--> ABSTRACT: Genetic Sequence Analysis of Reservoir Facies Distributions and Gas Production, Eagle Sandstone (Campanian), Montana, by Mark S. Hanson and Laird D. Little; #91038 (2010)

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Genetic Sequence Analysis of Reservoir Facies Distributions and Gas Production, Eagle Sandstone (Campanian), Montana

Mark S. Hanson, Laird D. Little

Integration of numerical stratigraphic models with outcrop observations has proven effective in predicting the geometry, continuity, and geographic position of reservoir facies in the subsurface. The numerical models simulate marine shelf to coastal-plain stratigraphic successions by adding the interactive effects of eustatic fluctuations, tectonic movement, and quantity of sediment delivered to a sedimentary basin. The models predict that such successions are composed of multiple, time-bounded progradational wedges. These wedges are arranged systematically in three geometric patterns which are, in stratigraphic order, seaward stepping, vertical stacking, and landward stepping. In the first and last case, similar facies are laterally displaced on opposite sides of wedge b undaries. In the other case, facies are superposed across boundaries.

The 300-ft thick Eagle Sandstone exposed near Billings, Montana, comprises lower shoreface through coastal-plain facies and consists of seven progradational wedges arranged in the predicted hierarchical stacking pattern. Wedge bounding surfaces were traced physically over tens of miles, thus establishing their utility for high-resolution chronostratigraphic correlation. Upper shoreface sandstones, potentially the best reservoir facies, are laterally displaced across wedge boundaries in the lower and upper part of the formation but are superposed in the middle part.

This method of dividing thick sandstones into progradational events and using event boundaries as the basis for chronostratigraphic correlation was applied to the Eagle Sandstone in natural gas fields in north-central Montana. Discrete, time-correlative progradational wedges comprising shallow marine strata were recognized from well log signatures. The positions of reservoir facies within the wedges and, therefore, production across the field are controlled by the hierarchical stacking geometry. Vertical fluid communication between reservoirs occurs where favorable facies are vertically stacked. Potential for additional production beyond existing field limits is suggested by observed lateral facies shifts that occur with the seaward or landward stepping of wedges.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.