Spain's first hydrocarbon discovery, the No. 1 Castillo drilled by CIEPSA near the northern city of Vitoria, was completed as a gas well in 1961 and continues to produce a small but locally important amount of gas from a thick fractured marl. The field is not significant economically but does provide nearby industry with needed fuel and gives information on the behavior of a type of reservoir which will become more important in the future as demand and prices increase.

Located on the south flank of the Cantabrian trough, the reservoir section of Turonian and Cenomanian age is between 2,000 and 3,000 m deep on a large anticlinal structure formed during Alpine orogenic events. More critical than structural closure are the various fracture systems which create both reservoir volume and permeability in the 1,000-m column of marls, thin limestones, and minor quartzitic sandstones. Effective primary porosity is negligible.

The field has produced over 1.2 Bcf since 1963 out of an estimated ultimate reserve of 2 Bcf. A variety of drilling, completion, and stimulation techniques have been used in attempting to extend production without marked success; however, higher prices for gas may result in a reevaluation of these methods. It is anticipated that modest reserves such as these will be needed in the future.

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