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The Cedar Creek anticline is an elongate northwest trending flexure approximately 100 miles long and 10 miles wide located in eastern Montana on the northwest flank of the Williston basin.
As expressed in surface beds of Upper Cretaceous age, the structure is an asymmetric fold with
the steep side on the west. Minor structural closures occur along the axis. The general appearance is one of a long, narrow flexure typical of the compression-type Rocky Mountain anticlines.
Increased exploration for oil since the discovery of the Glendive field in late 1951, has made available geologic data which reveal a considerably different structural development than most geologists expected. The preparation and interpretation of isopachous maps, paleogeologic maps and cross sections indicate that the Cedar Creek flexure is not the result of compressive forces, but due to a sequence of events, i.e., vertical uplift and eastward tilting, normal faulting, extreme truncation, and finally rejuvenation. The growth commenced in early Mississippian time and continued intermittently into Eocene time. The present structure is the resultant of several stages of structural movement which in many ways is similar to the structural development of the Nemaha ridge of Oklahoma and ansas.
Until 1936 when the first discovery of oil was made in the Williston basin by the Montana Dakota Utilities Company near the south end of the Cedar Creek anticline, the trend was thought of as gas-producing only. Gas was first discovered in 1913 on Gas City dome about 10 miles south of Glendive, Montana. Since this date, approximately 350 gas wells have been drilled. Most of the gas is being produced from the Judith River sandstone at an average depth of 900 feet. According to Petroleum Information, the present gas reserves on Cedar Creek anticline are approximately 60 billion cubic feet. Several of the wells had an initial potential of 25 million cubic feet of gas daily.
The 1936 oil discovery is reported to have flowed at the rate of 125 barrels of oil per day with a small amount of water from the Madison limestone at a depth of 6,800 feet. The test was finally abandoned because of water trouble. Four other deep tests were drilled prior to 1951. All of the tests had good shows or actually produced some oil from either the Mississippian or Ordovician carbonates, but were soon abandoned because of water shut-off difficulties. Altogether, several thousand barrels of oil were produced from these five tests.
It was not until the discovery by The Texas Company, late in 1951, that impetus was given the Baker-Glendive area. Since that time 50 tests have been drilled, of which 37 have been successful. These tests have revealed a thick stratigraphic column of potential reservoirs which includes the Winnipeg sandstone, Red River dolomite, and Gunton dolomite of Ordovician age; Interlake dolomite of Silurian age; Jefferson dolomite of Devonian age; and limestone and dolomite of the Madison group of Mississippian age. Discoveries have been made in the Charles and Mission Canyon formations of Mississippian age, the Interlake group of Silurian age, and the Gunton and Red River formations of Ordovician age. Currently most of the production is from the Red River formation.
At present seven of the several surface or seismic structures have been found productive. Numerous possible traps have not been tested and it is probable that during the next few years more discoveries will be made.
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