Horizontal Drilling in the Bakken Formation--The Hunt for an Elephant That Never Left the Source System
PRICE, LEIGH, U.S. Geological Survey, Denver, CO, and JULIE LE FEVER, North Dakota Geological Survey, Grand Forks, ND
New organic-geochemical studies show that bitumen extracted from the upper and lower shale members of the Mississippian-Devonian Bakken Formation does not correlate to Mississippian Madison Group oils, and that the Bakken shales have contributed only a minor percentage of the conventionally produced oil in the Williston basin. Instead, organic-rich Madison marls are an adequate source for the Madison oils. Also, few pathways exist for vertical migration of Bakken-generated oil to shallower Madison reservoirs. Published calculations suggest that the Bakken shales have generated 100-150 billion bbl of oil. If the conventional reservoirs were not charged by Bakken-generated oil, this oil would not have leaked from the basin. Thus, an in-place resource of 100-150 billion bbl of Bakken-gen rated oil may exist in fractures in the Bakken "source system," that is, within: (1) all three Bakken members, (2) the lowermost Lodgepole, and (3) the uppermost Three Forks.
Vertical wells in older "Bakken" oil pools (Antelope, Elkhorn Ranch, etc.) are perforated in one or all of the three units (Three Forks, Lodgepole, and the Bakken middle siltstone member) adjacent to the two Bakken shales but are not necessarily perforated in the Bakken shales. Rock-Eval analyses of 6- to 12-in. spaced core samples through all these units show that where Bakken shales are thermally mature, the three adjacent organic-poor units contain 10-20 times the hydrocarbons (HCs) they could have generated. Thus, Bakken-generated HCs appear to have moved into the three adjacent units, probably via fractures created by volume expansion of organic matter during HC generation in the Bakken shales. Mature Bakken shales are oil-wet systems with no water in them; any water introduced i to these shales during drilling, completion, or workover operations will decrease or ruin the shales' oil-productive capabilities.
Bakken well histories reveal that unsuccessful Bakken wells appear due to questionable techniques during these operations and not a lack of fractures. If a large in-place resource base exists in the Bakken source system, its commercial recovery will depend on new exploration, drilling, completion, and production technologies and on how much of the generated oil is in fractures rather than dispersed throughout the rocks.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)