Anatomy and Sequence Architecture of the Early Post-(Argentina): Implications for Unconventional Reservoir

SCHWARZ, E.; VEIGA, G.D.; SPALLETTI, L.A.; MASSAFERRO, J.L

Congreso; AAPG ACE 2013; 2013

AAPG

A detailed facies and sequence-stratigraphy analysis was carried out in the early post-rift succession of central Neuquén Basin (Middle Jurassic Cuyo Group), integrating outcrop and subsurface information from a 2.500 km2 area. More than three/fourths of the total post-rift strata (< to 320 m thick) comprise five marine facies associations representing a storm- and wave-dominated shoreface to offshore system (Late Toarcian-Early Bathonian). The remaining stratigraphy (< 30 m) is dominated by a delta-front facies association interpreted to reflect a fluvio-dominated deltaic system (Late Bathonian-Early Callovian). The post-rift succession has a general wedge shape with abrupt changes in thickness and pinches out within the study area. Nine parasequences grouped in four parasequence sets (PSS) where identified in the Late Toarcian-Early Bathonian interval. The basal three PSS, mostly composed of muddy sandstones and sandy mudstones representing offshore-transition to upper-offshore settings, conform a log-term transgressive trend (~ 10 ma) in which retrogradational and aggradation/retrogradational stacking patterns alternate. Organic-rich shale facies are associated with the maximum transgression, and they are overlain by the last PSS, which represents highstand conditions. This, in turn, is cut by a regressive surface of erosion, indicating an abrupt basinward shift and the onset of the fluvio-deltaic system. The long-term transgression is interpreted to represent a steady accommodation creation produced by the combination of thermal subsidence, differential compaction of syn-rift deposits, and eustatic rise, under a relatively low sediment supply. In turn, variations in the stacking pattern of retrogradational PSS are attributed to changes in the gradient of the topography being flooded, that was probably a remnant of ramps and flats regions generated during the syn-rift stage. These elements, coupled with the wedge-shape architecture of the study succession, suggest a gradual infill of the inherited, terraced syn-rift topography during the early post-rift. The complex interaction of global and basin-related factors reported for the Cuyo Group has controlled the localization and timing of organic-rich shales (i.e. potential source rock/shale play), as well as the preservation of significant volumes of low-permeability sediments (potential poor-quality reservoirs). The assessment of that complexity in any post-rift subsurface analogue (e.g. North Sea, Magallanes Basin) would therefore enhance the understanding of its unconventional plays