About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 43 (1959)

Issue: 5. (May)

First Page: 1103

Last Page: 1104

Title: Stratigraphy of Amsden Formation in Wolf Springs-Delphia Area, Central Montana: ABSTRACT

Author(s): R. D. Ramsey

Article Type: Meeting abstract


The Amsden formation thins progressively northward by pre-Piper truncation. This can be demonstrated

End_Page 1103------------------------------

by stratigraphic cross sections. By means of the same media it can be shown that the Custer anticline and the Delphia structure were both relatively high in pre-Piper time, because the Amsden at both places is thinner than that in nearby off-structure wells. This gives evidence for postulating that these two Laramide structures had some expression in post-Amsden, pre-Piper time. The same "thins" show up in Amsden isopachous maps.

The Amsden formation in central Montana produces commercial quantities of oil at the Wolf Springs, Delphia, Gage, and Big Wall fields. Stratigraphic studies of the productive intervals at Wolf Springs and Delphia and the surrounding area show that reservoir space in the form of vugs and fractures has been developed in the dense upper dolomite part of the Amsden. Development of such vuggy porosity came about as a result of the dolomite being exposed to a prolonged period of pre-Piper sub-aerial erosion in this general area. Fracturing along pre-Laramide structural highs (later modified by Laramide folding) provided solution channels for ground-water movement. Typically, the Amsden pay is a dense, light-colored dolomite, commonly cherty, vuggy, and fractured. In some places it is brecci -like in appearance and may include chert in the form of irregular inclusions and vug linings. Very little, if any, porosity is intercrystalline.

Subdivision of the Amsden is possible by the use of electric logs. The Amsden can be divided into upper and lower zones stratigraphically on the basis of using a persistent low-resistivity kick as the dividing line. Both zones produce oil. Either can be considered a possible reservoir, depending on whether that part of the section is subjacent to the Piper and hence was exposed at the unconformity surface by erosion.

End_of_Article - Last_Page 1104------------

Copyright 1997 American Association of Petroleum Geologists