Summary

We have studied the link between lithofacies and rock physics properties in North Sea deep water clastic systems of Late Paleocene age. We introduce the term seismic lithofacies representing seismic scale sedimentary units with characteristic rock physics properties. We define six different facies (I-VI) based on clay content, grain size and bedding configuration, which are primarily determined from well logs (gamma ray, density and sonic logs). Sub-facies of thick-bedded sandstones (facies II) are defined by certain textural parameters (clay location, cementation, etc.), as well as sedimentary structures, which are determined from core and thin-section analyses. Rock physics analysis shows that both thick-bedded uncemented sands and pure shales tend to have relatively low acoustic impedance. In contrast, slightly cemented thick-bedded sandstones and interbedded sand-shale units show relatively high impedance values, while silty shales have intermediate values. The ambiguity between clean sand and pure shale observed in acoustic impedance can be resolved by considering the Vp/Vs ratio which is almost linearly proportional to clay content. We suggest that since seismic lithofacies in the studied North Sea deep water elastic systems are best distinguished by different Vp/Vs ratios and not by acoustic impedances, one should apply AVO analysis to predict the distribution of seismic lithofacies from seismic data in this area.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah