THE USE OF AVO IN LITHOLOGY DETERMINATION, NW ATLANTIC MARGIN

Richard Morgan, Dave Went Veritas DGC Ltd.

Early successes using AVO to support prospects in the NW Atlantic margin have not been maintained in more recent exploration drilling and outcomes have been disappointing, hese results have raised doubts in many exploration is minds as to whether AVO techniques can be used as a prospect finding technique in this region.

To better understand the fundamentals underpinning the seismic response multiple well log-based rock property studies have been performed using released exploration wells in the Faroe-Shetland and Voring Basins. Analysis of this database including generation of forward models of the AVO signature, indicates that AVO responses can indeed be used to characterise aspects of lithology and pore fluid. However, there are important mitigating factors that can also result in ambiguity and misinterpretation when this approach is applied.

Key amongst these are:
1. The near offset reflection co-efficient associated with a shale over sandstone interface is typically non-unique.
2. The strong AVO effect associated with some brine sands can commonly overprint other oil-bearing responses.
3. The effects of 'noise1 in the AVO inversion procedure limit the practical usefulness of traditional pre-stack AVO attributes and results in mismatches between rock property models and inversion results.

The first two factors limit the usefulness of the stack, partial stacks and elastic impedance inversions (generated from partial stacks) as anomaly indicators.

The third limits the usefulness of pre-stack attributes that purport to invert for rock properties such as Poissons ratio etc. but which in reality are less powerful.

An appreciation of these key factors can significantly reduce risk in itself It also allows for the design of seismic attributes that are less susceptible to misinterpretation. Building upon the results of the rock property work, an AVO based inversion, here termed Facies Finder, has been produced that, when used with appropriate controls, provides improved discrimination of lithology and fluid differences. Advantages of this particular AVO procedure are independence from well log-derived models and an output that allows representation of lithology differences basin-wide. Because well logs are not an input to the inversion, the validity of the technique can be demonstrated through blind tests using discovery well and dry hole locations. Furthermore, favourable indicators can be placed in context within the basin and validated geologically, linking the seismic anomaly to the prospect. Examples of drilled and undrilled prospects are shown from Faroe-Shetland, Voring and North Sea Basins.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004

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