Abstract

Savageton Field was discovered in 2002 by Ensign Oil + Gas through subsurface mapping of a discrete zone in the lower part of the Upper Cretaceous Parkman Sandstone. Numerous shows, including minor production from a completion in 1976, highlighted the potential for an accumulation. Fifty-one horizontal and four vertical wells currently comprise the field, recently unitized for secondary recovery; Savageton will be the first horizontal waterflood in Wyoming. The primary EUR is 7.4 MMBO with an additional 5.6 MMBO anticipated through secondary recovery.

Savageton Field lies on the gently southwest dipping east flank of the Powder River Basin. Regional mapping revealed a classic stratigraphic trap; a convex, up-dip bulge of the lower Parkman at the transition from shallow marine sandstone to shelf mudstone. This unit is overlain by about 45 feet of mudstone providing a top seal and distinct break from the overlying upper Parkman.

The Parkman Sandstone is composed of multiple parasequences arranged in a west to east, progradational stacking pattern. The pay zone is the distal portion of a single parasequence at the base of the lower Parkman sandstone. The average reservoir thickness is 10 feet with average porosity and permeability of 13% and 5.8 millidarcies. The upper Parkman is composed of a massive amalgamation of wet shoreface deposits over 100 feet thick. In the Savageton field area the top of the Parkman is marked by a transgressive surface of erosion, overlain locally by the Ferguson sandstone member and the Pierre Shale.

Hydraulic stimulation of the lower Parkman pay zone in the discovery well and subsequent vertical well offsets resulted in communication with the water-bearing sandstone of the upper Parkman and high water cuts. Geosteering of horizontal wells to maximize wellbore contact in the thin lower Parkman pay zone eliminated the need for stimulation and prevented communication with the upper Parkman water zones. The successful application of horizontal drilling to both avoid water production and improve economics in tight reservoirs has potential application in many other reservoirs and should enhance waterflood sweep efficiency.