Abstract
The continuous demand for energy and advancement in technology has accelerated the research in the areas of unconventional petroleum resources. The Devonian Three Forks Formation consisting of carbonate and clastic sediments is an unconventional reservoir with about 3.73 billion barrels of technically recoverable oil. However, major gaps remain in the knowledge of how mineralogy, structures and fabrics within the various lithofacies have influenced the hydrocarbon storage and transport capacity. This work integrate petrographic (physical rock description, XRD bulk mineralogy and thin section) and petrophysical (Helium pycnometry and NMR) studies in characterizing the reservoir potentials of the Three Forks lithofaies. Physical core description and wireline logs were used to identify and correlate seven (7) lithofacies within the Three Forks Formation. They are: 1) green - grey massive mudstone; 2) tan massive dolostone; 3) grey - tan laminated mudstone and dolostone; 4) tan - dark brown mottled dolostone; 5) grey and tan mottled mudstone; 6) grey and tan conglomerated mudstone; and 7) grey and tan brecciated mudstone. Diverse types of pores were identified within various lithofacies, with microporosity pores dominating the mudstone rich lithofacies and interparticle pores dominating the dolostone rich lithofacies. Preliminary results showed the dolostone lithofacies have relatively lower total porosities while the mudstone lithofacies have relatively higher total porosity values.
The Three Forks Formation is a tight reservoir in the Bakken-Three Forks unconventional petroleum accumulation in the Williston basin (Lefever et al. 1991, Nordeng et al. 2010). The Devonian Three Forks Formation directly underlies the Bakken Formation and overlies the Bird Bear Formation. It is mainly an association of tight carbonates, clastic sediments and anhydrites. The amount of oil in place within the Bakken- Three Forks system is estimated to be about 7.8 billion barrels with almost a half of it from the Bakken Formation while the other half is from the Three Forks Formation (Gaswirth and Marra 2015). Apart from being a thermally matured and prolific a source rock, the Bakken Formation has produced an appreciable amount of oil through advanced drilling, completion and stimulation techniques (Lefever and Nordeng 2015). Oil discovery and production in the Bakken-Three Forks system began in 1953 from the Antelope with vertical wells (Lefever and Nordeng 2015).
