Application of Nonmarine Genetic Sequence Stratigraphic Concepts to Reservoir Characterization in the Fluvial-Lacustrine Westbourne Formation, Eromanga Basin, Australia

Douglas S. Hamilton, Mark H. Holtz, Joseph Yeh, Tom Lonergan, Phillip Ryles, Michael Hillyer, and Tim O'Sullivan

A high-resolution sequence stratigraphic analysis of the Westbourne Formation identified five chronostratigraphic genetic units each separated by thin, but laterally extensive, shale markers interpreted as maximum lacustrine flooding surfaces. The flooding surfaces were primarily identified by their lateral persistence and high gamma-ray log response, but marked changes in bedding architecture across these surfaces also facilitated their identification. The changes in bedding architecture reflect reorganization of the depositional systems from one depositional episode to the next.

The Westbourne Formation is interpreted as a series of fluvially-dominated lacustrine delta sequences. Although the genetic units generally display lobate to digitate sand body geometries, sediment transport directions between successive units is highly variable. The complex morphology of the distributary network and accompanying high degree of facies variability indicates shallow lacustrine sedimentation similar to that in the modern inland Niger River delta which provides a modern analog.

Westbourne fluid flow trends were established by mapping water encroachment during field development, observing differential depletion in repeat formation test data, and monitoring production response to water shut-off workovers. The fluid flow trends emphasized the highly layered character of the Westbourne reservoirs and integrating these trends with the geologic architecture defined stratigraphic controls on Westbourne flow units.

The stratigraphic framework proved essential for unraveling sediment transport patterns and thus, predicting reservoir sandstone distribution. The stratigraphic framework also provided the key to understanding water encroachment and pressure depletion which, when combined with predicted sandstone geometries, identified several step-out drilling and recompletion opportunities.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California