An oil field became California’s first carbon vault. In May, California’s largest oil producer, California Resources Corp., began burying carbon at Elk Hills, a vast, century-old oil field in rural Kern County. It’s not clear which agency is in charge if carbon escapes through one of the oil field’s older wells, for instance. For decades, oil companies have injected carbon dioxide into aging fields to squeeze out more crude, a practice banned in California called enhanced oil recovery. The company also intends to draw carbon from a nearby natural-gas power plant, opening another source of carbon for the project.