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

Abstract

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.