Abstract The Ayoluengo Field, commonly cited as the only onshore oil field in Spain, was discovered in June 1964 by Amospain, a joint venture of Chevron, Texaco and CAMPSA, by then the state-owned company. Ayoluengo is located about 300 km north of Madrid in the southern part of the Basque–Cantabrian Basin, a geological region where natural oil seeps, tar sands and asphalt have been recognized in outcrops since the early twentieth century. Now, 50 years after the first oil in 1967, the field has a cumulated production of 17 million barrels of oil. The 50-year production concession expired at the end of January 2017 and the field is now closed, awaiting a bidding process for a new concession to be awarded. The Ayoluengo Field consists of a NE–SW-orientated salt-cored anticline, related to Triassic salt movements. The field is divided into two large structural blocks by a normal fault. Oil and gas production comes from a series of thin lenticular fluvio-lacustrine sandstone packages of Late Jurassic–Early Cretaceous age. More than 50 separate oil and gas sandstone beds have been identified by drilling. This multilayer reservoir, together with the structural component, means that Ayoluengo is considered to be a grouping of hundreds of small oil and gas fields. After years of intense exploration activity, the Ayoluengo Field still, surprisingly, remains a unique oil discovery and is the only onshore commercial oil field in Spain and also the only one in the entire Iberian Peninsula. This geological singularity has resulted in recurrent discussions between petroleum geologists because it is difficult to explain why a petroleum system is working uniquely at this particular spot and nowhere else within such a vast territory.

Abstract The knowledge that societies have of their geological environment and of the materials used in their heritage is of great importance for the sustainable use and conservation of traditional natural stone architecture. Similar weathering mechanisms occur in natural environments and the built environment and, in both cases, weathering may endanger both natural and built heritage. The Sierra de Guadarrama (Spanish Central System) is a privileged area in which natural and built heritage coexist. This paper describes an area of particular monumental and geological interest on the northern slopes of the Sierra de Guadarrama; its main aim is to propose an itinerary for members of the public, presented as a route through the area around Segovia, in which architecture/cultural heritage and an Earth sciences approach are brought together with an educational purpose. To this end, some connections are established between weathering forms in natural and built environments, as well as between the lithological characteristics of the rocks used in construction of ancient monuments in the area and the distribution of stresses and loads of walls.