1999 Conference Abstracts

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1999 Abstract: McIlroy et al.

Applications of High-Resolution Sequence Stratigraphy to Reservoir Prediction and Flow
Unit Definition in Aggradational Tidal Successions

Duncan McIlroy, Stephen Flint, John Howell
Department of Earth Sciences
University of Liverpool
Liverpool L69 3BX, UK


A common element of sequence stratigraphic models for ancient shallow marine systems is the shift from wave- dominated shoreface deposition during highstand to tidal/ estuarine deposition within incised valleys during early base-level rise. In this widely used model, tide-dominated sedimentation is thus restricted to specific base-level positions, which has important implications for prediction of reservoir geometry and quality. However, this model neither adequately explains nor allows reservoir prediction in aggradational tide-dominated reservoir successions. The Lower-Middle Jurassic Lajas Formation of the Neuquén Basin, Argentina, comprises 500 m of well-exposed tide- dominated facies deposited within low-frequency unconformity-bounded sequences, which are tide dominated throughout. The maintenance of macrotidal conditions over several complete base-level cycles during the 8 m.y. within which the Lajas was deposited is interpreted as being due to the structural topography inherited from rifting, in which the whole sub-basin behaved as a structurally controlled estuary. Facies associations include prograding tidal deltas, stacked sandy tidal channel fills, extensive lagoonal deposits, and tidal-flat successions, which are locally cut by heterolithic tidal-channel-fill facies. Despite the narrow bathymetric depositional range of the Lajas sediments and the complex facies variability, parasequences can be defined and correlated on a kilometer scale. Unlike simpler fluvial or wave-dominated shallow marine systems, there are two types of parasequence, each with different net:gross sandstone ratios and different vertical/lateral reservoir-quality trends. These parasequences form the building blocks for accurate reservoir modeling in complex tidal reservoirs.