STRATIGRAPHIC- TO FACIES-SCALE HETEROGENEITY IN A TIGHT-GAS FLUVIAL RESERVOIR ANALOG (MULICHINCO FORMATION, NEUQUÉN BASIN, ARGENTINA)

KIETZMANN, D.A.; TAHA, N.; OLIVO, M.S.; ESPÍNOLA, J.; WALDMAN, N.

Congreso; International Convention & Exhibition 2019; 2019

Fluvial multiscale sedimentary heterogeneity may result in reservoir compartmentalization defined by separation of flow units with different porosity and permeability distribution. The Mulichinco Formation exposed in the southern-central Neuquén Basin represents an ideal case study of fluvial and inner estuarine systems to attend these issues. Although these deposits are included among the main tight-gas reservoirs of the basin, variations between large- to small-scale sedimentary elements and their potential role as flow barriers were not studied in an integrated analysis. The aim of this work is to characterize the sedimentary heterogeneity of the unit, integrating detailed sections from reservoir-scale outcrops, together with facies associations and architectural analysis as well as three-dimensional variations in facies and petrophysics of the different channel-body types. The unit includes six main facies associations: braided channels, meandering channels, terminal channels, floodplain deposits and subaqueous plains deposits, which represent the deposition in a proximal to distal fluvial system, and in an inner-estuarine sub-system. The channel bodies vary according to width/thickness ratios, scale of bounding surfaces, storey amalgamation, in-channel accretion style, facies and petrophysics. Four potential scales of heterogeneity are identified: stratigraphic distribution of channel bodies, facies association, facies and lamina. The stratigraphic architecture shows proximal fluvial deposits pass vertical and laterally to distal fluvial and estuarine units. Moreover, a vertical decrease in net-to-gross ratio and the amalgamation degree of channel bodies suggests channel body connectivity would be sensitive to channel-body type. Facies associations and facies scale seem to have a reduced influence on porosity and permeability distributions, whereas at the lamina scale, grain-size, matrix fraction and cement composition represent the main controls on the reservoir properties.Depositional architecture, qualitative and quantitative data of sedimentary units allow developing a conceptual depositional model, which provide an invaluable basis for updating potential 3D geological models. This work shows that multiscale characterization approach should be implemented to evaluate possible carrier and barrier distributions in fluvial reservoirs, allowing prioritize which scale of heterogeneity requires to be focused on subsurface reservoir modelling.