Integrated characterization of middle Bakken diagenesis, Williston Basin, North Dakota, U.S.A.

Abstract

Diagenetic calcite cements have a detrimental impact on reservoir properties within the Middle Member of the Bakken Formation. These cemented zones appear to have grown early in the post depositional history of the Middle Bakken. Hydrocarbon charge is significantly inhibited where calcite cements are present. This indicates that calcite cementation preceded the regional migration of hydrocarbons from the Upper and Lower Bakken Shale source rocks. In addition, the cements appear to have shielded large pore volumes from dolomitizing fluids, which inhibited reservoir enhancement. The integration of whole core analysis, plane polarized and Cathodoluminescence petrography, scanning electron microscopy, and stable isotope analysis was used to develop a comprehensive characterization of calcite cements. Qualitative results demonstrate an association between the morphologies of cements and the individual facies of the Middle Bakken. Depositional porosity and permeability and post depositional alterations from bioturbation influenced the preferential pathways of cementing fluids. Quantitative results establish constraints on the timing and environments of cementation and indicate the source of cementing fluids that were oversaturated with respect to calcium carbonate. Isotope data collected from calcite cements in Middle Bakken cores is consistent with Late Devonian-Early Mississippian marine values. The calcite cements must have been sourced from the dissolution of internal marine carbonate sediments. This study establishes a link between reservoir heterogeneity and the producibility of individual Middle Bakken wells. The regional abundance of carbonate is associated with the pervasiveness of calcite cements, which have the greatest influence on reservoir quality distribution. The measured porosity and permeability of cemented zones are well below the economic threshold for tight oil reservoirs.