"If this outage goes past the end of the week ... prices could spike pretty dramatically," said Kevin Book of Clearview Energy Partners.
"It's very difficult to pin the exact amount prices may rise, but for now, it appears to be a few cents per gallon, possibly growing more significant if the pipeline remains shut down for more than 2-3 more days," GasBuddy said in a blog post on Monday.
Prices were already edging up before the cyber intrusion, due to growing consumer demand and resurgent economic growth. Over the last week, the national average for a regular gallon of gas has risen six cents to $2.96, according to AAA. Should that trend continue, an increase of three more cents would bring the average to $2.99, which would be the priciest gas since November 2014, the auto club said.