Geology's big issue

David Knott
London
[email protected] Conferences offer a chance to learn about parts of the industry one does not normally encounter, and a chance to put questions to the experts.

At the Offshore West Africa conference in Abidjan, Ivory Coast, Mar. 23-25, came an opportunity to dispel some of the ignorance of geology that has landed me in trouble in the past (OGJ, Nov. 10, 1997, p. 41).

The big question for geologists is how to identify hydrocarbons from a mass of seismic and well log data in which oil and gas reservoirs look much the same as dry structures.

Cathy Coon, staff geologist at PanCanadian Petroleum Ltd., Calgary, presented a paper describing how she and colleagues at PanCanadian and Integra Geoservices Inc., Calgary, tackled this problem.

Coon told delegates how conventional amplitude versus offset (AVO) and petrophysical analysis tries to identify hydrocarbon reservoirs from variations in seismic compressional wave velocity and shear wave velocity.

At PanCanadian, however, a team led by Bill Goodway has developed a method to convert velocity data into readings of formation rigidity and incompressibility, as a means of gaining new insights.

Better targeting

Coon said that plotting rigidity and incompressibility data rather than compressional and shear wave velocities improves reservoir zone identification.

She showed data from a gas field in Western Canada. Because PanCanadian built up a wealth of seismic and well log data in the field over the years, Coon and colleagues had an ideal test bed for their ideas.

The records showed that conventional AVO techniques could identify drilling targets, but also that the new method could separate out those that contain good gas sands.

Coon was able to cite proof of her theory's validity, which even the most hard-bitten driller would find convincing: drilling records showed gas finds where the new method predicted, and dry holes where the AVO technique identified prospects but Coon's method did not.

Angola search

While PanCanadian has not yet tried the new method on its acreage off West Africa, another speaker gave delegates details of work in the region.

Justin Morrison, senior geologist at Ranger Oil Ltd., Guildford, U.K., told delegates how the company thought Miocene turbidite sands on its Block 4 license area off Angola had been formed.

Morrison explained how gravity-driven extensional tectonics were the main factor behind the structures and sediment formation in Angola's Lower Congo basin.

With a series of cross-sections and maps, Morrison painted a detailed picture of how major growth faulting helped to channel turbidity currents into hanging-wall half-grabens, creating the structures Ranger is probing.

At the end of his paper, Morrison invited questions. When the delegates appeared shy, he said, "So I'm in for an easy ride?"

This proved too tempting for one member of the audience-a drilling engineer, I suspect-who asked, "So where's the oil, then?"

But like Coon, Morrison was able to back up his claims, telling his inquisitor that Ranger had tested 7,000 b/d from one of the structures he had just detailed.