Large-Scale Depositional and Palaeogeographic Reconstruction of the Pilmatué Member (Neuquén Basin): Understanding from the Source to the Sink

SCHWARZ, E.; VEIGA, G.D.; ECHEVARRIA, C.; SPALLETTI, L.A.

Congreso; AAPG International Convention and Exhibition; 2019

The Pilmatué Member (Agrio Fm., Valanginian-Hauterivian) has recently received significant attention due to both unconventional exploration and tight gas development. This paper presents facies, depositional and sequence-stratigraphic analysis of the Pilmatué Member, both in time and space, using outcrop and subsurface information, in order to provide with better regional exploration models. Deltaic deposits representing fluvial/delta plain, delta front and prodelta settings have been recognized in the subsurface near Zapala. Sediments are commonly well sorted and typically no coarser than medium sand and the unit is 400 m thick. Delta-front facies indicate that river deltas were dominant, but wave influence to dominance is also recorded, particularly toward the top. In outcrops located to the west of the previous region, the unit represents mostly prodelta to shelf settings, with a dominant siliciclastic contribution. This area is interpreted to represent the distal expression of the deltaic setting. In northern Neuquén, the unit shows a significant increment of carbonate contribution in the proximal marine facies. In this region the unit is considered to represent a mixed (siliciclastic-carbonate) shoreface-offshore system dominated by normal and storm waves, fed by an effective dispersal system bringing sediments from the southern deltas. Several parasequences are recorded within the 700-m thick unit. Progradation of the system is mostly from south to north, although the younger parasequences show an east-west progradation orientation. In the northernmost Neuquén and southern Mendoza the unit (350-m thick) solely represents accumulation in carbonate-dominated offshore to basinal settings. Organic content in significantly high in several stratigraphic intervals. The large-scale, but detailed palaeogeographic reconstruction of the Pilmatué Member provides a robust framework for understanding and predicting reservoir unconventional (tight and shale oil) properties.