The incident, which began May 10, led to an order from state oil and gas regulators for Chevron to "take all measures" to stop the spill.
The flow reactivated this week, pouring about 62,000 additional gallons of liquid into a dry creek bed.
The spill, which state officials have said consists of about one-third crude oil, was first reported by KQED last week and prompted calls from state legislators who want to know what happened at the well site and why it so long for the incident to become public.
Chevron officials say they've traced the spill — formally called a surface expression and which they refer to as a seep — to work the company says it routinely does to keep tabs on wells that have been taken out of production.
"We continue to evaluate the source of the seep, but believe the likely cause to be related to the abandonment of a well," Chevron spokesman Morgan Crinklaw said in an email.
The well was one of the 40 or so that Chevron typically abandons each year in the Cymric oil field, just outside the town of McKittrick and 35 miles west of Bakersfield.
Chevron says the well in question had been taken out of production and "plugged and abandoned" -- filled with concrete -- because of damage in the bore hole that posed a risk of material flowing to the surface from the Cymric's underground reservoir of crude oil and water.
To understand the abandonment process and why crude oil might have begun flowing again from a plugged well requires a short detour into some of what's going on inside an oil well.
Petroleum well bores include a casing -- basically a steel pipe -- that is run down into a hole after it's drilled. Cement is pumped into the hole to hold the casing in place and create a barrier around the casing to minimize the possibility of an uncontrolled flow of crude oil, natural gas and subterranean water.
That flow could enter groundwater or adjacent layers of rock and gradually seep to the surface. In an extreme case, an uncontrolled flow could occur as a blowout, an explosive release of oil and gas that manifests itself as a fire or oil gusher.
Chevron says that when it abandons a well, it plugs the bore hole with cement to a depth intended to ensure it's permanently sealed. If data from oil field monitoring raises questions about the seal — perhaps because of cracks in the cement plug — it may be "re-entered."
"Re-entry" is essentially a "redo" of the abandonment process: Crews use instruments to investigate whether cement in the abandoned well is faulty. If it is, they use a special rig to remove the old cement from the bore hole, then replace it with new cement.
Chevron says that in May it was in the process of re-entering some of its abandoned Cymric wells to provide new cement barriers.
The company says that the initial flows from the previously damaged well, first reported to state officials on May 10, began when it was re-entered. The releases occurred again at a larger scale on June 8 when Chevron conducted a pressure test in the area to confirm the cause of the May flow. Even more oil and water began pouring from the site on June 23 when crews attempted to complete the job of replacing cement in the re-entered well.
