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

Gary P. A. Muscio, Brian Horsfield, Dietrich H. Welte

The lack of direct and unequivocal evidence for the existence of fractures in the Bakken petroleum system (Williston basin, USA/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% R₀ have revealed that the main phase of petroleum formation took place very early during catagenesis (Ca. 0.4 - 0.8% R₀) and that its potential has already been realized in those areas of the basin which currently produce from Bakken reservoirs. Therefore, present-day abnormally high pressures encountered in these regions might represent overpressures created during this stage of petroleum generation and maintained over geologic time.

Alternatively, results of the present study suggest that overpressuring and subsequent fracturing may be a post-generation effect: (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 be 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. It is this which may have induced volu e expansion of crude oil and subsequent pressure build-up. This process was supported by the restricted lateral and vertical migration ranges in the Bakken petroleum system. (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. However, the significance of this process has to be balanced against other factors like e.g the lack of appropriate expulsion pathways or maturity anomalies.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France