Structurally Caused Reservoir Heterogeneity--Its Influence on Reservoir Performance

LEWIS, HELEN, Resource Consulting Group, Helensburg, Dunbartonshire, Scotland

Geologic reservoir heterogeneity is the consequence of an original sedimentary framework and diagenetic and structural alterations to that framework. Heterogeneity in the fluid system can also occur, due both to original reservoir charging characteristics and to production practices. This paper addresses the structural elements of reservoir heterogeneity--determination of structural reservoir partitioning distribution and its effect on porosity and permiability values. For any particular reservoir, the resulting structural model can be tested using reservoir performances.

Anschutz Ranch East is a large asymmetric anticline in the thrust belt of Utah and Wyoming. The reservoir unit is the 300-m-thick aeolian Jurassic Nugget Sandstone. Reservoir operation is challenging, pressure maintenance having been required from the start of production.

Sedimentology studies demonstrate stacked dune/interdune sequences, with the interdunes being sufficiently extensive to be significant barriers to vertical communication. Because of the combination of moderate initial porosity and the stress states and temperatures imposed during tectonism, the deformation mechanisms are typified by compactive modes, with accompanying reductions in porosity and permeability. The exception is the overturned forelimb, where deformation has progressed to the dilitaut stage. Thus, most of this "fractured" reservoir is actually degraded by the fracturing, not enhanced. However, parts of the highly deformed forelimb have been greatly enhanced.

Fortunately, structural methods allow prediction of deformation patterns, including division of the Nugget into several layer-bounded mechanical units, with layer-normal flow barriers within these units, and compartmental faults offsetting the fold crest. Use of modern structural concepts has allowed for improved prediction of pressures, fluid types, and connectivity of injectors and producers, and so an increase in efficiency of reservoir management.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)