A detailed gravity study over a known Devonian carbonate buildup: the Shell Golden Reef, northwestern North Dakota
A detailed gravity study was conducted over the Shell Golden reef in the northwestern North Dakota portion of the Williston Basin. The Shell Golden reef has been previously interpreted to be reefal facies within the Winnipegosis Formation (Devonian) located at a depth of about 2400 meters. Based only on information from wireline logs and core from two boreholes, the Shell Golden well #34X-34 has been previously interpreted as a pinnacle reef with 70 meters of relief and undetermined diameter. The basin regime of the Elk Point shelf created an environment that allowed the growth and development of the Shell Golden reef, numerous pinnacle reefs, and mounds in northwestern North Dakota and southeastern Saskatchewan. Carbonate buildups within the Elk Point Basin have previously been interpreted to be 70 to 105 m thick with diameters ranging from 0.5 to 5.0 km. For this study, vertical and horizontal positions were surveyed to +/- 3 cm and+/- 1 m, respectively, providing gravity measurement accuracy of .01 mgal. Five profiles were surveyed with three east-west lines intersecting two north-south lines forming a cross over the expected center of the reef. The distance between each line was 1.6 km and gravity-station spacing along each line was 0.4 km. The additional information provided by the. analysis of the residual Bouguer gravity anomalies generated in this study indicate that the Shell Golden reef consists of a north-south trending elongate carbonate buildup rather than a pinnacle reef. The length of the elongated carbonate buildup is about 4.0 km both north and south of the location of the two drill holes, the width is between 850 and 1500 m, and the relief ranges from about 90 to 105 m, Analysis of the residual Bouguer gravity anomaly further indicates that the Shell Golden well #34X-34 is on the south flank of the reef and not on the apex.
Reefs--North Dakota; Geology, Stratigraphic--Devonian; Gravity--North Dakota, Geology
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