Diagenesis & Reservoir Quality of the Middle Bakken Formation

Abstract

The largest continuous oil accumulation ever assessed by the USGS, the Bakken Formation in the Williston Basin, USA, is a tight oil resource with low recovery factors. Increasing the recovery factors from tight hydrocarbon formation has significant economic implications.

In this study we show that the effects of diagenesis have significantly affected the reservoir quality of the middle Bakken Formation. Diagenetic processes in the middle Bakken are complex and vary across several scales. Early carbonate cementation exerts the primary control on porosity loss in the middle Bakken. Detailed petrographic examination by electron microscopy reveals multiple textural phases of calcite and dolomite, each of which contributes to changes in the reservoir quality. Cementation by calcite and dolomite has resulted in the low porosity and low permeability in the middle Bakken. Depositional facies have been defined within the middle Bakken. Cementation by carbonate minerals exert a key control on reservoir quality at scales below that of a facies.

Porosity in the middle Bakken varies from ~2-10%. Low pressure nitrogen gas sorption experiments show this is composed of pores which are small; predominantly mesoporous in size. Mercury intrusion experiments show pores in the middle Bakken typically have pore throats with radii <100 nm and low pressure nitrogen gas sorption analysis shows pore bodies are <135 nm in size. A significant component of porosity forms as intragranular pores, predominantly in quartz, calcite and dolomite.

Previous studies have struggled to unravel the timing and history of porosity-occluding carbonate cement due to its exceptionally fine-grained nature. We have taken a novel approach, using both sequential acid dissolution and in-situ Secondary Ion Mass Spectrometry (SIMS), to determine the carbon and oxygen isotope composition of individual carbonate phases. Coupled with grain-scale compositional analysis by SEM-WDS we show that diagenetic dolomite formed at 5-64°C by replacement of early calcite cement in low temperature, near surface ocean conditions at temperatures of 8-51°C. The results of this PhD show that multi-stage carbonate diagenesis has destroyed, preserved and created porosity in the middle member of the Bakken Formation.