Petroleum Generation and Expulsion in the Constrained Bakken Petroleum System and Its Relevance to the Formation of Fractures: ABSTRACT

Abstract

The lack of direct and unequivocal evidence for the existence of fractures in the Bakken petroleum system (Williston Basin, U.S.A./Canada) has led to controversial discussions with respect to the magnitude, orientation and frequency of fractures and especially their formation mechanism. Amongst other hypotheses, the creation of overpressuring as a result of high petroleum generation rates was called upon to induce fracturing. On the basis of a comprehensive organic geochemical study incorporating source rock and reservoir data, the present contribution provides evidence for an alternative pressure-controlled fracture formation mechanism: Mass balance calculations performed on Bakken Shale samples covering a maturity spectrum from 0.3% to 1.1% R{sub o} has revealed that the main phase of petroleum formation took place very early during catagenesis (ca. 0.4 - 0.8% R{sub o}) and that its potential has already been realized in those areas of the basin which currently produce from Bakken reservoirs. (1) It has been shown recently that the Bakken petroleum system represents a closed system, i.e. the Bakken sourced oils have not entered the Madison reservoirs. Hence, overall high expulsion efficiencies indicate that Bakken Shale over- and underlying units might represent the principal reservoirs and, most importantly, the strata where overpressuring should bemore » expected. (2) Detailed analysis of Bakken sourced oils which are produced from Bakken reservoirs imply that their bulk and molecular composition may have been altered by significant in-situ thermal alteration which took place after the main phase of expulsion. (3) The hypothesized process of reimpregnation of petroleum from the reservoir back into the source rock system may account for locally occurring high concentrations of solvent extractable organic matter in Bakken Shales.« less