Abstract
The Three Forks Formation is an important part of the Bakken Petroleum System of the Williston Basin. The USGS estimates that 3.7 billion barrels and 3.5 trillion cubic feet of gas can be technically recovered from the Three Forks. The formation is currently being developed with horizontal drilling and multi-stage hydraulic fracturing. Many geological and technological factors control production. The principal source bed for the Three Forks is the Lower Bakken Shale (LBS). Source rock maturity is a primary control on the extent of Three Forks production across the basin. Poor, if any, production exists outside the maturity boundary. Other geologic factors controlling production include matrix and fracture porosity and permeability, depositional environment, reservoir facies, overpressure, structure, mechanical stratigraphy, and thickness. Technology factors controlling production include lateral length, number of hydraulic fracture stages, proppant volume and type, and well spacing.
The Three Forks is a very fine-grained to silty dolostone unit with interbeds of dolomudstone and anhydrite. Siliciclastic content ranges from 10 to 40%; dolomite content ranges from 20 to 60%; clay content is low in the dolostones (<10%) but increases in the dolomudstone units (>30%). The depositional environment of the Three Forks ranges from subtidal to supratidal. This peritidal sequence is clastic dominated with no or very little microbial carbonates being observed. The dominance of silt-sized dolomite, quartz, and feldspars suggests transportation of silt-sized material by streams and/or wind into a tide flat and then reworking by tidal energies. A portion of the dolomite may be detrital. Bi-directional ripples, wave ripples, mud cracks, mud drapes, and flaser to lenticular bedding all support the tidal interpretation.
Much of the lower and middle Three Forks is deposited in a highly oxidizing continental to coastal environment. This interval is characterized by intense red color resulting from hematite staining of clastic material. Anhydrites (nodular, bedded, and disseminated) are common. Anhydrites increase in abundance in the lower Three Forks. The anhydrites are nodular "chicken-wire" types which were deposited in coastal to supratidal sabkhas. Thin layers of anhydrite or coalesced nodules occur in more landward parts of the sabkha. Some halite content (1-2% XRD) is present throughout the Three Forks which supports the strongly evaporitic environment interpretation.
Producing reservoir facies have porosities which range from 5 to 10%; permeabilities are generally less than 0.05 md. Microfractures are common in the reservoir facies. The upper Three Forks unit is the primary objective; however, newer wells are also completing in the middle or second Three Forks unit suggesting additional reserves. Faults and fracture corridors may provide migration pathways to middle and lower Three Forks units. Lateral length and fracture stimulation stages vary from operator to operator.
