--> Where Are the Hydrocarbons? Siliclastic Versus Carbonate Micropores in the Middle Member of the Bakken Formation

AAPG ACE 2018

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Where Are the Hydrocarbons? Siliclastic Versus Carbonate Micropores in the Middle Member of the Bakken Formation

Abstract

The middle Bakken member in the Willison Basin of North Dakota represents a mixed carbonate-siliciclastic unit consisting of predominantly siltstones. This oil-rich interval is sandwiched between two black shales, the upper and lower Bakken, both considered source rocks for the middle member. Previous petrographic studies of the middle Bakken have identified a variety of pore types, with intercrystalline and dissolution porosity being ascribed to the process of dolomitization, largely due to the observation that in thin sections, large pores are associated with the presence of dolomite.

In this study we combine thin section petrography, SEM imaging of Ar-ion milled samples, and hyperspectral core imaging to characterize the types of porosity in the middle Bakken and identify the location of oil-containing pores. We found that the observation of visible porosity being contained within the dolomite-rich laminations is likely an artifact of thin section sample preparation. SEM imaging our samples indicates that porosity was not produced by of dolomitization, but rather preserved within the clay-rich laminations. The oil is contained in pores of these clay-rich laminations, residing in the spaces between clay lathes and around dolomite rhombs that they abut. The pores in the carbonate-rich laminations, originally thought to be filled with oil, are occluded by multiple phases of calcite, dolomite and ferroan dolomite cement, all of them degrading reservoir quality. Hyperspectral imaging of middle Bakken core enabled us to produce mineral and hydrocarbon maps of the middle Bakken, which clearly show that the presence of hydrocarbons is limited to the clay-rich laminations. While previous studies have attempted to show that porosity in the middle Bakken is associated with dolomitization, we can illustrate that the oil-containing pores are limited to the clay-rich laminations and that cementation in the carbonate-rich laminations occludes, rather than preserves or creates, porosity.