders et al., 2008; Sluis et al., 2009) and). In combination with the history of basal heat flow within the Dutch Central Graben, estimated by Abdul Fattah et al. (2008) with the in-house developed 1D tectonic heat flow predictor (Petroprob), this results in a new temperature curve for the
Next to the basin modeling, a detailed biostratigraphic and biofacies study was carried out. In oil and gas exploration, microfossils, especially foraminifera, are of great importance since they occur in a variety of marine environments and throughout the geological time. Biostratigraphy and paleoenvironmental reconstructions are the most common applications of microfossils. Whereas biostratigraphy provides the temporal constraint of rock units based on the fossil content, paleoenvironmental reconstruction provides the interpretation of the depositional environment in which the rock was formed. Because these microfossils live in different environments they can provide paleoenvironmental information for paleodepth reconstruction, paleoproductivity and oxygen at the sea-floor. In this study microfossils are used as a stratigraphic tool to recognize paleoenvironmental changes and unconformities (Guasti et al. 2009).
Key conditions for a successful Chalk play
Including all these new results in the Hanze Field study revealed that three conditions seem to be crucial for a successful Chalk play next to traditional play and reservoir conditions:
Late Tertiary/Quaternary hydrocarbon generation
Recent 3D petroleum systems modeling (Verweij et al. 2009) of the Dutch Central Graben revealed a large variation in burial history of the main source rock (Posidonia Fm.), resulting in very different temperature and associated maturity histories, depending on structural position. Together with the newly estimated history of basal heat flow by Abdul Fattah et al. (2008) a novel temperature curve for the
This new temperature model has a big effect on the Paleogene and Neogene temperature and maturity evolution of the Posidonia Shale Formation. Two different temperature and maturity trends could be observed. In the area south of the Hanze field, the present-day depth of the Posidonia Shale Formation is 4.5-5km. In this area hydrocarbon generation from the source rock started already in the Late Jurassic and reached transformation ratios (TR) of 90% (Figure 2). Higher temperatures in the Mesozoic did not affect the hydrocarbon generation from this area. To the west of the Hanze field the present-day depth of the Posidonia Shale Formation is around 4km. In this region the hydrocarbon generation from the Posidonia Shale started in the Early Cretaceous and continued until the Late Cretaceous. Transformation ratios of 40-50% were reached at that time. The new temperature history for the Mesozoic results in continued generation from the Late Cretaceous and, in combination with the rapid sedimentation during the Neogene, the transformation of another 10% of its potential (Figure 2).
F03-05 Extraction 1 Extraction 2
Figure 2: a) Location of the three extraction points (F03-05, Extraction 1 and Extraction 2) and b) transformation ratio of the Posidonia Shale Formation at three points south-east of the Hanze field. The transformation ratio describes how much of a source-rocks potential has been transformed in %.
Pore fluid pressure data indicate that the Chalk Group is overpressured in the Dutch Central Graben and Step Graben (Verweij et al. 2008). The Tertiary shales cap the Chalk Group and in the inverted centre of the graben the Rijnland Group and Jurassic units. 1D-2D basin modeling studies in the southern and northern part of the Central Graben showed that present-day pressure conditions in the Chalk, and underlying Jurassic reservoirs, are related to the Pliocene and Quaternary increased rates of sedimentary loading. The increasing overpressures during the Quaternary have influenced the hydrocarbon migration and remigration in the Jurassic-Cretaceous system to a greater or lesser extent.
Paleoenvironmental conditions during deposition
Although the monotonous lithology of the Chalk, characterized by often subtle variations in the logs, changes in the foraminiferal assemblages represent an important stratigraphic tool, able to recognize changes such as paleobathymetric differences through time and also laterally. Several wells located in the reservoir of the Hanze field (e.g, F02-06, Figure 3) and in the surrounding area have been chosen for the micropaleontological studies, in order to compare biofacies and stratigraphy. The depositional depth in the Hanze field is fairly shallow. The microfossil assemblages suggest a paleodepth between 100m-150m in the upper part of the Chalk. Whereas, the lower part of the Chalk present in the surrounding area (e.g. F02-07) is generally deposited at a deeper setting. Such variations in paleoenvironmental conditions influence the biogenic origin of the sediments, i.e. porosity and permeability, and it might have influenced early diagenesis (Guasti et al. 2009).
Absence of Lower Chalk and right structural environment
Most Chalk reservoirs within the
Abdul Fattah, R., Van Wees, J.D.,
Abdul Fattah, R., Van Wees, J.D.,
Donders, T.H., Weijers, J.W.H., Munsterman, D.K., Kloosterboer-van Hoeve, M.L., Buckles, L.K., Pancost, R.D., Schouten, S., Sinnighe Damste, J.S., and Brinkhuis, H. 2008. Miocene climatic cooling in terrestrial and marine environments of northwestern
Price, A., Hofmann, A., Van Dalen, E., McKellar, D. and Kaffenberger, G. 2002. Hanze field in the Dutch North Sea. Oil Gas European Magazine 2/2002, 15-20.
Sluijs, A., Schouten, S.
Verweij, Hanneke, Erik Simmelink, Jim Underschultz. 2008. Application of integrated hydrodynamic analysis of overpressure distributions in offshore