Hiperpicnites (Inundites) as reservoir analogs: an example in the Triassic Gondwanic deposits of Arroio Moirão Graben, Rio Grande do Sul

Abstract

Sandstones associated with episodic flooding models, such as inundites from hyperpicnal flows in shallow water bodies, can be hydrocarbon reservoirs. However, there are few studies to understand the architecture and heterogeneity of these models, resulting in difficulties to explore and exploit these reservoirs. The current work aimed to analyze Triassic strata from the Paraná Basin belonging to the Santa Maria Formation, which occur as isolated fragments over the Sul-Rio-Grandense Shield in the region of Arroio Moirão Graben (RS). Thus, a comparative study of flood models was made, as a methodology for data collection, systematic mapping, facies heterogeneity analysis and sequence stratigraphy were used, including columnar profiles survey, identification of facies associations and sequences, and key surfaces. Secondarily, some petrographic analyses were performed. It was possible to define the main deposits of sandstones, in which tabular geometry, poorly selected grains and clay matrix predominate. Five ascending granodecrescent depositional cycles were distinguished, limited at the base and the top by erosive surfaces, which mark recurrent heterogeneities. Reservoir facies are subarkoses, with clay matrix aggregates of kaolinite, intergranular porosity and shrinkage. Potential reservoir facies formed by coarse to conglomerate sandstones with tangential and planar bed cross stratifications were identified. Data integration resulted in the elaboration of a model of lateral and vertical facies variation of lake hyperpicnites, as a potential analogous model of compartmentalized-type reservoir. These results allow the prospection of other sandy deposits from this stratigraphic unit of the Paraná Basin, in order to regionally evaluate the reservoir analogue.