Old basins, new ideas

April 14, 2014
The accepted paradigms in today's exploration industry seem obvious yet in their origins were often difficult to understand. Historically, new geologic concepts have experienced relatively slow acceptance due to perceived high risks, unfavorable price environments, and a lack of enabling technologies.

Tayvis Dunnahoe
Exploration Editor

The accepted paradigms in today's exploration industry seem obvious yet in their origins were often difficult to understand. Historically, new geologic concepts have experienced relatively slow acceptance due to perceived high risks, unfavorable price environments, and a lack of enabling technologies.

Once proven, the resulting paradigm shift often leads to a growth in opportunity. New opportunity types introduce new questions that need answers.

"Many of the questions before us today are very similar, if not the same, to the questions we have been trying to answer for decades," said Carlos A. Dengo, director of the Berg-Hughes Center for Petroleum and Sedimentary Systems at Texas A&M University. "Time will show us again that what we accept today to be our paradigms will be proven wrong," he said.

Speaking at AAPG's Annual Convention in Houston on Apr. 7, Dengo said, "Exploration success is driven by the competitive advantage of seeing first what others have not." New resource opportunity growth resulted from applying new concepts and new innovative technologies. "But certainly we've also seen some paradigm shifts that can be attributed to accidental discoveries," he said.

The discovery of the East Texas oil field in 1930 highlighted the importance of stratigraphic traps. The field has produced 7 billion bbl of oil. "At the time, many companies discounted this [stratigraphic traps] as an exploration concept. They did so because those who were exploring the area were caught in the paradigm that the only types of traps you could find were salt dome related anticlines," Dengo said.

Shifting continents, ideas

"Plate tectonics when combined with new appreciation for the potential of continental carbonate reservoirs could further change the opportunity space, and this is best illustrated with the presalt examples offshore Brazil and Angola," Dengo said.

The accepted paradigm states that as continents break up and form rifts and eventually passive margin basins, the demarcation of the continental and ocean boundary is very sharp. "We had convinced ourselves that we knew where the ocean and continental boundaries lie all around the world," Dengo said. "The consequence of this paradigm, of course, is that it limited exploration plays to those areas underlain predominantly by continental crust."

Recent observations based on modern geophysical data, deep offshore exploration, and onshore analogs are challenging the classical concepts for the evolution of rift basins. Recent discoveries of how rift basins are formed are changing the accepted paradigms about the controls and processes that thin out continental lithosphere under rift basins. This new understanding has shown that ocean and continental boundaries are not equivalent everywhere around the globe. "With these new concepts, it is clear that the opportunities lie further outboard than what we previously thought with our existing paradigms," he said.

Dengo pointed out that this new concept may be a reasonable interpretation that explains the initial rift history underlying the Tupi and other presalt discoveries offshore Brazil. Since 2007, deepwater, presalt discoveries in Brazil's Santos basin have revolutionized offshore exploration. The following year, the complementary margin offshore Angola also led to deepwater discoveries. Brazil and Angola share a common rift history, according to Dengo. The total resource base for Angola's Kwanza basin is not well understood, but some analysts have estimated at least 30 billion bbl of oil in place, if not more.

New understanding

Rift history is initially characterized by isolated basin formation followed by the widespread deposition of salt and evaporites. "What was not fully appreciated until the Tupi discovery was the stratigraphy of the presalt section," Dengo said. Most are now recognized as continental carbonates. The paradigm for years has been that most of the world's producing reservoirs involved marine carbonates—about 60%. For decades, Dengo said, all research was directed at understanding marine carbonate reservoir characterization, facies distribution, and physical properties. Recent discoveries off Brazil have again changed the industry's paradigm. "We are now going back to asking the same questions we have all along," he said. Meanwhile, he added, "There are still questions we need to keep answering if we are to continue identifying new opportunities, which should abound if we continue to shift our paradigms."

With the example of hyperextended margins and the possibility of new reservoir types, the industry is revisiting many of the passive margins around the world and this could open up additional resource opportunities. The 21st century is as good a time as any to keep an open mind.