ABSTRACT

The Middle to Upper Ordovician Winnipeg Sand has attracted Williston Basin explorationists as a potential hydrocarbon reservoir ever since early drilling in the 1950's found oil and gas shows and fair to good porosities in some areas of the Basin. Deeper pool drilling in Beaver Lodge Field in 1956-1958 found Winnipeg pay zones via production tests which flowed from 3000 to 8000 MCFGPD. Although these wells were completed at the time in younger zones, one was later (1963) recompleted in the Winnipeg for 6540 MCFGPD and 3.5 BCPD, establishing the first Winnipeg Sand production in the Basin.

Although there were numerous shows in subsequent years, including some apparently commercial production tests in the late 1970's, no further wells were completed successfully in the Winnipeg Sand until the Gulf 1-21-IB Leviathan in eastern Stark County, North Dakota. This well discovered a Winnipeg productive zone in early 1981 which was completed for 3500 MCFGPD with 1 BCPD and sparked renewed interest in the Winnipeg across the entire Basin. Further drilling in the area north of the Leviathan well resulted in several additional wells which are either completed as gas producers or are still testing the Winnipeg reservoir.

The Winnipeg Sand is a blanket marine sand which thickens to a maximum of about 180 ft near Williston, North Dakota. Although present over most the of the Williston Basin, it does have a depositional edge southward at about the South Dakota state line, and is absent from all but the northernmost end of the Cedar Creek Anticline. The sand is fine to medium grained, subrounded, well sorted and commonly frosted. Grains are predominantly quartz, but dolomitic sands also occur as does a variable clay fraction which adversely affects reservoir quality and requires careful selection of stimulation fluids during completion. Locally, the sand has poor to no porosity, due to silica cement (making the rock a near-quartzite). The depositional environments range from beach and barrier bar to near-shore eolian.

Regional porosity trends show the sand to be economically most prospective in the west-central area and on the eastern flank of the Basin. In northeast Montana, in North Dakota just west of the Nesson Anticline and in southwest North Dakota the sand is generally tightly cemented. Stratigraphic traps may occur where matrix permeability is lost updip or where the sand itself pinches out between the overlying Winnipeg Shale and underlying Deadwood Formation.