--> Facies Variability and Stratal Architecture of a Large-Scale Mississippi River Point Bar, False River, Louisiana

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Facies Variability and Stratal Architecture of a Large-Scale Mississippi River Point Bar, False River, Louisiana

Abstract

Although the general processes by which meandering river point bars are constructed have been known for some time, there is a lack of clarity regarding why small scale heterogeneities develop. This is especially true in the largest systems, equivalent to some of the most important hydrocarbon reservoirs, such as the Alberta oil sands. In this study, we combined shallow coring and geophysical logging to examine the 80 km2 False River Point Bar on the Lower Mississippi River. We evaluate the stratigraphic architecture of the point bar and associated channel tie system and use the knowledge gained to better comprehend point bar construction at different phases and in different parts of the bar. Previous studies of small to medium-scale systems have not been able to predict the degree of lateral heterogeneity of larger systems— where data on a scale of 1 m to approximately 300 m is needed. Guided by a high resolution DEM, this study combines sediment coring and gamma ray, hydraulic profiling, and electrical conductivity logging at 19 sites (12 sites with sediment coring) in the bar head, apex and tail, and channel plug. Petrel is utilized to create a model which recognizes variability in grain size and facies across the bar. Results show not only large scale differences in lithology and permeability between these areas but also on a finer scale within these locations. This is especially true at the bar apex where an abandonment facies is observed. At the bar tail, where data was collected from closely spaced sites (approximately 30 m apart), data shows a rapid deterioration in permeability towards the youngest parts of the bar. The channel fill deposits that once connected the present oxbow lake to the Mississippi River show even more pronounced changes over 10's of meters and generally poor reservoir characteristics. The False River system— and presumably other major river point bar deposits— show coherent differences across their width with a fining and reduction of reservoir quality going downstream and a sharp decrease immediately before the final abandonment of the channel.