As most oil and gas exploration starts with seismic surveys — think sound waves that create images like CAT scans of the subsurface — discovering the anomalies in those surveys might indicate the presence of hydrocarbons. Such irregularities are called “direct hydrocarbon indicators” and they include:
- Bright spots: areas of anomalously high seismic amplitude
- Flat spots: areas where the seismic reflection cuts across the depositional layering
- Dim spots: areas of unusually low seismic amplitude
- Amplitude versus offset anomalies: changes in the seismic response as a function of the source-receiver offset
The better scientists get at detecting these anomalies, the more accurate industry professionals will be in evaluating projects that can turn into successful prospects and plays.
When coupled with artificial intelligence algorithms, which automate and speed up identification of DHI characteristics, scientists can more efficiently and accurately predict potential oil and gas reserves in four specific areas:
- Identifying the presence of potential reserves
- Improved drilling decisions
- Assessing specific insights of the prospect in terms of trap area, pay thickness and sometimes porosity
- Reducing overall geological risk
Henry S. Pettingill, chairman of the Rose and Associates’ DHI Interpretation and Risking Consortium, and someone with more than 40 years’ experience in the industry, said it is important to remember that a prospect is only known to contain DHI once it is drilled. The subtlety, he said, is that those bright, dim and flat spots are just components – and none of them alone are proof that hydrocarbons exist.
“There is a rather unfortunate tendency in our industry to equate the term ‘AVO’ (the amplitude variation with offset) with DHI,” he said.
Further, there are direct hydrocarbon indicators – anomalies caused by fluids within the reservoir, as well as indirect hydrocarbon indicators observed above the reservoir, such as gas chimneys.
Once DHI is suspected, further investigation of that area, possibly through more advanced seismic processing, interpretation techniques or eventual drilling, is then undertaken.
A Brief History of DHIs
DHI is not a new phenomenon – it’s been around for over five decades – but operators have seen some impressive results in onshore, shallow water and deepwater, most recently in Alaska (North Slope), Angola, Colombia, Cyprus, Egypt, Indonesia, Israel, Ghana, Guyana, Mexico, Namibia, South Africa, Suriname and Turkey.
Antonio José Velásquez Espejo of Ecopetrol, Colombia’s largest petroleum company, said, “The offshore Caribbean is a great recent example of how to unlock exploration potential by improving the portfolio decisions implementing a DHI-based exploration approach in a frontier basin.”
At present, Ecopetrol, along with Petrobras, announced the most significant gas discovery in the country’s history, which is projected to increase Colombia’s reserves by 200 percent.
Pettingill, an AAPG Certified Petroleum Geologist and an active member of AAPG, SEG and SPE, spoke of the impact this has had in the industry.
“Approximately two-thirds of the reserves from all deepwater giant discoveries were found using DHIs,” he said.
Specifically, when looking at global DHI giant discoveries (see the accompanying graphic), the discovery of gas through DHI since the 1970s has outperformed that of oil.
The reason, said Pettingill, is that “In general, gas is much easier to detect than oil.”
And that is because in many basins, oil’s seismic response is closer to that of water.
“So DHIs find mostly gas,” he said. “Many frontier wells discover gas, whether by accident or not.”
But it’s not always the case.
“In some basins with high GOR oils,” he added, “oil is detected almost as easily as gas,” and he pointed to the Gulf of Mexico, Kutai Basin in Indonesia and Guyana as good examples.
“But the trick is telling the two apart,” he noted.
Pettingill wants to make clear, though, that there have been successful outcomes the old-fashioned way.
“Plenty of basins and plays were broken open without DHIs,” he said.
There is a simple explanation for that.
“Mainly because DHIs would not work,” he said. Either the seismic data quality (imaging) was not good enough to resolve DHIs or because the rock properties of the sands and shales show insufficient contrast in acoustic impedance, hence no amplitude anomaly.
“Most often this occurs as a function of depth and consequent rock compaction, typically around 18,000 feet below the mud line in Neogene basins, or as little as 10,000 feet for Cretaceous basins,” said Pettingill.
As mentioned, more and more evidence is proving how effective DHI is becoming.
“Typical frontier wildcat success rates,” Pettingill said, “are 25 percent without DHIs, and 50 percent and upward with DHIs. In some plays it can be 80 percent.”
