California is one of the states that has the largest number of refineries, but access to them is quite limited due to the general lack of pipelines leading to the West Coast from the middle of the country. In order to secure the crude they need for their operations, Californian refineries have been importing increasing amounts of crude from Canada, mostly by rail.

According to data from the state's Energy Commission, 709,014 barrels of crude from Canada were shipped by rail to California in December 2013. This was 4.9 percent higher than the previous month. Just a year earlier there was practically no crude transportation from Canada at all, while two months ago the country accounted for two-thirds of the rail shipments of crude to California.

RELATED: Regulator fines companies for misclassifying crude oil

Refineries on the West Coast have been forced to look for alternative supplies as demand for crude has been continuously growing. In the final quarter of last year the state imported 2.83 million barrels of oil by rail, which was almost double the amount recorded in the previous quarter.

Meanwhile, Wyoming accounted for 21 percent of the oil-by-rail supplies to California in December, totaling 221,793 for the month. It was followed by North Dakota, which delivered 62,325 barrels, or 5.9 percent of the rail shipments, and New Mexico, at 12,927, the figures show.