Understanding source rock provides a key to evaluating the hydrocarbon potential of super basins and an important tool for basin exploration. Geologists get the basics right, in general, but sometimes they might be thinking about source rock the wrong way.
They are definitely talking about it in the wrong way.
“The term ‘world-class source rock’ has been devalued by casual usage and by people promoting prospects and basins,” said Andrew Pepper, founder and managing director of This is Petroleum Systems LLC in Houston.
“As a professional in this area, I’m fed up with people using language they can’t justify,” he said.
Pepper is pursuing a project to analyze, accurately date and assess source rocks in basins around the world. He utilizes the concept of “ultimate expellable potential,” or UEP, representing the cumulative mass of oil and gas that can be expelled upon complete source-rock maturation, and their proportions, system gas/oil ratio.
“So then, the challenge is to map the source rock. Historically, people have not been very good at mapping the UEP,” he said. “There are all these ways people have done the analyses in a very clumsy manner because the modern workflows were not published.”
In analyzing, mapping and dating source rocks, accuracy is a must, Pepper noted. If a geologist is working a prospect “and you want the reservoir to be in close association with the source rock, the age of the source rock really matters,” he said.
“Knowing the true age of a source rock instead of just lumping an age is going to be really important in prospecting, even in a super basin,” he added. “Then the ability to extrapolate in going from a known area to a frontier basin gives you much more confidence.”
“An interesting factoid is that you can correlate acmes (what Pepper calls times of maximum organic deposition) in the Gulf of Mexico that were probably deposited in a kilometer or more of water with carbonates in Kurdistan that were deposited in less than 100 meters of water,” he said.
Pepper thinks that kind of correlation across basins wouldn’t be possible without a climate commonality, a global effect of climate on geological processes.
“You have to come back to a strong climate correlation,” he said.
As an example of imprecise age labeling, Pepper cited the La Luna formation source rock in South America. Once the La Luna was identified, the geological literature started to be filled with references to “La Luna-equivalent” or “La Luna-type” sourcing, a not particularly helpful designation, he said.
What’s been called La Luna actually covers a broad expanse of time and multiple ages and climatic events, from the early Cretaceous Albian all the way up to the Maastrichtian in Trinidad, he noted.
“Another example is the Kimmeridge Clay, the source rock for the North Sea. That is somewhere I fooled myself – I thought I knew the age of the Kimmeridge Clay. To think that it’s Kimmeridgian is a very logical thing to think,” Pepper said.
The reality turned out to be considerably more complex. To classify a source rock formation as late Jurassic to early Cretaceous in age is not especially descriptive because so much can happen in a 20- million-year interval – including, clearly, the end of the Jurassic period and the beginning of the Cretaceous.
Rightly Defining ‘World Class’
Labeling every super basin source rock as “world class” is also a pet peeve for Pepper, even though, by definition, every super basin has cumulative production of at least 5 billion barrels of oil equivalent and future production potential of more than 5 billion boe.
Sourcing for the Bakken in the United States is adequate to fill the formation’s middle reservoir member, but hardly world-class, he said. In fact, U.S. unconventionals tend not to make the world-class cut for Pepper, not even the source rocks of the Permian Basin.
“They’re respectable source rocks, probably not top tier, but there are seven source rocks that do an adequate job of sourcing that system,” he said.
“It doesn’t mean because I have more source rocks I have more potential,” he noted. “If your UEP is large enough and your reservoir storage is large enough, you just need one source and reservoir in the system.”
Pepper outlined his approach in the presentation “Estimating the Ultimate Expellable Potential of Source Rocks: Defining ‘World Class’ for Aquatic Organofacies with Examples from the Arabian, West Siberian, Bohai, and Williston Basins,” available on AAPG/Datapages Search and Discovery and to be published in an upcoming Super Basin issue of the AAPG Bulletin.
He prepared the presentation with co-author Elizabeth Roller, now senior geologist at Belmont Technology in Houston. It includes several posters or illustrations describing source-rock analysis, assessment and classification.
The presentation gives examples of UEP mapping of several source rocks that have charged major conventional petroleum systems – the Arabian Basin in Saudi Arabia, the Bazhenov formation of the West Siberian Basin, the lacustrine Shahejie formation of the Bohai Basin in China and the Bakken formation of the Williston Basin in the United States.
Comparing Source Rocks
Differences among the source rocks are noticeable, not the least in their extent. They range from about 61,000 square kilometers for the Shahejie to 1.75 million square kilometers for the Bazhinov.
“We have to remember that the West Siberian Basin is three times the size of Texas,” Pepper noted.
Expellable potential comes into focus in this analysis – for instance, in comparing the spotty patching of the Shahejie to the regionally-continuous, truly world-class source rocks in the Middle East and Russia.
“The lacustrine in the Bohai Basin map looks like a small case of measles compared to the huge expanse of the charge in the Bazhenov,” Pepper said.
Geologists are generally on the right track when they assess the total organic carbon of source rock and study maturation windows. Along with formation thickness, UEP is driven by those parameters, Pepper said.
Organic carbon and thickness “are essentially the scalars. The other thing is the hydrogen index,” he explained. The hydrogen index controls the expulsion efficiency of the organic matter and therefore the amount of gas from cracking of unexpelled oil.
World-class source rocks are often condensed sections, so “they can only last a certain amount of geological time,” Pepper noted. For a thick source rock formation, repeated acmes have to keep feeding organic deposition in a basin, he said.
“In many of these basins you can see the richest source is actually the thinnest. Geologists tend to overemphasize (source rock) thickness,” he observed.
Catching Up To Technology
Pepper began his career with BP in 1981 as a geologist in international exploration, later working in BP’s Geochemistry Research Lab and then serving as a petroleum systems analyst.
“In the 1980s when I used to do this stuff, I used to average the TOCs and put that in a primitive spreadsheet and do some calculations,” he recalled.
Computer power and analytical abilities have come a long way since then, but the methodology still needs work, Pepper noted
“Now that we have the capability to do these calculations, I don’t see an industry-wide workflow to tackle the analyses,” he said.
Pepper said his work has brought surprises in determining and defining the stratigraphic position and age of super-basin source rocks, “compared to the average perception.”
“The first surprise in stratigraphy for me was that I didn’t know what the age was of the Kimmeridge Clay. And I’m from Britain, so I’m supposed to know that,” he said.
Pepper called it a learning experience that informed his subsequent work in analyzing source rocks in basins around the world.
“It’s that learning that spurred me onto correlating the ages of the acmes in different basins,” Pepper explained. “The ultimate motivation is to put right some of the crazy things I and others have thought about source rock.”