Formação Guarda Velha, Caçapava do Sul, RS: modelo de afloramento análogo em sistemas aluviais

Abstract

Caçapava do Sul, Rio Grande do Sul, is characterized by outcrops of the basal unit of the Guaritas Group in the Camaquã Basin, named the Guarda Velha Formation. This formation is formed by neoproterozoic alluvial-dominated coarse-grained sandstones, pebbly sandstones and conglomerates. This work presents virtual oucrop models (VOM) of four outcrops of this Formation to build an analogue reservoir model. The VOM were built in PhotoScan Software and integrated with field data for the stratigraphic and faciological interpretation. In the field, sequence stratigraphy was used to perform the analysis of the facies and the description of the depositional sequences. On the VOM, photofacies and photoestratigraphic interpretations were performed. The associations of facies identified were alluvial fans, conglomerates and fluvial sandstones. The facies are: conglomeratic breccia with planar cross bedding (Gm); polymorphic conglomerate with conglomerates and sandstone lenses (Gms); sandy conglomerate with gravel and pebbles (Gt); solid conglomerate with clast imbrication (Gp); medium to thick sandstones with planar cross stratification (St); sandstones with planar stratification (Sh); The interpretation of the reservoir model was performed in the MOVE Software, The surfaces of the four MVA were integrated to determine the connectivity between the sequences, which occur in successive upward ascending cycles, being succeeded or laterally distributed with lower energy facies. The highest frequency cycles range from a few tens of centimeters up to 5 meters and were termed elementary cycles. The lower frequency cycles are defined by the stacking pattern increasing the volume of the St and Sh facies to the top. Finally, an analogy was made between the rocks of the Guarda Velha formation and the braided river systems reservoir rocks of the Prudhoe oil and gas field in Alaska.