# Evaluation and Economics of Shale Gas Reserves in the Flysch-Eocene Formation of the Jaca Basin
## 2 Materials and Methods  
### Geology. Discovery of the Jaca Flysch  
The exploration and development of shale gas fields requires the integration of numerous disciplines such as geology, geophysics, hydrogeology, geochemistry, and petrophysics [24]. Although Spain's geography has been little explored with respect to its hydrocarbon reserves, it has favorable depositional environments and pressure regimes similar to other countries. The prospective possibilities of Shale Gas are located in the Basque-Cantabrian, Pyrenean, Ebro, Guadalquivir, and Betic basins, which correspond to Paleogene (Eocene), Cretaceous (Upper and Lower), Lower Jurassic (Lias), and Carboniferous (Westphalian-Eastphalian) areas. Effective shale volumes are conditioned by organic matter richness, reservoir quality, and adsorption capacity. These parameters are affected by age, depth, organic content, maturity, and mineralogy, with great variation among different formations worldwide. Major shale geological formations include Goldwyer and Shahejie (Paleogene), Vaca Muerta (Cretaceous-Jurassic), Barnett, Bakken and Bowland (Carboniferous), Marcellus, Muskwa, Otter Park and Evie (Devonian), Qiongzhusi, Llandovery, and Longmaci (Cambrian and Silurian). Geographically, the area studied covers the pre-Pyrenean zone, framed between the meridians of Pamplona to the W and Bolata to the E, to the S it is limited by the Ebro depression (Somontano de Huesca and Ribera de Navarra) and to the N by the loamy depression of the Canal de Berdun-Val Ancha and the Aoiz area. Important features include the Sierras Oscenses, which form a continuous elongated front from ESE to WNW. To the north of these mountain ranges, the Tertiary Continental folding gives rise to a moderately rugged relief in which the conglomeratic masses of Cancias, Pena Oroel, San Juan de la Pena, and Pena Izaga stand out due to their elevation [25]. The Jaca Flysch is located in the eastern part of the Pamplona-Jaca Basin (Figure 2). The producing units are the fractured carbonate megabreccias of Aurin and Jaca, intercalated in the Flysch units (Eocene) that reach a total thickness of 2500 m, and are divided into (Figure 3) [26]:  
* Lower carbonate Flysch. Carbonate breccias intercalated with sands and clays, 150-300 m thick. Recognized as the source rock of the Serrablo Field. * Sandy Flysch. Submarine turbiditic sands with intercalations of carbonate mega-layers (Gallego, Aurin, Suprajaca and Jaca), 10-100 m thick. * Upper Flysch. Similar to sandy Flysch, but without megalayers. * Blue marls. They are interfingered with the Flysch acting as a seal. The Serrablo field is a case of a gas trap in carbonate megabasins formed by a conglomerated and brecciated basal zone, where gas is produced mainly through fractures. Each producing megabasin has an independent gas-water contact. The marl and clay deposits along with other levels of the Flysch sequence constitute the source rock for the generated hydrocarbons, which migrated and were trapped in structures created during the later stages of thrust [27]. The field began production in the 1980s, when several exploratory boreholes were drilled near the town of Sabinanigo (Huesca) [28]. The Flysch and the carbonate series were identified, both with the presence of natural gas with an approximate methane content of 98%. The Serrablo 1-5 wells allowed estimating Flysch reserves of at least 25 Bsm3. Twenty three drillings were carried out to characterize the reservoir, with companies such as Hispanoil and Eniepsa interested in its possible exploitation. At the beginning of 1984, the area that was put into production was the carbonate rocks. The Zaragoza-Serrablo gas pipeline was built to connect the field with the existing gas pipeline network managed by Enagas [29]. Figure 2: Geological map of the Jaca Basin (modified from the “Instituto Geológico y Minero de España”, cartographic viewer). At the end of the 1990s, the field ceased to be exploited because the reserves proved to be lower than expected.