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

¹ Department of Environment and Resources, Technical University of Denmark, Bygningstorvet 115, DK-2800 Kgs. Lyngby, Denmark; ilf{at}er.dtu.dk
² Ødegaard A/S, Titangade 15, DK-2200 København N, Denmark; present address: Mærsk Olie og Gas AS, Esplanaden 50, DK-1263 København K, Denmark
³ Department of Civil Engineering, Technical University of Denmark, Brovej 118, DK-2800, Kgs. Lyngby, Denmark
⁴ GEUS, Øster Voldgade 10, DK-1350 København K, Denmark

Ida Lykke Fabricius is an associate professor at the Department of Environment and Resources, Technical University of Denmark. She works within petrophysics and rock physics of chalk, with the aim of integrating petrographic, geochemical, geophysical, and geotechnical information.

Lars Gommesen is a geophysicist with the Petroleum Engineering Department, Maersk Olie og Gas AS. He works with reservoir geophysics, with a current focus on four-dimensional analysis of North Sea chalk fields.

Anette Krogsbøll is an associate professor at the Department of Civil Engineering, Technical University of Denmark. She works within soil and soft rock material modeling. Ongoing research deals with laboratory tests on various aspects of compaction of chalk and clay. The consequences of the simultaneous processes of consolidation and creep regarding interpretation of tests are investigated, and constitutive models are established.

Dan Olsen is a senior research petrophysicist at the Geological Survey of Denmark and Greenland (GEUS). Since 1985, he has worked with a broad range of petrophysical methods applied to rock samples, mainly related to petroleum engineering. Acoustic measurements have been an important research area for him since 2001. He holds an M.S. degree in petrology from the University of Copenhagen.

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.