Anatomy and Sequence Architecture of the Early Post-Rift in Central Neuquén Basin (Argentina): Implications for Unconventional Reservoirs
Schwarz, Ernesto; Veiga, Gonzalo; Spalletti, Luis A.; Massaferro, Jose L.
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. More than three/fourths of the total post-rift strata (< to 320 m thick) comprise fine-grained sandstones to mudstones representing a storm-dominated shoreface to offshore system (Late Toarcian-Early Bathonian). The remaining stratigraphy (< 30 m) is dominated by sand-rich deposits with clinoforms interpreted to be 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 aggradational/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 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, which 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 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 controlled the deposition of organic-rich shales (potential source rock/shale play) and 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 enhance the understanding of its unconventional plays.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013