A Fractal Model for the Vertical Distribution of Sands in the Perales Oil Fields in the Basin of Golfo San Jorge (Argentina)

Abstract

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.