Salt tectonics may be “an outlier in this country’s university training in structural geology,” but geologists entering the petroleum industry will almost certainly find themselves dealing with the topic, explained Mark G. Rowan of Rowan Consulting in Boulder, Colo.
Rowan discussed “Salt Diapirs – What Are They, How Do They Form and What is Their Role in Hydrocarbon Exploration?” at the recent Visiting Geoscientist Program Super Session organized by AAPG and the AAPG Foundation.
Salt diapirs are mostly enormous rocksalt bodies that crosscut surrounding strata. They are important factors in exploration and development in many basins, including the Gulf of Mexico, North Sea, Precaspian Basin, Zagros Mountains, margins of Angola-Gabon and offshore Brazil, Rowan explained. Some can be up to 45 miles long and five miles wide and can be up to 30,000 feet in height – higher than Everest, Rowan said.
How They’re Formed and Why They’re Useful
A growing understanding of how they form – especially since the 1980s – has been helpful in increasing their importance to exploration and production, he explained. They were originally thought to be buoyancy-driven, “bubbling up like lava” and piercing the overlying strata, because over geologic time, salt flows as a viscous fluid. Rowan said they are now understood to be structures that grow at or near the surface as sediments are deposited around them. Hydrocarbons in the sediments become trapped against the salt. The diapirs can keep growing for up to 300 million years.
“At some point, extension, shortening or differential loading triggers some of the salt to move up while nearby areas are going downward. The pressure may be great enough for them to break through and grow as more sediments are deposited,” he said. “There are many variations.”
In addition to their importance in oil exploration, the structures have also been found to be useful for other purposes, such as storage of CO₂ and nuclear waste, because besides helping to trap hydrocarbons, salt diapirs impact many other aspects of the petroleum system, Rowan said.
“Diapir growth creates traps; diapir-related topographic relief at the depositional surface controls sediment transport and reservoir development; salt influences hydrocarbon maturation because it is an excellent conductor of heat; and salt controls the migration and sealing of fluids because it is largely impermeable,” he explained.
A better understanding of diapirs proves useful in appraisal and development as well as during exploration, Rowan said.
Appraisal wells provide data that, combined with modern concepts, helps determine the size of the reservoirs and in the creation of 3-D models, he said.
Salt basins can be very complex. Scanning through data from areas without diapirs shows gradual changes, while diapirs can cause rapid structural-style changes, Rowan said.
There also are problems associated with the diapirs in the petroleum industry, he said.
“It can be hard to image that part right up against the diapirs – you usually encounter surprises in drilling,” Rowan said.
When images are poor, geoscientists can study outcrops to better understand what to expect, he said.
Also, there are unknowns. “Do the hydrocarbons go right up against the diapirs? … How good is the seal? Does the diapir deform the surrounding rocks or cause fractures?” Rowan noted as examples.
He added that geoscientists should “’ground-truth’ all models by observations in the field – field work is fundamental.”
“People have known about these structures for over 100 years,” he said.
The famed Spindletop was drilled into a diapir, in what Rowan called an “unusual scenario.”
Meanwhile, industry understanding of salt diapirs and their role in the petroleum system is improving, he said.
The main reasons are that better seismic produces “fantastic” images, more wells provide more data to examine and produce more ideas, and increased outcrop studies.
Rowan said the ultimate puzzle is revealed in studying salt diapirs in four dimensions – the 3-D geometry and its evolution – and called the subject “endlessly fascinating.”
Rowan is a structural geologist who worked in industry and academia before founding his own company that specializes in training, consulting and research on salt tectonics. He has authored more than 100 papers, has been an AAPG Distinguished Lecturer and International Distinguished instructor and is the recipient of the GCSSEPM Doris M. Curtis Medal and the AAPG Robert J. Berg Outstanding Research Award.