TY - GEN
T1 - Chalk background velocity
T2 - 67th European Association of Geoscientists and Engineers, EAGE Conference and Exhibition, incorporating SPE EUROPEC 2005 - Extended Abstracts
AU - Japsen, Peter
AU - Mavko, Gary
AU - Gommesen, Lars
AU - Fabricius, Ida L.
AU - Jacobsen, Finn
AU - Vejbæk, Ole
AU - Rasmussen, Rasmus
AU - Schiøtt, Christian Rau
PY - 2005
Y1 - 2005
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=33645116026&partnerID=8YFLogxK
M3 - Conference article in proceedings
AN - SCOPUS:33645116026
SN - 9073781981
SN - 9789073781986
T3 - 67th European Association of Geoscientists and Engineers, EAGE Conference and Exhibition, incorporating SPE EUROPE2005 - Extended Abstracts
SP - 1829
EP - 1832
BT - 67th European Association of Geoscientists and Engineers, EAGE Conference and Exhibition, incorporating SPE EUROPEC 2005 - Extended Abstracts
PB - Society of Petroleum Engineers
Y2 - 13 June 2005 through 16 June 2005
ER -