An Early Cretaceous age is considered to be most probable for the group of productive sandstones which are found in the northern Sweetgrass arch and which extend from a basal unit (Cut Bank), that lies directly on the deeply eroded Jurassic Rierdon Shale, upward 200 ft. in the section to include the main productive sandstones at Fred and George Creek and Flat Coulee fields (possible "Moulton" stratigraphic equivalents). Areas of substantial sandstone development, having good reservoir characteristics and oil "shows," occur at stratigraphic positions intermediate between the Cut Bank and "Moulton" and can be found at other localities on the northern Sweetgrass arch.

All of these sandstones have a general relationship with the underlying erosional surface on the top of the Jurassic Rierdon, and thus are indirectly related to each other.

Study of the oil and gas fields from the Lower Cretaceous of the northern Sweetgrass arch is confined for practical reasons to the more recently discovered fields for which electric logs are available. The three most important of these fields are Fred and George Creek, Flat Coulee, and Red Creek. Each produces from basal or near-basal Cretaceous sandstones but differs in detail

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regarding sandstone deposition and reservoir quality.

The Cut Bank Sandstone of the Red Creek area consists typically of black chert and quartz; it is conglomeratic at the base, grading upward into fine-grained, commonly clay-cemented sandstone. It is largely a blanket sandstone throughout the field. However a definite thinning of the major basal unit takes place on the eastern side of the field. The area of thinning coincides with development of a stratigraphically separate, relatively "tight" upper unit. The accumulation is largely structural, and the reservoir is filled nearly to the spill point. Approximately 45 ft. of critical closure is mapped on the reservoir beds, but shallower beds indicate only a north-plunging nose.

The main reservoir sandstone at Fred and George Creek shows evidence of having been deposited in a deeply eroded channel, probably at or near drainage base-level. Evidence of channel scour is prominent here as it is in some areas of "Moulton" deposition on the northwestern side of the arch.

The reservoir sandstone at Flat Coulee is, in depositional detail, considerably different from the sandstones at Red Creek and Fred and George Creek, although it may be nearly equivalent stratigraphically to the latter. At Flat Coulee, the reservoir sandstone appears to be a part of a major sandy shale unit (Ribbon) from which the shale has been removed, probably by shallow nearshore current activity.

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