Chalk background velocity - Influence of effective stress and texture

Peter Japsen, Gary Mavko, Lars Gommesen, Ida L. Fabricius, Finn Jacobsen, Ole Vejbæk, Rasmus Rasmussen, Christian Rau Schiøtt

Research output: Chapter in Book/Report/Conference proceedingConference article in proceedingspeer-review

8 Citations (Scopus)

Abstract

The relative variations in acoustic impedance and hence porosity may be determined by inversion of seismic data. The estimation of the absolute level of these parametres, however, requires a priori knowledge about the low-frequent, background velocity. We find that the background velocity of the North Sea chalk is primarily controlled by effective stress: velocity and porosity log data for chalk in two wells plotted versus effective depth match published reference lines for normally compacted chalk above c. 1.8 km. Effective depth is depth corrected for the effect of overpressure. Below that depth we observe a significant drop in porosity and an increase in velocity that possibly may be explained by a decrease in the Biot factor leading to an increase in effective stress and hence in pore-filling cementation originating from pressure dissolution along stylolites. Moreover, we observe different velocity-porosity relation for the chalk in the two wells leading to differences in porosity of some 5% for identical values of velocity for apparently pure chalk. These variations may be related to differences in contact cementation between the poorly cemented and suffer pore shapes (possibly affected by quartz-overcoating). The non-unique velocity-porosity relation for chalk will lead to variability in the estimation of porosity from seismic data because one impedance value may represent a range of porosities.

Original languageEnglish
Title of host publication67th European Association of Geoscientists and Engineers, EAGE Conference and Exhibition, incorporating SPE EUROPEC 2005 - Extended Abstracts
PublisherSociety of Petroleum Engineers
Pages1829-1832
Number of pages4
ISBN (Print)9073781981, 9789073781986
DOIs
Publication statusPublished - 2005
Event67th European Association of Geoscientists and Engineers, EAGE Conference and Exhibition, incorporating SPE EUROPEC 2005 - Feria de Madrid, Spain
Duration: 13 Jun 200516 Jun 2005

Conference

Conference67th European Association of Geoscientists and Engineers, EAGE Conference and Exhibition, incorporating SPE EUROPEC 2005
Country/TerritorySpain
CityFeria de Madrid
Period13/06/0516/06/05

Programme Area

  • Programme Area 3: Energy Resources

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