Shell discovered a loss of pressure in the pipeline, filed a report with the Governor's Office of Emergency Services and then shut the line down.
San Joaquin County officials believe the spill covered an area 250 feet long by 40 feet wide, Medina said in an interview.
A preliminary test of the pipeline found a split of approximately 18 to 20 inches in length, said company spokesman Ray Fisher in an email.
Fisher also confirmed that the same pipeline ruptured and caused an oil spill in the same vicinity, near West Patterson Pass Road, last Sept. 17.
Here's a link to Shell's report on that incident that found the rupture spilled 21,000 gallons of oil, about the same amount as Friday's break.
Fisher said Shell inspects its pipelines every three years, and the company conducted an inspection of the line after the September incident.
He added that the line has no history of corrosion problems.
It's unclear what caused the most recent spill.
On Tuesday, state fire officials confirmed that the Office of the State Fire Marshal had opened a probe into the pipeline rupture.
Federal regulators are not investigating the break, but are providing technical support to the state, said an official with the Pipeline and Hazardous Materials Safety Administration.
The spill prompted concerns from environmentalists.
Sierra Club representatives pointed out that the spill near the Altamont Pass came weeks after Shell spilled about 90,000 gallons of oil into the Gulf of Mexico and a year after a major spill involving another company's pipeline on the Santa Barbara County coast.
"Sadly, it's become undeniable that oil spills will remain the status quo if we continue our dependence on dirty fuels," said the Sierra Club's Lena Moffitt in a statement. "This is just Shell's latest disaster and the company has done nothing to assuage fears that it can stop its reckless actions."
"The environmental impacts could be very serious," Patrick Sullivan, a spokesman for the Center for Biological Diversity, said in an interview. Sullivan said the spill could hurt birds and other animals in the area and could contaminate nearby groundwater.
State water regulators, though, say they're not concerned the spill could affect water in the area.
"Given the location and the relatively limited extent of the spill, it is highly unlikely that the spill would affect underlying groundwater and even more unlikely that it would impact any drinking water supplies," said Miryam Baras, a spokeswoman for the State Water Resources Control Board, in an email.
Sullivan also questioned whether Shell's statements on the size of the oil spill were correct.
"We don't know how much oil has been spilled," Sullivan said. "With previous pipeline spills the initial estimates have sometimes turned out to be wrong. They've turned out to be under-estimates."
Fisher, the Shell spokesman, said the company had not revised its estimates.