The discovery of the Groningen field and the rapid introduction of natural gas not only increased the popularity of natural gas but also acted as an incentive to find more. This led to active efforts to find new fields in the 1970s. Subsurface mapping of the Netherlands revealed more locations where natural gas could be expected to be found. In Drenthe, Overijssel, Friesland and dozens of locations under the North Sea, gas fields were discovered and drilled. Nowhere was such as large field found as that under Groningen, but still when taken as a whole hundreds of billions of cubic metres of extra gas were found below the surface of Dutch territory. These smaller fields were relatively expensive to exploit. The cost of the many test drills, the need to drill more often in order to hit gas once and production costs were high relative to the small volumes of natural gas that could be produced. However, the government really wanted to save the Groningen field as a strategic reserve, which is why it introduced the small fields policy. This policy offered producers attractive tax benefits and a guaranteed market for the gas they produced. The producers were not obliged to sell, but found in Gasunie had a buyer that would always take their gas at the going price. As the years have gone by, this policy has led to the addition of dozens more gas fields to the Dutch stock. The small fields have been producing more gas for sale than the Groningen field for many years in succession.