I have long suspected that interpretation of seismic data is a branch of voodoo. In a presentation at Offshore Northern Seas '96 conference in Stavanger Aug. 29, I found firm evidence.
Hans Christian Roennevik, vice-president of exploration with Norway's Saga Petroleum AS, told delegates: "Exploration is a combination of theology, psychology, and geology:
"Theology in the sense that it is necessary to believe in something that cannot be seen or changed; psychology because others must be convinced to believe in the same; and geology because the perception of the subsurface has to be changed continuously in light of new information."
Roennevik reckons investment in Norwegian offshore exploration should be based on the aim to double present proved reserves, provided new play types are established in frontier and mature areas.
Uncertainties
"Uncertainties about the subsurface have to be accepted as well as the fact that the major reserve growth phases will occur as breakthrough events," said Roennevik. "These events will take place unpredicted or when observations that do not fit the models are appreciated."
Having come clean about the significance to geologists of keeping fingers firmly crossed, Roennevik provided an example of where oil has been found off Norway in an area where simple black oil migration models indicated that was impossible.
"An example was discovery of Tardis East field," said Roennevik, "which before drilling was argued to be in the migration shadow, because Tardis oil could be assumed to spill (migrate) directly into Gullfaks field.
"Two vintages of 3D seismic existed; the older with an excellent flatspot and the newer without. This difference was related to sampling direction but is poorly understood and demonstrates that seismic hydrocarbon indications are not necessary to find petroleum."
Resolution problem
Roennevik said the 34/7-18 well, which found Tardis East, was drilled into an assumed sealing fault prospect.
But analysis of well results showed a 5m deep pay zone of upper Cretaceous had draped over Jurassic period faults. The 5 m oil layer was too thin to be spotted with seismic.
"Future reserves in the North Sea will, to a larger degree than in the past, be related to stratigraphic traps below seismic resolution," said Roennevik. "Tardis East discovery was made in a seismic sequence where pre-drilling general wisdom said there would be deepwater shale."
Exploration technology will continue to grow more precise, said Roennevik, and the renewal rate for petroleum resources will continue to be linked to global information technology development. There will be more useful technology available than any company can fully use, he said. Yet there will still be room for voodoo.
"Hindsight analysis demonstrates surprises related to most discoveries," said Roennevik. "Major discoveries have usually been considered impossible at some stage, and it is important to remember that an area has to be frontier before it becomes mature."
Copyright 1996 Oil & Gas Journal. All Rights Reserved.