He said, for example, when Noble Energy (now part of Chevron), began exploration in the Tamar gas prospect in the Eastern Mediterranean Sea, which ultimately became a major natural gas discovery, they gave it a 40-percent chance of success, despite having no well control in the basin.
What eventually happened: it de-risked the entire play and unleashed a trend of follow-up gas discoveries in Israel/Cyprus.
“In retrospect we should have given it much a much higher chance of success because the DHI was that good (my boss at the time, Senior Vice President Susan Cunningham, loves to remind me that I was too conservative!). We had a 100-percent success rate in the play after drilling eight wildcats,” Pettingill related.
Overall, in the Rose 405-well DHI database, the success rate is 53 percent, from wells in over 45 basins around the globe, some super-mature, others still frontiers with few or no wells.
What a DHIs are not – Pettingill wants underscored – is a shortcut.
“You need to do your homework as to whether or not the play should be expected to have DHI,” he said.
He said that 20 percent of the prospects in their database were actually downgraded in the final analysis because, even though they should exhibit strong DHI characteristics, they were actually shown to lack such characteristics, making the prospect high-risk.
Discovery Thinking Forum
The history of DHI as a concept – its successes and the people at its forefront – will be on display at the Discovery Thinking forum on Aug. 27, chaired by Charles Sternbach and co-chaired Mike Forrest, at the upcoming 2025 International Meeting for Applied Geoscience and Energy in Houston.
“This year’s focus on DHI driven discoveries is a great way to celebrate the 30th Discovery Thinking forum,” said Sternbach.
Pettingill will be speaking, along with Rose’s Rocky Roden and Roger Holeywell, on:“Direct Hydrocarbon Indicators in Carbonates: Current Landscape, Future Potential?”
For his part, on the geophysics front, Roden said there is also great promise, specifically in three main areas:
- Ocean bottom node seismic: “It’s very expensive but for imaging-challenged plays like sub salt, there is nothing better, and the results keep getting better,” said Roden.
- Full waveform inversion seismic processing, including elastic FWI (eFWI): “Again, it’s all about better imaging, and the advancements have been incredible,” he said.
- Computing power: “This follows Moore’s Law of integrated circuits, which states that the semiconductor compute power doubles every two years,” Roden added.
The artificial elephant in the room is artificial intelligence and machine learning which, Roden said – as with all data-driven sciences – will “continue to change everything for us.”
Other presentations at the Discovery Thinking forum at IMAGE will include: “From Moosehead to Mopane,” “Colombia and Sirius Discovery,” “Sirius Gas Discovery – Geological Setting and Caribbean Offshore Exploration Historical Perspective” and “New Frontiers in the Last Frontier – Pantheon Progress on the North Slope of Alaska.”
A Bright Future
Teresa Martins, a geologist at Galp specializing in quantitative interpretation, will be involved with the “Moosehead to Mopane” presentation.
“Right now, the role of DHIs is shifting from being a ‘cherry on top’ to a quantitative, probabilistic input in full-cycle risk analysis,” she said. “Instead of being used as isolated ‘bright spots,’ DHIs are increasingly embedded in workflows that combine geology, petrophysics, and seismic attributes to generate predictive, statistically backed models of reservoir presence and quality.”
She said the future of DHI is not about “finding bright spots – it’s about building confidence in subsurface models through integrated, data-driven workflows.”
When looking at what’s ahead for DHI (and where it’s been), Pettingill is both humbled and excited.
“As late as 10 years ago I hardly could have imagined DHIs subsalt. But then we started getting better illumination and imaging, and things started to pop out,” he said.
As non-DHI prospects started to be revealed as having DHIs, Pettingill said, Consortium members asked his team – and this started during the pandemic – to work on a subsalt DHI module.
The post-pandemic kickoff meeting was December 2021.
By 2023, he said, everything was fully functional in that software module, with subsalt DHIs now being “business as usual.”
Future breakthroughs, he believes, will include continued resurgence of onshore DHIs, subsalt DHIs, dim spots and other subtle DHIs, electromagnetic surveying in appropriate setting, advances in seismic acquisition and processing and, that elephant in the room – AI/ machine learning.
“Hard to believe for an old fart like me who thought he’d seen everything, but the current generation of explorers will have a bright future!” said Pettingill.