HOUSTON—Attention offshore Guyana has been focused on the Stabroek Block, where Exxon Mobil Corp. and partners Hess Corp. and CNOOC Ltd. have made 13 discoveries with recoverable resources at more than an estimated 5.5 billion barrels of oil in turbidite fan and channel systems.

But Dribus Geologic Consulting’s John Dribus, a geologist and Atlantic Margin expert, is excited about the potential of the 13,500-sq-km Kaieteur Block, north of the prolific Stabroek, as well. He pointed out debris flows after highlighting similar ones near the Ranger discovery, the carbonate reservoir find that proved a new play concept on the 26,800-sq-km Stabroek Block.

Of the discoveries ranging in age from Cretaceous to Tertiary announced so far offshore Guyana, Ranger is closest to the Kaieteur Block. The other dozen are in clusters in the southeast part of the block, where the Cretaceous-aged reservoirs of the Liza sandstone play marked the 2015 start of a successful run.

“I think the bioclastic debris flow is the play in Guyana that nobody’s talking about and nobody’s looking for yet that I think is highly significant,” Dribus told attendees recently at the Guyana Petroleum Summit in Houston.

Of all the exploration prospects in the Guyana-Suriname Basin, Dribus said he is most excited about the Kaieteur Block. But he cautions “it’s very high risk. I give that 20-25% chance of success. But it could be billions of barrels huge.”

If the concept is proven there, it could be proven in other areas nearby.

Exxon Mobil is the operator of the Kaieteur Block. Partners include Israel-based Ratio Petroleum (25%), Guyana-based Cataleya Energy (25%) and Hess (15%). Interpretation of seismic data gathered in 2017 across 5,750 sq km on the block is expected to be completed during first-half 2019, Ratio Petroleum said on its website.

Ratio also said Exxon Mobil informed the company in February that it had decided to carry out an exploratory well at a prospect in the southern part of the block, having identified “a number of leads [in several stages of maturity].”

Exxon Mobil was not immediately available for comment about its work program on the Kaieteur Block. But the company has said it plans to drill a second well at the Ranger discovery in 2019.

The Guyana-Suriname Basin has an estimated resource potential of more than 13 billion barrels of oil, according to the U.S. Geological Survey, and is considered one of the world’s top unexplored basins. The mainly offshore basin partially lies onshore.

The area offshore Guyana is more than one play, Dribus said, pointing out basin floor fans, turbidites and carbonate buildups. “These are the two most important rocks,” he said, referring to turbidites and carbonates.

“Deepwater turbidites tend to be more liquid systems. They are clastic sandstone reservoirs. They come in varieties of basin four fans like Jubilee and Shell’s Perdido in the Gulf of Mexico,” Dribus explained, noting the water supported but can be confined as well. “If they’re pressure depletion then you get lower recoveries.”

When companies go into these plays, he said it’s important to test for things like flow rate, drawdown and flow assurance plus look at how much paraffin is in the oil and how much compartmentalization is present.

Carbonate reservoirs, on the other hand, tend to have more gas systems in reef-related deposits but oil is more prevalent with microbialites such as presalt, which he said isn’t present offshore Guyana. With carbonate plays, companies should test for contaminant gas, connectivity, asphaltene segregation and flow assurance. Microbialites and bioclastic grainstones though long-lived with high flow rates also have high CO₂ and possible H₂S, he said.

The geologist also pointed out the importance of having seals with amalgamated systems. Without a top seal, “it’s like an open jar and everything can leave,” he said. The same applies to the bottom seal. “We not only have to pay attention to the top seal, we must pay attention to the bottom seal and a lot of people haven’t. And dry holes have been drilled as a result.”

Turning to Suriname, Dribus questioned whether the reservoir fan system offshore Guyana is also present offshore Suriname and whether the necessary seals are present.

“There are wells being planned that I can tell you right now have a much higher chance of failure than success,” he said. “That’s because the total petroleum system analysis has not been given its due and people haven’t considered the bottom seal sufficiently.”

Dribus passed on learnings from wells drilled and regional geology. Among these were that silt in reservoirs degrades quality, amalgamated reservoirs are better than layered and pelagic ductile shales seal, whereas chaotic mass transport deposits may not.

In addition, “Distance from the paleo shelf is something you should not ignore,” he added. Looking at plays across the world, Dribus said turbidites tend to extend out about 30 km before stopping. “There are no major discoveries outside 30 km away from the paleo shelf margin. So, be careful in Guyana and follow your paleo shelf margin. If your prospect is beyond that 32 km, you need to explain it.”

Velda Addison can be reached at vaddison@hartenergy.com.

Offshore