The geology of the Newburg-South Westhope oil fields, Bottineau County, North Dakota

The Newburg and South Westhope oil fields of the northeastern part of the Williston Basin are essentially one continuous secondary stratigraphic oil trap as determined by well log studies. The reservoir rocks in the two fields are carbonates present near the top of the Mississippian Ratcliffe interval and sandstone present in the basal unit of the Triassic Spearfish Formation. The two pays” are separated by the post-Mississippian erosional unconformity which truncated the Ratcliffe beds. The two fields are marked with depressions formed as a result of the dissolving of deep seated, presumably Devonian, salt beds and complementary gradual subsidence of overlying sediments. Two stages of salt solution are recognized, a pre-Spearfish stage which is reflected in thickened Spearfish sediments in depressions in the New burg Field and a later post-Spearfish solution stage which deformed but did not affect the thickness of the Spearfish thickness in the South Westhope Field. Post-Spearfish beds are thickened in the latter field. During the post-Mississippian erosion interval the South Westhope area was higher than the Newburg area and was subjected to more erosion. At present, the two fields are sloping slightly to the northwest such that the formerly higher South Westhope Field is Lower than the Newburg Field. It is possible that during the first stage of salt solution the Newburg area had subsided slightly, relatively elevating the South Westhope area. During the second stage of salt solution this area subsided slightly, producing the present low position of the formerly high and eroded South Westhope area