Rock physics of chalk
The acoustic properties of the pure chalk of the Tor Formation have been studied on core material, well logs and surface seismic data from the Dan and South Arne fiels.
To establish a systematic basis for interpretation of Chalk porosity and fluid content from seismic measurements over Chalk fields and potential accumulations. Through laboratory measurements on core samples from Danish Chalk fields, the effect of texture, mineralogy, cementation and fluid content on the relation between Chalk porosity and seismic velocity will be examined. Which variations in the petro- physical properties of Chalk that are detectable from seismic data will be examined through seismic modelling of log data, calibrated by measured reflection seismic data. The basis for modelling of fluid movements related to the monitoring of Chalk fields will thus be established. The applicability of seismic measurements (AVO) for estimating fluid content near the well bore will furthermore be investigated. Finally, an improved theoretical understanding of the rock physical properties of Chalk will be established from the empirical investigation
In particular, detailed investigations have been carried out on 56 vertical plug samples, i.e. microtextural image analysis on backscatter micrographs and measurement of P- and S- wave velocities of dry and water-wet samples. Porosity is the primary control on sonic velocities and elastic moduli of chalk with identical pore fluids. For chalk samples dominated by fine carbonate particles (mud) we obtain a well-defined relation between elastic moduli and porosity, which can be modelled using a modified upper Hashin-Shtrikman bound. Variations in chalk particle size distribution and the content of non-carbonates influence the relation between elastic properties and porosity for the samples investigated South Arne samples are found to be stiffer than Dan/Gorm samples for identical porosities. The presence of large grains (microfossils) in chalk has a stronger tendency to reduce porosity than to enlarge elastic moduli. Kaolinite has small influence on elastic properties according to our preliminary interpretation; whereas smectite tends to reduce elastic moduli for a given porosity. A reduction of porosity with little effect on velocity was observed for relatively clay-rich intervals in the Tor Formation based on well log data from the Dan field. Gassmann's relations for fluid substitution give valid first order predictions of the elastic properties of dry chalk samples based on observed properties of water-wet samples. Differences in oil/brine saturation were found to have a clear influence on the acoustic properties expressed as varitions in V_P-V_S ratio based on well log data from two wells by applying Gassmann's equations. Comparison between surface seismic data and downhole information could only be carried out for one Dan field well. Through inversion of surface seismic data we find it possible to predict both acoustic and shear impedance within units of c. 20 m. We find that the surface seismic data can be appropriately described by full waveform modelling performed on well log data blocked into eight layers. Comparison between AVO-behaviour for modelled and measured seismic data showed that subsurface properties could be resolved within #+-# 5%
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| Danmarks Tekniske Universitet (DTU) | ||
| Ødegaard og Danneskiold-Samsø S/S | ||
| Stanford University |
Contact
Thoravej 8
DK-2400 København NV, Denmark
Japsen, Peter (seniorforsker), 38142514, pj@geus.dk
Øvr. Partnere: Danmarks Tekniske Universitet; Ødegaard og Dannerskiold-Samsø A/S; Stanfort University (US)
