# Evaluation and Economics of Shale Gas Reserves in the Flysch-Eocene Formation of the Jaca Basin
## 1 Introduction  
Shale gas reservoirs are located in multiple Paleozoic and Mesozoic formations from the Cambrian to the Cretaceous. These formations have given rise to reservoirs with different properties, depending on the geological environment in which they sedimented. Shale gas is stored in shales, marks, or gas siltstones, all of which are fine-grained sedimentary rocks rich in organic matter, capable of generating and retaining gas, which can be exploited through unconventional methods. These rocks act both as a source and reservoir of gas, with no traps or seals, which generally gives them a regional distribution. In these reservoirs, methane is present in two different forms, adsorbed on the surface of organic components or in the clay minerals of the shales. They also have very low permeability (micro-nano darcy), making gas extraction difficult and requiring the use of stimulation methods such as hydraulic fracturing. Shale gas is usually found in very large areas that laterally present highly variable petrophysical properties, requiring significant \"in situ\" volumes due to their low recovery factor [1]. The advances in horizontal drilling techniques and in well stimulation have made it possible to obtain economically profitable production and to place the significant resources of unconventional gas reservoirs in the reserves category. As an alternative to traditional sources of natural gas, depending on their geological characteristics, quality and volume, as well as other factors (accessibility, ease of extraction, location, cost, and production technology), there are four types of unconventional gas [2,3] (Figure 1). The hydraulic fracturing is a method used to increase the flow rate of oil and gas wells. The production wells are drilled to a depth of 8000-10,000 ft [4,5], and may have horizontal or directional sections. It is performed by pumping into the geological formation the fracturing fluid at high pressure, commonly composed of water (more than 90%), proppant with sand and chemical additives [6]. The injection rate must be sufficient to increase the downhole pressure such that the fracture gradient/pressure gradient equilibrium of the surrounding rock is broken [7,8]. The width of the fractures created is maintained by introducing a proppant into the fluid to prevent them from closing when injection stops and pressure is reduced. The permeability generated allows the flow of formation fluids into the wellbore [9]. The internal pressure of the geological formation causes the injected fracturing fluid to rise to the surface and can be stored in tanks and wells prior to disposal or recycling. The recovered fluid is known as return flow or produced water, which can be managed by discharge to surface water or by underground injection [4, 5, 6, 7, 8, 9, 10]. Worldwide, shale gas resources are estimated at 16,000 Tcf (450 Tcm), of which 510 Tcf (14 Tcm) would be found in Europe: Germany, France, Holland, Austria, and Poland; and about 2000 Tcf (57 Tcm) in North America: USA and Canada. Considering that world conventional gas reserves are on the order of 6400 Tcf (180 Tcm), the potential for shale gas is enormous. On the other hand, they require significant investments, mainly due to the high number of wells that are necessary, usually horizontal, and may be multilateral, requiring multi-stage fracturing. U.S. conventional natural gas production peaked in early 21 st century. The supply-side solution seemed to lie in the development of LNG in the Middle East, Africa, Australia, and Russia for import to the U.S. market. The IEA expected that the U.S. would need a high level of LNG imports to meet gas demand [11]. However, shale gas production in the U.S. grew significantly in the 2000-2010 period, from 0.39 Bcf to 4.87 Bcf, representing a 22% increase in dry gas production. The forecast increase in shale gas production to 13.6 Tcf by 2035 would account for 49% of total U.S. gas production [12].