Heath-Amsden Boundary on Central Montana Uplift at Big Wall Field, Musselshell County, Montana: ABSTRACT

Abstract

The Big Wall field was discovered by The Texas Company in 1948. The main producing reservoir is a stratigraphic unit within which are irregularly bedded sands having a linear distribution. These sands also produce at the Melstone and Northwest Sumatra fields. Workers agree on most aspects of the Heath formation of the Big Snowy group (Chester) and the Amsden formation, but the relationship of the sand-shale producing zone to these formations has remained conjectural. Initially an Amsden affinity was held, and of late the zone has been placed in the upper Heath formation. Both the lower Amsden and Heath formations are considered Upper Mississippian in age by the U. S. Geological Survey. One factor has probably caused more confusion than any other in differentiating the producing unit from the Heath. The dark gray to black shale within which the producing sands are found is differentiated from the Heath shales only with difficulty, particularly when examined as cuttings. The sands and shales of the producing zone contain much carbonaceous plant material, the only discernible fossil, as compared with the distinctive marine fauna of conodonts and brachiopods found in the top of the Heath. Compaction slickensides are common in the carbonaceous shale. Cross-bedded sandstones and conglomeratic material indicate a more turbulent environment during deposition than there was in Heath time. Thin pebble conglomerates at the base in many places sharply mark the boundary with the Heath formation. All of the evidence mentioned indicates a hiatus, however brief, which does not favor correlation with the Heath. End_of_Article - Last_Page 532------------