ABSTRACT

The Newburg Field which lies on the east flank of the Williston Basin has obtained the first significant production from the shallow Jura-Triassic "Spearfish" section in this basin. This red clastic section lies unconformably upon truncated Charles (Mississippian) and the two formations serve as a common reservoir through stratigraphic entrapment.

The reservoir of the "Spearfish" section is a very fine grained quartz sandstone, interbedded with silts and shales, which ranges in thickness from 40 feet to an updip wedge out. The Charles portion of the reservoir is an 18-foot bed of buff, fragmental, partly oolitic, and argillaceous limestone. Porosities and permeabilities vary greatly but are fairly low.

The Charles at the Newburg Field lies in a synclinal position, and the transgressing basal sands of the "Spearfish" section were deposited in this trough, wedging out updip and laterally to form a stratigraphic trap. The Charles pay section, which also wedges out by truncation, is believed to be the carrier beds from which the oil "leaked" into the overlying "Spearfish".

Currently there are 25 wells in the Newburg Field, the majority of which are producing from both the "Spearfish" and underlying Charles. Some producers are completed only in either the "Spearfish" or Charles. Production rates vary from 75 to 125 barrels of oil per day.