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Ayoluengo Oil Field
The Ayoluengo oil field is Spain's first and only commercial onshore oil field. It is located in Sargentes de la Lora, Province of Burgos (Northern Spain). This is the Southern part of the Basque-Cantabrian Basin. It was discovered in June of 1964 by Amospain, which was the union of the Standard Oil Company of California (which is known as Chevron today), Texaco, and Compañía Arrendataria del Monopolio de Petróleos Sociedad Anonima (CAMPSA), thanks to modern seismic data which indicated that there was a faulted anticline below an Upper Cretaceous carbonate plateau which kept hydrocarbons entrapped in place. Commercial production took place in 1967, the year in which they were granted a 50-year production concession. Since its expiration in January of 2017, a new concession is waiting to be awarded. The total proven reserves of the Ayoluengo oil field are around 104 million barrels (13.9×106tonnes), and production is centered on 225 oilbbl/d. The last well was drilled in 1990, making for a total of 52. Only 10 wells were active by 2016, when the cumulative production had reached 17 million barrels of oil.

Historical Context
Prior to the discovery of the Ayoluengo oil field, Spain was involved in the Spanish Civil War from the years 1936-1939 which left them in a severe state of economic depression. To add to the crumbling economy, Spain was facing international rejection. Because of these reasons, Ruler Francisco Franco's idea of an autonomous economy intensified. One of the ways he believed Spain could become self-sufficient was by hydrocarbon exploration. During the 1940’s, oil exploration was prominent however, the first exploratory wells drilled were scarce in hydrocarbons. By the late 1950's, the failure of the self sufficiency plan was evident. Because of this, Spain repaired its relations with the US and lowered restrictions they had previously placed on international economic activity, allowing foreign investment. This helped repair international relations, eventually leading to the Hydrocarbons Law of 1958 being enacted. The law promoted hydrocarbon research and exploration in Spain. It also encouraged foreign companies to join in on the exploration.

Petroleum Exploration
The last well was drilled in 1990, making for a total of 52. Only 10 wells were active by 2016, when the cumulative production had reached 17 million barrels of oil.

Ayoluengo-1

The first major oil discovery in Spain after more than 100 exploratory wells that were either dry holes or produced scarce oil production.


 * Discovered: June 6, 1964
 * Produced: 85 barrels/day from a 5m thick sandstone bed (Late Cretaceous - early Jurassic age)
 * Last Production Rates Recorded: 100-150 barrels of oil/day and 37,000 cubic feet of gas/day

Stratigraphy
The 10 km2 field consists of a Triassic salt-cored anticline with 200 m of vertical closure. Oil and gas are produced from more than 50 lenticular sandstone bodies of Late Jurassic to Early Cretaceous age. These sandstone bodies lie at a depth of at least 1300 m.

The Lower and Middle Jurassic are dominated with limestone units (apart from the Carniolas dolomite, Middle Marl Zone black marl, and a couple of dark grey shales located in the upper Middle Marl Zone and upper Marine Limestone unit in the Dogger rock formation). The Late Jurassic consists of the Purbeckian facies which we can divide into two phases, the Early Purbeckian and the Late Purbeckian. The Early Purbeckian facies, an Upper Jurassic to Lower Cretaceous lithostratigraphic group, are composed of claystones, sandstones, dark grey shales, black shales/coals, and a small unit of anhydrite. A thin, organic rich limestone unit located at the bottom of the late Purbeck unit serves as a seismic horizon. The late Purbeckian facies after that consist of sandstones, claystones, and shales (both dark grey and black/coals). The Purbeck stratigraphy is responsible for the hydrocarbon production in the Ayoluengo Oil Field. The Wealden facies are made up of alternating sandstone and red-brownish shales. The three red-brownish shales towards the top of the Wealden facies contain black shale and sub-bituminous coal beds in their lithology. The Utrillas formation is composed of mainly sandstones, with thin layers of coal and dark grey coaly shales. Lastly, the Upper Cretaceous is essentially limestones apart from a dark grey shale unit.

Source
The source rocks for the oil field include Lower Jurassic (Pliensbachian-Toarcian) black shales and organic rich marls as well as Upper Jurassic Purbeck sandstones.

Seal
Mid-Cretaceous shales make up the seal.

Trap
Early diapiric salt structure (due to Triassic salt movements) along with faults due to the compressive alpine fold form traps.

Migration Pathway
The hydrocarbons from the Lower Jurassic source is believed to have went through a slightly difficult migration path. First it moved horizontally, ranging from 5- 20 km long along bedding planes. Then, it did a vertical migration ranging from 1,000 to 1,500 m through faults before it reached reservoir rocks.