Chapter 21: Does Bakken Horizontal Drilling Imply a Huge Oil-Resource Base in Fractured Shales?

Abstract Previous assumptions regarding high efficiencies of primary petroleum migration from mature organic-rich source rocks appear invalid. These assumptions were based on large declines in Rock Eval hydrogen indices of source rocks with progressive burial, without equivalent increases in either Rock Eval S1 peaks or Soxhlet-extractable hydrocarbons. Thus, it followed that the generated hydrocarbons must have escaped the source rocks by efficient primary migration. However, horizontal drilling in fractured, self-sourced shales shows that this is an erroneous hypothesis, because these shales contain high hydrocarbon concentrations. A better explanation for these missing hydrocarbons is that they were lost during recovery of rock samples from the bottom of the hole. Basin richness for basins worldwide increases with intensity of faulting over and adjacent to basin deeps. Thus, faulting and fracturing of mature source rocks may be necessary for efficient expulsion of generated hydrocarbons from such rocks. Without such faulting, generated hydrocarbons may largely remain in cracks, fractures, parting laminae, and matrix porosity in both source rocks and in the rocks immediately adjacent to them. It follows that a huge oil-resource base may be present in and adjacent to fractured, mature, self-sourced shales. The shales of the Upper Devonian and Lower Mississippian Bakken Formation provide insight into this proposed resource base. Both historic production data from vertical Bakken wells and Rock Eval analyses of close-spaced core and cuttings from mature basinal Bakken shales demonstrate significant movement of hydrocarbons from Bakken shales for at least 50 ft (15 m) into adjacent units. Migration probably occurred through vertical fractures created by fluid overpressuring during intense hydrocarbon generation. It is widely believed that the Bakken shales are the source of most of the conventional oil in the Williston Basin. However, newer geochemical analyses suggest that this is not the case. The Bakken shales are calculated to have generated over 100 billion BO (15.9 × 109 m3). If this oil did not charge the conventional oil reservoirs, we can only conclude that it remains in the shales and in the rocks adjacent to them. Numerous other self-sourced, fractured shale reservoirs produce oil commercially. If the thin Bakken shales have generated 100 billion BO, then it is possible that the lower 48 states of the U.S. contain an unrecognized oil-resource base in the trillions of barrels. However, economic recovery of this possible resource base would depend on development of new exploration, drilling, completion, and production techniques appropriate to the non-classical reservoir characteristics of this oil resource base.