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A San Juan University team blends old and new data to better understand an important sedimentary basin in the foothills of Argentina
By Graham Chandler
As the crow flies, it’s a good 900 kilometres from the country’s nearest ocean frontage, but the Argentines call this eastern forefront of the Andes the ‘coast’ of La Rioja. They so named the popular Provincial Route 75 because it borders many different hills of the Velasco Mountain Range as it winds its way along valleys, rivers and through character towns set in a dry and sunny microclimate perfect for growing the world renowned Rioja wines.
The name La Rioja is found in its capital city, too—originally called Ciudad de Todos los Santos de la Nueva Rioja—with its mix of old and modern. Narrow streets and history complement the city’s dynamic commercial centre. Tourist brochures list lots to do here.
But it is what lies beneath this wine country that brought Dr. Mario Gimenez, Professor of General Geophysics at San Juan University and his colleagues including Teresa Jordan of Cornell University, New York, and Argentine graduate student Gabriela Bustos to the La Rioja Valley. “This area was explored for oil on the 80s,” says Gimenez. “And our study was to determine more precisely the depth and geometry of this basin.” Of particular interest, he says, were the Mesozoic strata.
The La Rioja Valley Basin lies west of the capital La Rioja. It covers 10,000 sq km in the north central portion of South America’s Pampean Ranges, between 66°–67°W longitude and 29°–30°S latitude. The Pampean Ranges is a geologic province in the easternmost part of the Central Andes, constructed of a series of elongated mountain chains predominantly trending north-south. These mountains were uplifted during the last 10 million years as a result of a reverse faulting, and expose crystalline basement rocks.
The boundaries of the Pampean Ranges region are nearly coincident with boundaries of the segment of the Nazca Plate, ranging between 30°–33°S latitude where plate subduction dip is practically horizontal. Consequently, the deformation which created the modern topographic ranges and basins of the Pampean Ranges province is interpreted to result from the interaction between the overriding plate and nearly horizontal subducted plate during the last several million years.
The La Rioja Valley is a sedimentary basin set in these Pampean Ranges. But the team was faced with a lack of firm information about the geometry, magnitude, and ages of the basin, as well as most of the other sedimentary basins of the Pampean Ranges, which has left limited understanding of its deformation. So they set out to show that analysis of the gravity field, from data that can be obtained more economically, efficiently, and comprehensively than many types of geophysical data, can markedly improve knowledge of the structural segmentation of the Pampean Ranges.
To do this, Gimenez and his team first took gravity and topographic data, including a newly completed gravity survey, and integrated it to create a complete Bouguer anomaly map. They then summed Euler convolutions (which identify a variety of geologic structures—such as faults, contacts, and intrusive dikes—and estimates their depth) and Werner convolutions (which allow the automated detection of depths) techniques with traditional potential fields methods and independent data from seismic and magnetotelluric methods. They used the combination to test and constrain the subsurface geophysical characterization of the basin.
From this, the team could make a new hypothesis. The original 280 km of 2D seismic exploration lines undertaken by the petroleum company YPF (Yacimientos Petroliferos Fiscales) in the La Rioja Valley area in the 1980s had revealed an approximately 3900 meter thick sedimentary cover above the crystalline basement. But from their research, Gimenez and his team suspected otherwise: that the depth to basement in the basin is considerably deeper than that interpreted from the seismic data, so they tested it using 2D and 3D gravity inversion.
The magnetotelluric (MT) surveys were along three profiles, two running from east to west and the third northeast to southwest. These were existing data. “The MT studies were conducted in 2000 in order to search for hot springs,” says Gimenez. For the 2D inversion, the recorded MT points were identified, from north to south. Their interpretation showed the Cenozoic cover thickness would be 3250 m in the south and 3800 m in the north.
Earlier gravity studies the team used included a complete Bouguer anomaly map of the Velasco Range, a simple Bouguer anomaly map for the province of La Rioja and a study of the isostatic compensation of the Velasco Range, all from 1990s research. To ensure adequate spatial distribution, the team added to these by making new gravity measurements, primarily for locations in the La Rioja Valley Basin. Gravity data were reduced topographically by means of Hayford zones with 167-km diameters. The topographic correction adjusted the gravity effect produced by a mass excess (such as a mountain) or deficit (valley) with respect to the elevation of the observation point. Two regional and local elevation models were used to correct this effect; elevation models were obtained from the Shuttle Radar Topography Mission of the United States Geological Survey.
To separate gravity effects, the team used two common filtering techniques, wavelength filtering and upward continuation, to remove long wavelength features attributable to Moho depth variation and highlight the shorter wavelength features attributable to sedimentary basin geometry and structures. Geosoft Oasis montaj software was used to analyze upward continuations of the gravity field for a set of different heights, from 10 to 40 km ASL. Results that best reflect the regional effects were obtained with filtering by upward continuation at 30 km height. So to estimate the depth to the crystalline basement of the La Rioja Valley Basin, they calculated an inversion of the residual Bouguer anomaly residual from upward continuation at 30 km.
Gravity modeling requires knowledge of densities of subsurface bodies, which can be approximated by using standard relationships between densities and seismic wave velocities of igneous and metamorphic rocks. Usually one can extrapolate densities from wells. But as no deep boreholes existed in the area, the team obtained the velocity from seismic refraction profiles.
Gimenez and his team then overlaid a series of layers, defined by grids overlying a half-space; each layer assigned density values. GM-SYS 3D modeling software was used to compute the subsurface model response: a map of the top of the crystalline basement of the basin.
Final analysis: basin geometry is dominated by a depocenter in the south-western part of the basin where there is a maximum thickness of nearly 6000 m of sedimentary rock. “The asymmetry of sedimentary fill coincides with estimates based on seismic reflection profiles but clearly exceeds magnitudes estimated by either seismic or MT methods,” concluded their report.
Gimenez and his team like the Geosoft software they use—he says the University of San Juan bought Oasis montaj and several modules eight years ago. “We have modules for processing of gravity and magnetism, filtered and Modeling 2 and 3D,” he says. “The Oasis montaj platform is very comprehensive. It enables a variety of processes in the same environment.”
As a result, they’re very satisfied with the La Rioja basin project. “The study contributed to a better understanding of the basin, covering an area greater than that analyzed by the seismic exploration,” says Gimenez. They plan to carry on with exploration and analysis of adjacent basins now. “We are continuing to work towards the north of the basin, to compare development of the other basins.”
And, he says, further oil and gas exploration is now imminent.“Repsol YPF has several blocks in the north of Argentina with exploratory expectations,” says Gimenez. “Among these is the La Rioja Valley.”
Bustos, the graduate student, has made excellent use of this, her thesis work, too—she now works with privately owned Argentinian oil and gas company Capsa in Buenos Aires. The company’s fields are located in the province of Chubut in southern Argentina, far south along the foothills from the La Rioja wine country.
The writer acknowledges with gratitude the assistance provided by Dr. Mario Gimenez in preparation of this article, including “Gravity characterization of the La Rioja Valley Basin, Argentina” by Gimenez et al, which appeared in GEOPHYSICS,VOL. 74, NO. 3 MAY-JUNE 2009.