The Ayoluengo field, commonly cited as Spain’s only onshore oil field, was discovered in June 1964. Today, 50 years later, the field is still active, with an average production of some 100 barrels oil per day and a total cumulated oil production of nearly 17 million barrels of oil.
This June marks the 50th anniversary of the discovery. The celebration’s main sponsor the municipality of the nearby village of Sargentes de la Lora (Burgos), along with several other groups, including the University of Burgos, Fundación Repsol and the AAPG-affiliated Spanish Association of Petroleum Geologists and Geophysicists (AGGEP).
A number of events are planned, including the inauguration of an oil museum in Sargentes de la Lora – the first of its kind in Spain.
The museum is planned to provide information about the oil industry and its products, focused mainly on the upstream, introducing into the petroleum system concept and the wide variety of geological, geophysical and engineering techniques used on the exploration and production industry.
An important part of the exhibition is dedicated to the Ayoluengo field geology and its history, captured in an excellent collection of photos provided by the villagers and local newspapers, together with press clippings, documentaries of the mid-1960s, educative panels, geological 3-D models, drilling and production material and an authentic working rod pump.
The Big Boom
The Ayoluengo field is located about 300 kilometers north of Madrid, in the Basque-Cantabrian Basin, a geological region where natural oil seeps, tar and asphalts have been recognized since the early 20th century.
The region was considered highly promising and most of the hydrocarbon exploration effort in Spain during the 1940s and 1950s was focused in this area. Some basic underground mining was carried in the region during the 1940s to exploit the tar sands, but eventually abandoned because of poor economic results.
In the early 1960s, surface geological mapping and modern reflection seismic equipment allowed identification of a faulted anticline in an Upper Cretaceous carbonate flat plateau—an agricultural terrain mostly dedicated to growing potatoes—where the exploration well Ayoluengo-1 was located with the main objective of testing the Lower Jurassic carbonates at some 4,000 meters depth.
On June 6, 1964, the Ayoluengo-1 oil discovery well tested 85 barrels oil per day from an unexpected, five-meter thick sandstone bed of Late Jurassic-Early Cretaceous age located at 1,350 meters depth. It was the first oil discovered in Spain after more than 100 exploration dry holes – and it brought great expectations in the region, presumed to become a prolific “black gold” region.
The oil discovery gained national attention, with extensive media coverage that helped attract many curious visitors to the wellsite. A “Texas Oil Boom in Spain” was the headline in some national newspapers.
The discovery also revitalized the seismic and drilling activity in the region, but subsequent exploration drilling only tested uncommercial oil flow rates.
Geologic History
Surprisingly, even after years of intense exploration activity, the Ayoluengo field remains Spain’s only onshore commercial oil field – and also the only one in the entire Iberian Peninsula.
This anomalous geological singularity has brought recurrent discussions among petroleum geologists, all trying to explain why it is the only one within a vast territory.
The Ayoluengo field consists of a NE-SW-oriented and fractured anticline with a series of thin lenticular sandstones packages of Late Jurassic-Early Cretaceous age. More than 50 separated oil and gas sandstones beds have been identified. Some are as thick as 10 meters, but the average is only two-three meters.
Areal extent of these lenticular sandstone bodies varies widely. Some are quite restricted, while others are laterally continuous. The sandstones have mean porosity values of 18 percent and permeability up to 1 Darcy.
Most of the individual reservoirs layers are isolated by shales and compartmentalized by faults; thus, Ayoluengo is considered to be not a single field but the grouping of more than 100 independent small fields.
The organic-rich marls and black shales of Liassic age have been largely considered as the only source of the oil, but this is still far from clear. The deep erosion by rivers in nearby areas allows observation on outcrops of most of the elements of the Ayoluengo petroleum system: tar impregnated sandstones, the claimed Liassic source rock and text-book faulted anticlines.
The first Ayoluengo oil production started in 1967, reaching the peak production at 5,200 barrels of oil per day in 1969 before gradually declining. Oil is produced by rod pumps, locally and popularly known in Spanish as “caballitos.” The small amount of produced natural gas is used to power the rod pumps motors and to generate the electricity used in the field.
A total of 52 wells have been drilled in the field, the last one in 1990. Currently only 10 wells are active. Many of the infill wells encountered undepleted oil-bearing sandstone beds, indicating the field complexity. A 3-D seismic of 390 square kilometers was acquired in 1988 to identify undrained reservoir beds and better estimate remaining reserves; unfortunately, the results were poor.