Lisandro Hernández

Late Cenozoic Geohydrology of Extra-Andean Patagonia, Argentina

Publisher Summary This chapter focuses on the role played by Late Cenozoic rocks and sediments of continental extra-Andean Patagonia in the geohydrological processes. They act not only as carriers of usable underground water but mainly as participant in recharge mechanisms to other formations of older age as well as support for highly fragile ecosystems, such as the regional wetlands known as “mallines.” The methodological concepts discussed in this chapter are deductive, making use of evidence provided by indicators to define the most accurate hypothesis. Consequently, it is a model in which the hypothesis that explains most of the evidence is more plausible. The work was oriented to identify the geohydrological conditions to ascertain the dominant regional characteristics. It comprised critical reviewing of existing information, pointing out the hydrolithological properties of the geological materials and the outcrop characteristics, in a geomorphological approach. The chapter focuses on their capacity of receiving, storing, and transmitting water, as the underground circuit of the hydrological cycle. Different physical conditions of the geohydrological system have thus been determined.

Management and Governance

The major conflicts between water uses, mainly domestic and mining uses, are addressed, analyzing the surface and groundwater availability—as well as the reuse of water in the oil industry—and their harmonious use , taking into consideration the legal regulations in force. The surface water is limited to the Senguerr River (discharge: 48.6 m3/s), since the Chubut and Deseado rivers are in a marginal location, and the environment of the great lakes is highly compromised. The groundwater sources available are the water of the SGS that is unfit for human consumption, all of the water of the SGI, the underflow of the Senguerr River, the reusable water from the flowback of unconventional HC exploitation, and the water remaining from conventional secondary recovery, favoured by the progressive decrease in oil/water ratio. The management of water is outlined and its governance is analyzed, focusing on the difficulties in its implementation, given the great variety of social, sectoral and institutional actors. Besides, the situation is complicated further by the overlapping national, provincial and municipal jurisdictions . Added to this, there is a lack of specialized knowledge on the subsurface hydrology derived from scientific and technical research, the mobility imposed by the market on the corporations and trade unions , and a wide range of resource administrators . All of this is analyzed in this chapter, taking into account the necessary dynamics that would guarantee the sustainability in different scenarios.

Hydrogeology in Oil and Gas Production

The evolution of HC production since 1907, the economic transformation of the basin, and the increase in water demand due to the exploitation of new oil deposits and the use of secondary recovery are discussed. The conventional methods have produced a cumulative total of 734,508,000 m3 of oil and 101,567,000,000 m3 of gas, with 39,300 active wells in 2014. Today, secondary recovery with water injection amounts to 48% of the total extraction, initially using fresh groundwater (Patagoniano aquifer) and, at present, lower quality water (from the aquifers of the SGI) or from the underflow of the Senguerr River, as well as the reuse of production water. As regard water for domestic supply, the aqueduct from Musters Lake is used; at a provincial level, the extraction from aquifers fit for human consumption was regulated. Considering the prospects of the recently undertaken unconventional HC exploitation (tight and shale gas/oil in the Pozo D-129 Formation and, in the future, the Pozo Anticlinal Aguada Bandera and Pozo Cerro Guadal formations), a significant demand can be foreseen, of the order of 7500–30,000 m3/well in the case of fracking and 2250–9000 m3/well for drilling . Given the legal restrictions imposed for the aquifers of the SGS, a comprehensive management of the basin must be implemented—which is discussed in Chap. 8—including the aquifers of the SGI, the underflow of the Senguerr River, and the recovery of the flowback , in order to guarantee the sustainability of the exploitation and the continuity of the operations.

San Jorge Gulf Basin

The basin is first characterized by its hydrometeorology: using as reference the Comodoro Rivadavia Aero weather station, the main variables are analyzed statistically and graphically. The mean annual precipitation is 227 mm, with a decadal upward trend , and the mean annual temperature reaches 12.7 °C. Westerly winds prevail (32 km/h), the relative humidity is 49%, and the mean barometric pressure is 1000.2–1006.0 hPa. Potential evapotranspiration reaches 7004 mm/yr, with an annual deficit of 477 mm. The climate is arid, mesothermal with no water excess. The basin is a part of the South American Plate and the regional sedimentary environment developed during the late Carboniferous–Permian . In the Jurassic , an extensional tectonic event occurred, filled with Cretaceous and Palaeogene sediments in a late-rift stage . The major positive landforms are the Patagonian Tablelands, Bernardides, low hills, littoral ridges and dunes. The negative ones, the depression of the central lakes, the valleys of the Senguerr, Chico, Deseado, and Chubut rivers and endorheic depressions. The Senguerr is the only intra-basin perennial watercourse. Aridisols dominate over Entisols and Mollisols , with xerophytic vegetation . The deepest rocks are metamorphic and Palaeozoic–Precambrian intrusives ; Devonian granites and schists ; Carboniferous–Permian sedimentary rocks; Permian–Triassic igneous rocks; Triassic pelites, psammites and pyroclastites; and Jurassic Volcano-Sedimentary Complex rocks. Then, Lower Cretaceous and Chubut Group deposits occur, hosting the main oil-bearing formations. A Palaeogene–Neogene sequence follows, topped by the Patagonia and Santa Cruz formation s, where brackish or freshwater aquifers occur. The column ends with Cenozoic sediments and/or modern basalt layers.

Hydrogeology of a Large Oil-and-Gas Basin in Central Patagonia

A comprehensive regional study of the groundwater hydrology of the San Jorge Gulf Basin (Argentina) is presented for the first time. It is an onshore area of 40,530 km in an arid region, representative of the Extra-Andean Patagonia, between the Andes and the Atlantic Ocean. The geohydrology and its basic components—hydrogeology, hydrodynamics, hydrochemistry, and environmental geohydrology—are analysed in a region with precipitations of barely 200 mm/year, a water deficit of 477 mm/year, and a limited occurrence of allochthonous watercourses. Groundwater is therefore essential, especially because it is the most important oil basin in the country, active since 1907, and with excellent prospects regarding the production of unconventional hydrocarbons (shale gas/oil, tight oil), still under development. A lower or passive geohydrologic system and an upper or active one were recognized. The latter coincides with the most hydrocarbon-productive levels, in sites with ages that span from Early Cretaceous to Pleistocene. Water in most of the aquifers ranges from brackish to saline, with the Tertiary ones being the only low-salinity units. In order to manage conflicts between uses— in particular between mining and domestic uses—a management and governance plan is proposed for the limited water resources, including low-quality groundwater, the underflow of the Senguerr River, which is the only relatively important watercourse, the reusable water of the injection for secondary recovery and the flow back of the exploitation of unconventional oil and gas (fracking). The use of freshwater aquifers is regulated by provincial protective legislation. This book also discusses the indispensable groundwater resource protection policies, which are key for the region, and, above all, the need for hydrogeological studies, as at present they are limited or partial.