Depositional environments of the lower cretaceous Muddy Sandstone, southeastern Montana

The interpretation of the environments of deposition is an important aspect in the prediction of reservoir potential and trapping mechanisms. If trends and morphology are accurately predicted, then exploration techniques are greatly enhanced. It has been suggested that the use of composition and texture alone might be sufficient to identify environments within a single basin. As a test of this concept, the Muddy Sandstone in southeastern Montana was studied, using both full-diameter and sidewall core data. The six Muddy zones represented a complex of fluvial-deltaic environments. The lowermost zones represented strong flu vial influences that gradually gave way to deltaic complexes as the Muddy transgressed eastward. These environments were interpreted from full-diameter cored wells and extended throughout the area by use of isopach maps and sidewall core data. As a test of the concept of identifying environments from only petrographic data, all of the data from the full-diameter cored wells was used. Discriminant-function analysis was made to determine if it was possible to separate the already known environments into groups. It did not prove possible to do this, except in an extreme case or two. This is believed to be due to the fact that the environments represented are gradational and no sharp breaks exist between them. However, environmental interpretation and mapping was possible with limited amounts of data as long as a controlling factor such as full-diameter cores was present to give a starting point