The Northwest Sumatra oil field, located on the Central Montana uplift, was discovered in July, 1952. The discovery well, The Texas Company's Grebe No. 1, was located on the basis of seismic investigations by The Texas Company. This geophysical work indicated slight seismic closure on one of the en echelon folds characterizing the Central Montana uplift between Ragged Point Dome on the west and Ingomar Dome on the east.

The surface formation in the field is Upper Cretaceous in age. The stratigraphic section drilled in the area consists of Cretaceous, Jurassic, Pennsylvanian, and Mississippian rocks. Oil is obtained from lenticular sands developed in two zones in the Upper Heath transition zone of Upper Mississippian age. The origin of these lenticular sands is attributable to sand-bar and dune development associated with estuarine and lagoonal conditions of deposition.

The maximum thickness of effective sand penetrated in any well is 148 feet, and the average effective sand thickness throughout the field is 46 feet. The production of oil is stratigraphically controlled by sand development, rather than structural position.

On December 1, 1954, 58 wells had been drilled in and adjacent to the field, of which 42 were producing, 15 were abandoned, and 1 was drilling. The 42 productive wells were producing about 3,900 barrels of oil a day. Production was restricted due to market outlet. There were, at that time, 1,680 acres, with an estimated recoverable reserve in excess of 30,000,000 barrels of oil with the field limits still undefined.

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