Resumo

HECHEM, Jorge J. Modelo predictivo de reservorios en un sistema Fluvial efímero del Chubutiano de la cuenca del Golfo San Jorge, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 1994, vol.1, n.1, pp.3-14. ISSN 1853-6360.

The object of this paper is to develop a geological model intended to predict the spatial distribution of sandstone bodies (potential reservoirs) which originated in an ephemeral fluvial system draining into a lacustrine basin. Outcrop and subsurface data of the Lower Member of the Bajo Barreal Formation (Upper Cretaceous), of the Golfo San Jorge basin (Chubut and Santa Cruz provinces, Argentina) was used. The interpretation can be extrapolated to other units with similar or analogous tectonosedimentary conditions. From facies analysis and palaeoenvironmental interpretation it is considered that most of the sediments of the Lower Member of the Bajo Barreal Formation were deposited in shallow and temporary lacustrine environments, in mudflats and ephemeral fluvial systems (Hechem et al., 1990). In proximal areas alluvial fans developed with important volcaniclastic components grading downstream into ephemeral fluvial systems, controlled by sudden and episodic increments of discharge. The sand load was transported into the basin by currents which fluctuated between debris flows and normal tractive flows, with a predominance of high density flows because of abundant volcaniclastic input. Most parts of the currents were not channelized causing sheetlike or lobed sandy deposits (Fig. 1b, c, d), whereas others were channelized, depositing lenticular bodies (Fig. 1e). A great part of the sedimentation took place in shallow lakes and in flooded mudflats, with many deposits reworked by waves. The proposed model predicts that commemcing from a source of coarse clastics through an ephemeral fluvial system, taluses originated by active extensional faults favour the deposition of sandy lobes (Fig. 2). These lobes concentrated in bands parallel to the faults, with preference in the downthrown blocks. Upstream the number of sandstones decrease with a consequent increase in downstream shales resulting in facies deterioration in both senses. Parallel and perpendicular to the transport direction, the sandy layers wedge out into shales, although, by through amalgamation and compensation, the number of reservoirs may be maintained (Fig. 4). The bands with sandstone concentration are associated with ancient extensional faults, which will also be migration pathways from the source rock. This assures access of fluids to the reservoirs. The structures at different scales in the downthrown block from roll over structures to tectonic inversion anticlines (Fig. 3) will add up to the stratigraphical entrapment factor due to wedging-out of the lobes into shales. The model allows for the prediction of sectors with sandstone concentration and the spatial distribution of possible reservoirs (Fig. 4) and is a tool to improve correlation, thus avoiding erroneous interpretations (Fig. 5). It can be applied both to exploration and to development of hydrocarbon fields.

Palavras-chave : Predictive model; Fluvial; Reservoir.

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