Variability of marginal- and shallow-marine reservoirs within a low-order lowstand sequence (Mulichinco Fm, Aguada del Chivato Field, Central Neuquén Basin, Argentina)

SCHWARZ, E.; VEIGA, G.D.; LIBERMAN, A.; CORBERA, R.

Manchester

Congreso; 30th Meeting of Sedimentology; 2013

Stratigraphic units developed during low-accommodation conditions (i.e. forced-regression or lowstand) are commonly regarded as relative simple progradational packages in which deltaic deposits are dominant, particularly in interior seas with ramp-type profiles. The Valanginian Mulichinco Formation is a second-order lowstand unit in the Neuquén Basin, encased between offshore/basinal mudstones. It shows high vertical and along-strike variability of reservoir-type, marginal- and open-marine facies, precluding the application of simplistic models. The aim of this study is to reconstruct depositional systems and sequence-stratigraphic evolution of the unit at field scale, which ultimately could help to better understand and characterize conventional reservoirs. The Aguada del Chivato Block is located in east/central Neuquén Basin, and the field development of the Mulichinco Fm (140 m thick) is restricted to a 20-km2 area. For this study, 200 m of cores from six wells were described, subsequently integrated with petrographic and petrophysic data, well-log suites from 33 wells and seismic data. Nine facies associations were identified and interpreted in the unit: five are related to fluvio-deltaic depositional environments, two interpreted to represent marginal- and open-marine, carbonate systems and two attributed to estuarine conditions. This, together with key stratigraphic surfaces, allowed for the recognition of four sections (A-D). Section A (15 m thick) sharply overlies offshore shales and is composed of, sandstone-rich channel-fill facies and subordinated interdistributary plain deposits, collectively representing a delta plain. The base is interpreted as the second-order sequence boundary and the section represents an aggrading interval with moderate accommodation. This basal section is topped by a field-scale transgressive surface that marks the onset of a carbonate system (Section B). This section has an internal retrogradational stacking, in which dolomitic sandstones and heterolithics, interpreted to represent marginal-marine conditions, grade upward to open-marine, high-energy, grainstones and packstones. The top of this section is a regional marker and represents the restoration of pure siliciclastic conditions. The following interval (Section C, 80 m thick) is characterized by decameter-scale, coarsening-upward successions (mudstones to heterolithics to sandstones), likely reflecting prodelta to proximal delta-front settings in a fluvial-dominated (tidal influenced?) deltaic system. Delta-plain deposits at the top of the section suggest steady progradational conditions, which are eventually interrupted by a turnaround from the long-term lowstand to transgressive conditions. Under this new situation (Section D, 25-30 m), channel-fill facies with tidal influence (inner-estuary) grade upwards to bioclastic sandstones (outer-estuary), sharply capped by offshore shales. The results of this study allowed for a detailed characterization of different sandstone and carbonate reservoirs within a sequence-stratigraphic context. It also explained contrasting performances of similar sandstone reservoirs at similar stratigraphic location. Finally, it also illustrated that the evolution of marginal- and shallow-marine depositional systems during long-term, low-accommodation conditions can be highly intricate, and derived reservoir complexity might not be captured by simplistic models. Acknowledgements. We gratefully acknowledge Medanito S.A. for providing subsurface data and permission to publish this article.