Modeling 3-D Facies Architecture and Heterogeneity of Fluvial Point-Bar Elements Recording Meander-Bend Rotation: Implications for Reservoir Compartmentalization

Abstract

As fluvial point bars evolve, they commonly undertake rotations by different degrees, with the orientation of channel migration changing repeatedly through various stages of bar development. The change in rotating direction of meander bends may lead to the partial erosion of bar deposits accumulated in the earlier stages of bar development, and to the juxtaposition of complicated mosaics of scroll-bar units in planform. The associated migration of the in-channel pool zone may also result in variations in the dip direction and angle of bar-accretion surfaces, and in the thickness of bar deposits in three-dimensions. However, the relationship between rotational shifts of channel migration and the resulting stratigraphic architecture and lithofacies distribution within point-bar bodies remains poorly understood due to limited outcrop or subsurface evidence of the facies architecture of such evolutionary history.

To explore how multi-stage rotational shifts of channel migration may influence morphology of point bars in plan-view and stratigraphic architecture in cross sections, a forward numerical stratigraphic model has been employed. The model simulates the 3D architecture of point-bar elements with growth histories that comprise multiple stages of meander-bend rotation, using a combined geometric and stochastic approach.

The modeling approach is constrained by quantified sedimentological data from real-world case-study examples stored in a relational database. The model is able to simulate complex bar-growth histories and to incorporate facies architectures based on data from modern rivers and ancient successions that serve as geologic analogs. Informed by analog data, the model is used to examine the three-dimensional sedimentary architecture of point bars through multi-stage rotational shifts associated with changes in autocyclic controls on channel evolution at varying spatial and temporal scales. The modeling results show that accretion geometries are significantly influenced by rotation of bar development. In particular, channels undergoing abrupt shifts in migration direction and relatively large angles of shift can determine erosion of significant volumes of older bar deposits, and result in increased bar thickness along the most recent migration direction. The model is used to enhance reservoir modeling realism and to predict reservoir compartmentalization due to lithological heterogeneity associated with bar features such as bar-front mud drapes.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019