Rock Physics Modeling is Critical for AVO Analysis – A North Sea Paleocene Case Study

Hübert, Lars¹, Karsten Müller¹, Jack Dvorkin² (1) Rock Solid Images, Oslo, Norway (2) Stanford University, Stanford, CA

A thorough understanding of the geology of an area, especially the underlying rock physics is critical for getting the AVO right. The Lower Tertiary section of the Norwegian North Sea has proven up AVO as a viable exploration tool, however, as this case study shows, using AVO uncritically may lead to false positives, resulting in dry wells, or false negatives thereby missing opportunities.

The results of rock physics, AVO and seismic modeling in Block 24 and 25 indicate a complex system where the AVO signature varies considerably as a function of, the stratigraphy, depth and fluid content. This has an obvious effect on an exploration strategy for the area. In Well A, the Heimdal sand provides the reservoir. Here the AVO signature for hydrocarbons is Class III. Other reservoir sands (Hermod and Balder) are not developed in this well. In Well B we find reservoir quality sands in the Balder, Hermod and Heimdal formations. Modeling indicates that hydrocarbons in the Balder sand should exhibit a Class III AVO, in the Hermod sand a class IIP signature, and the Heimdal sands should exhibit a Class I AVO when hydrocarbon bearing. Note the contrast between Heimdal sand in Well A and Well B. Interference effects, due to thin layers, further complicate matters by clouding an expected AVO effect.

In conclusion, a thorough understanding of the rock physics, particularly as it relates to burial, and a thorough understanding of the stratigraphy of the area is critical for using AVO as an effective exploration tool.