Production evidence for geological heterogeneities in the Anschutz Ranch East Field, western USA

The Anschutz Ranch East Field is a large asymmetric anticlinal trap in the Wyoming-Idaho-Utah thrustbelt of the western USA. The aeolian Jurassic Nugget Sandstone reservoir unit is a layered sequence of dunes and stratigraphically flat interdunes. This interpretation is supported by patterns of reservoir performance in the gently-dipping, nearly planar backlimb of the fold where there is little structural deformation. Structural features which developed during emplacement of the Anschutz Ranch East truncation anticline are partially dependent on the distribution of lithofacies, and the changes in rock properties which result from the deformation are overprinted onto the pre-existing rock properties. An extensive coring programme demonstrates that most of the strain in the Nugget Sandstone is in the crest and forelimb of the fold. Throughout the structure, the deformation is primarily of the compactive (sealing) type, leading to lowered porosities and permeabilities, and thus both to degraded reservoir quality and to flow barriers. Structurally-created low-permeability zones are identified or interpreted at a range of scales, and production histories provide sufficient evidence to establish the role of the larger features as flow partitions.