About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


AAPG Bulletin, V. 92, No. 2 (February 2008), P. 201-223.

Copyright copy2008. The American Association of Petroleum Geologists. All rights reserved.


Chalk porosity and sonic velocity versus burial depth: Influence of fluid pressure, hydrocarbons, and mineralogy

Ida L. Fabricius,1 Lars Gommesen,2 Anette Krogsboll,3 Dan Olsen4

1Department of Environment and Resources, Technical University of Denmark, Bygningstorvet 115, DK-2800 Kgs. Lyngby, Denmark; [email protected]
2Odegaard A/S, Titangade 15, DK-2200 Kobenhavn N, Denmark; present address: Marsk Olie og Gas AS, Esplanaden 50, DK-1263 Kobenhavn K, Denmark
3Department of Civil Engineering, Technical University of Denmark, Brovej 118, DK-2800, Kgs. Lyngby, Denmark
4GEUS, Oster Voldgade 10, DK-1350 Kobenhavn K, Denmark


Seventy chalk samples from four formations in the overpressured Danish central North Sea have been analyzed to investigate how correlations of porosity and sonic velocity with burial depth are affected by varying mineralogy, fluid pressure, and early introduction of petroleum. The results show that porosity and sonic velocity follow the most consistent depth trends when fluid pressure and pore-volume compressibility are considered.

Quartz content up to 10% has no marked effect, but more than 5% clay causes lower porosity and velocity. The mineralogical effect differs between P-wave and shear velocity so that smectite-bearing chalk has a high Poisson's ratio in the water-saturated case, but a low value in the dry case. Oil-bearing chalk has up to 25 units higher porosity than water-saturated chalk at similar depth but similar velocity, probably because hydrocarbons prevent pore-filling cementation but not pore-structure stiffening cementation in this presumably water-wet chalk. These results should improve the modeling of chalk background velocity for seismic inversion analysis.

When describing the porosity-reducing process, pore-volume compressibility should probably be disregarded when correcting for fluid pressure because the cementing ions originate from stylolites, which are mechanically similar to fractures. We find that cementation occurs over a relatively short depth interval.

Pay-Per-View Purchase Options

The article is available through a document delivery service. Explain these Purchase Options.

Watermarked PDF Document: $14
Open PDF Document: $24

AAPG Member?

Please login with your Member username and password.

Members of AAPG receive access to the full AAPG Bulletin Archives as part of their membership. For more information, contact the AAPG Membership Department at [email protected].