The widespread sandstone members of the Lower Mesaverde Group (the Iles Formation) in the Piceance Creek basin have significant lateral variability within an overall blanket morphology. This variability can be seen in the Corcoran and Cozzette Sandstone members in the Department of Energy's closely-spaced Multi-Well Experiment (MWX) wells, and can be interpreted from nearby outcrops along the Grand Hogback. Much of the lateral variability consists of thickness changes of both the sandstones and the interbedded lithologies. The number and thickness of depositional units which comprise a bed may change, but the overall sedimentation event is always represented. This type of variation can be ascribed to local variations along widespread, regressive, wave-dominated delta and shoreline environments. Distinctly lenticular sandstones occur in the upper zones of each progradational sequence. These are best represented in the overlying Cameo Coal Member, and can be shown to be distributary channel and splay deposits of a lower delta plain.

The locations of the thickest blanket deposits (the best potential reservoirs) may be a function of the locations of principal distributary systems. For the delta plain deposits, however, the main source of natural gas is the interbedded coals which should be thickest where distributaries were least abundant. Therefore, the best reservoirs for this zone should be in those areas of intermediate sandstone and coal thicknesses.

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