Geology and fractures in the San Pedro Structure, Alto of Río Seco range, Subandean Ranges, Province of Salta

Abstract

The San Pedro structure is located at the north-central section of the Subandean Ranges morphostructural unit. Its stratigraphic column is characterized by a complex geologic history and a over position of a different origins sedimentary basins , with a total backfill of more than 10,000 m developed at different geological ages and under different environments. The San Pedrito reservoir (Devonian levels) located at the Acambuco Block of the Tarija basin can be studied like a naturally fractured reservoir due to the fracturing and faulting features present in the the anticline core. A fracture study has been done, focused on Las Peñas and San Telmo Formations outcrops to analyze the links with subsurface units (Carboniferous basal levels) which compose the reservoir rocks of the San Pedro field. The overlap of major zones of intense fractures on surface with high accumulated production values permit to postulate that the deformation affected all Carboniferous deposits like a single package developed in a consistent way. The Carboniferous deposits may show minor discontinuities but the tectonic stress does not decouple the upper and lower Carboniferous deposits. The orthogonal fractures (type I) have their major intensity values at the ridge sections and the parallel fractures (type II) have their mayor intensity values at the forelimb sections. This last set keeps a close relationship with the bend fold model and to the accumulated production of the San Pedro Oil field.