Shale Dependent Sediment Compaction Model Implemented in a Pressure Basin Simulator: Effects on Geo-Model and Modelled Pressure Build Up?

Arnt Grøver, Ane E. Lothe, and Hans Borge
SINTEF Petroleum Research, Trondheim, Norway

Sediment compaction curves are extensively used in basin analysis to set up burial histories using back-stripping techniques and further relating porosities to depth during fluid and gas migration. One difficult task has been to find proper values for the properties used in the compaction curves. The compaction properties not only control the porosity reduction, but also geo-mechanical properties and the geometry to the sedimentary layers of the basin. Burial history is thus a crucial input to basin models, affecting e.g. overpressure generation and fluid migration pathways. The compaction processes are also related to thermal history of the basin; the rate of digenesis is temperature-dependent while the thermal conductivity depends on the porosity.

Mudstones are the volumetrically most important rock types in sedimentary basins. A new compaction equation relating porosity versus depth has been developed by Fjær et al 2006. The method developed relates the initial porosity of a sediment layer to its shale content. The method has been tested on data from wells in different basins comparing calculated porosities with measured porosities (Lothe et al. 2008). See Figure 1. This compaction equation has now been incorporated in an in-house overpressure simulator; Pressim. This simulator tries to capture the 3D flow pattern in the basin on a geological time scale and the resulting pressure distribution.

We will in this work carry out a study to see if the presented compaction relation would improve the geo-model we simulate on and give a more proper porosity evolution, and thus improve the simulation results. This will be done by simulation test on a North Sea dataset where we try to match the simulated over-pressures with measured pressures from given well-data. We will also compare this method with other existing compaction relations (e.g. Sclater and Christie, 1980) within our simulator.

References

Fjær, E., Lothe, A.E. & Sylta, Ø. 2006: Porosity Versus Depth Relationship Derived from Rock Mechanical Arguments. Poster on AAPG Conference, Perth, November, 2006.

Lothe, A.E., Sylta, Ø., Fjær, E. & Helset, H.M. 2008: Shale dependent sediment compaction: is there a global porosity relationship? Scanning of parameters in wells. EAGE, Roma, Italy. June 2008, extended abstract.

Sclater, J.G. and Christie, P.A.F. 1980. Continental stretching: An explanation of the post-mid-Cretaceous subsidence of the central North Sea Basin. Journal of Geophysical Research, Volume 85, Issue B7, p. 3711-3739.

Yang, Y. & Aplin, A. C. 2004. Definition and application of mudstone porosity-effective stress relationships. Petroleum Geoscience 10, 153-162.

Figure 1. The porosity-depth relation with published data from Yang & Aplin (2004). Red graphs show estimated porosities, blue graphs show measured porosities. Results from Fjær et al. (2006).