The classical definition of 'Infracambrian' that refers strictly to sequences of Proterozoic age is not applicable in southwestern Gondwana. In this paper the term 'Infracambrian' is used to define the sequences deposited during the Pampean orogenic cycle, which extends until the Cambrian period.A classification of Infracambrian basins is proposed based on location, level of preservation and perceived petroleum potential. Only the eastern basins of the South American plate have potentially significant exploration potential.Two Neoproterozoic petroleum systems have been identified in the eastern basins: a Riphean system, developed on the western margin of the San Francisco Craton, in the San Francisco Basin of Brazil; and a Vendian system, developed on the eastern margin of the Amazonia -Río de la Plata cratons, in the Corumbá Basin of Brazil and Paraguay. The Riphean system is reportedly proven by a well test drilled by Petrobras. An active Vendian petroleum system is proven by the presence of oil seeps within fractured limestones. A Vendian petroleum system is proposed for the Claromecó Basin of Argentina based on the correlation of the Vendian and Riphean sequences.

This paper reports on the experience gained with a pilot project aimed at quantifying the value of geostatistical inversion for the detection and delineation of thinly bedded hydrocarbon reservoirs. The study interval is located within the lower member of the Bajo Barreal formation. Hydrocarbon reservoirs in this formation consist of fluvial sandstones with an average thickness of 4 meters. Acoustic impedances yielded by trace-based seismic inversion satisfactory resolved the petrophysical variation of stacks of individual sand layers. However, hydrocarbon-producing reservoirs are found in only a few of the sand layers contained within a sand stack. It is therefore highly desirable to resolve individual sands with a stack. Conventional seismic interpretation techniques are not applicable because of their lack of vertical resolution. Instead, an extrapolation of petrophysical data from existing wells was performed via geoestatistical inversion of acoustic impedance. The use of this technique has been facilitated by the generation of synthetic sonic in wells where that information was not available. A total of 20 wells were used in this study. Distances between wells ranged anywhere from 30 to 600 m. Wells were used selectively to analyze statistically the gain in vertical resolution in terms of individual sand layers as a function of the distance from key control wells. As a result, we were able to delineate individual sand layers otherwise poorly defined by trace-based inversion because of their reduced thickness and/or poor acoustic impedance contrast with the surrounding rocks. Examples are shown of how vertical resolution is improved in the vicinity of key wells to distances required by new development and step-out well proposals.

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The irregularity and discontinuity that the sands present in the vertical sections of the Bajo Barreal Formation lead us to the concept that these sands present a difficult and chaotic distribution. The tuning effects and lack of resolution of the seismic information leads us on a search for new models and methodologies to find a model for the distribution of these sands. The objective of this work is to demonstrate the fractal behavior of the sets of sands, and the need to keep this property in mind for any type of geostatistical analysis, since these types of analysis generally assume a normal distribution of the data population in opposition of the fractal concept. The term fractal was first defined in 1975 by Benoit Mandelbrot to describe the geometry of those objects which have an extremely irregular shape, interrupted and fragmented, and continue being so at any scale chosen for study. The fractal dimension (D) allows us to quantify the degree of irregularity of a geometrical set or of a natural object. An important consequence of this scale exponent is that a population of objects that respond to a fractal distribution will not have average values of its properties, being able to speak only of the distribution of sizes, of which a specific object or phenomenon occurs within a specific interval of time or space. An important concept is that the fractal behavior only occurs within a range of scales in nature, and no natural object is indefinitely fractal. The punctuated behavior of the sedimentation brings, as a consequence, the distribution of the sands presented in a discontinuous form, and for that reason describable from a fractal viewpoint. Based on these observations, there is a need to characterize the population of sands in the Bajo Barreal Formation as a fractal set. Likewise, these results of the subsurface have been verified with data from outcroppings, having obtained similar results. The fractal vision has contributed to the discrimination of parasequences, the recognition of a distribution model of the sands, and the conclusion that the most important problems in these oil fields are the pathways of migration and trapping of hydrocarbons and not the presence of sands as was previously assumed. Geological setting The San Jorge basin is located between the Andean range and the Atlantic passive margin. It's an intracratonic basin with an extensive dominant tectonics, limited with structural basement highs (Fig.1). The west edge of this basin (which is discussed in this paper) is characterized with a compressional system with a general north-south direction. This system comprehends asymmetric anticlines whose vergece changes along the strike (1). The area of study corresponds to Perales oil field. These oil fields are limited with WNW-ESE normal faults. All of these oil fields form part of a great structure associated to a master fault. This master fault generated rollovers, and crestal collapse grabens where the oil fields are (2). Figure(2). The classic statigraphic section is showed in the figure 2 (after Bellosi, 1998). But some modifications were done in this scheme for Bajo Barreal Fm as a consequence of the analysis done during this study. The Seccion Tobacea was separated from the Bajo Barreal Fm. And the rest of the formation was divided in three units, which can be recognized with electric logs and seismic sections. The lower unit can be assimilated to Bajo Barreal inferior described in outcrops of the San Bernardo range, while the second and third units (or the last only), could correspond to Laguna Palacios member.

El objetivo inicial de esta tesis fue el conocimiento de la fauna de equinodermos de la cuenca de Tepuel-Genoa del Paleozoico superior con fines bioestratigráficos y paleogeográficos. Durante el desarrollo de la misma me topé con el escaso conocimiento que existe de este grupo fósil para la Argentina, no solo en lo que hace a la descripción sistemática de los mismos en donde la mayoría de las citas corresponde a equinoideos del Cenozoico, sino a la falta de textos en habla hispana que traten los diferentes aspectos de los mismos. Los textos generales como el de Camacho (1966) y Clarkson (1986) son escuetos al tratar la problemática de este grupo y en parte desactualizados. Dado que para el desarrollo de la tesis me fue necesario abarcar el estudio de los equinodermos en extensión, consideré interesante el generar un texto general que trate este grupo con una sistemática y morfología actualizada, así como incorporar la problemática de los oscículos desarticulados los cuales son tratados en extensión en esta tesis, y lo cual no fue hecho en el Treatise of Paleontology (Moore, 1978). Este último tema fue iniciado casi sincrónicamente por la escuela americana y rusa, teniendo un gran empuje y continuidad de los estudios en la escuela rusa. Lamentablemente el grueso de estas publicaciones se hallan en ruso y son casi imposibles de conseguir. A fines de incorporar estos conocimientos al habla hispana se organizó los conocimientos que había de estas morfoespecies a fines de crear una guía sistemática de fácil acceso que ayude a iniciar este tipo de estudios en la Argentina.