As ExxonMobil announced discovery after discovery offshore Guyana, it sounded easy.
Beginning with the Liza-1 discovery well, the giant oil company announced 16 successful exploration wells. There were four or five successful wells per year from 2017 through 2019, ending with news of its 16th find in January—pushing its estimated oil in the ground to 8 billion bbl.
For those who found the oil, the evidence they used to pick those drilling targets was hardly clear cut, and it took more than 90 years to get to that point.
“In retrospect it wasn’t obvious, and that was why it took so long,” said Maria Guedez, exploration manager for Guyana and Suriname for ExxonMobil.
While a timeline of the Guyana project includes “initiated oil and gas exploration activities in Guyana in 2008,” the story ExxonMobil presented at the recent AAPG SuperBasins 2020 conference covered exploration starts and stops going back to 1928 by the company’s predecessor, Standard Oil Co.
One slide showed a typewritten report from the 1930s noting the test wells were “salted,” giving the impression of a rich reservoir, even though “there is no evidence of oil.”
But ExxonMobil and other oil companies kept coming back. There were dry holes in the 1970s off nearby Suriname—the eastern extension of the play. They saw some positive signs, but the source rock was judged to be not mature enough to fill the conventional oil deposits needed to justify development.
Finally, in the 1990s ExxonMobil geoscientists looking for new exploration opportunities used what had been learned, plus advanced seismic imaging, a better understanding of the movements of tectonic plates, and some simple analysis to outline a plan that ultimately led to the discovery.
Guedez showed a picture of a simple chart evaluating the pros and cons of the basin with handwritten notes, and a geologic cross section in which a geologist hand-counted layers in shallow and deep waters to identify the elements of a productive play.
“New data and technologies often trigger key insights, but you need to ask the right questions,” Guedez said. “Sometimes you don’t need massive amounts of data to make a difference.”
Executing the concept required developing the ability to drill wells in water 8,000 ft deep and high resolution to seismic that allowed interpreters to see the subtle signs of hydrocarbon-rich stratigraphic traps.
“The places where you have structure, it was hard [to spot anomalies]. Stratigraphic traps are normally just really difficult to explore. It was going to be difficult to hit without having some sort of high-resolution 3D seismic,” Guedez said.
These anomalies are never easy to spot, but she said that compared to her experience in offshore Africa, signs of traps off Guyana were “remarkably subtle.”
So far, they have hit on 89% of the wells drilled. “Once the eyes of the team got fine-tuned to that, they did a good job spotting them,” Guedez said.
