A British oil company offered a clarification of its estimate of the oil reserve potential at a site that sparked fears of a widespread fracking campaign.

U.K. Oil & Gas Investments said last week there may be as much as 100 billion barrels of oil onshore in the Horse Hill license area of the Weald basin in southern England. The company issued a statement aimed at clarifying "a number of points" about the discovery.

"The [oil in place] hydrocarbon volumes estimated should not be considered as either contingent or prospective resources or reserves," the company said.

In its announcement last week, the company said the reserves at Horse Hill are in shale beds that are fractured naturally, meaning extraction may be carried out using conventional techniques. Nevertheless, advocacy groups expressed alarm over the discovery, saying UKOG will face stiff opposition from environmentalists.

The discovery at Horse Hill came as the country contemplates the future of North Sea oil fields. Operating expenses in the North Sea are up 8 percent while revenues for oil companies working in the region are at their lowest levels since 1998.

UKOG said the Horse Hill license area covers about 5 percent of the entire Weald basin.

"The company has not undertaken work outside of its license areas sufficient to comment on the possible OIP in either the approximate 1,100 square miles or the whole of the Weald basin," the company said.

UKOG Chief Executive Stephen Sanderson said last week the company expected to recover as much as 15 percent of the oil in the Weald area, which he said would translate to the 2030 equivalent of 30 percent of the nation's oil demand.