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How an Integrated Reservoir Study has Totally Changed the Understanding and Development Plan of a Mature Alluvial Reservoir, the Case of the Papagayos Formation in the Vizcacheras Field, Cuyo Basin, Argentina


The Papagayos Formation is a continental siliciclastic alluvial succession (Lower Cretaceous?) deposited in the Cuyo Basin, Western Argentina. The Papagayos Fm. is the main producing reservoir of the mature Vizcacheras field (220 wells, STOIIP 76Mm3) with excellent reservoir properties (∼F 27%, 13D). New development opportunities were identified in 2011 after a new conceptual model revealed reservoir development to the west of the main field in very low structural positions. Understanding the depositional setting of the Papagayos Fm. has become a key issue for further field development. Unpredicted thicknesses and reservoir quality variations meant that only 60% of wells were successful. The objective of the reservoir characterization study was to reduce drilling risk, assess infill well placement and reservoir delineation. Based on the results of the integrated study, we subdivided the Papagayos Fm. into two depositional units. The lower unit is characterized by an alluvial-fan facies association dominated by debris-flow deposits (chaotic, matrix supported conglomerates and gravels). This unit shows low transport capacity and changes laterally into fine-grained floodplain deposits (fan toe deposits). The upper unit is characterized by fluvial-fan facies association showing amalgamated, poorly confined channel deposits in proximal areas changing towards variable to low N/G fluvial system in the far-from-source portions of the fluvial system. Changes in discharge and bedload capacity between the two units were linked predominantly to climate variation causing a transition from an episodic to a perennial flow regime. Implications of such changes are visible in terms of extension and development of both units. Short ranges for mass-transport deposits and limited extent of the alluvial fan reservoir facies are expected in the lower unit. Conversely, better reservoir quality and greater extent, together with thinning and lower N/G towards distal areas is expected in the upper unit. Detailed paleoflow study (FMI) together with sand maps and seismic control confirmed local migration of the system towards the west-northwest of the main field. Deterministic geological modeling generated from constraining the depositional model with seismic and well-data, enabled prediction of sand- facies distribution within and nearby to the field. As a result of this study, the infill drilling campaign was redesigned and the potential of future water-flooding demonstrated.