ABSTRACT The Williston Basin Bakken petroleum system is a giant continuous accumulation. The petroleum system is characterized by low-porosity and -permeability reservoirs, organic-rich source rocks, and regional hydrocarbon charge. Total Bakken and Three Forks production to December 2014 was 1.289 billion barrels (bbl) of oil and 1.3 trillion cubic feet of gas (TCFG) from 12,051 wells. U. S. Geological Survey (USGS) ( Gaswirth et al., 2013 ) mean technologically recoverable resource estimates for the Bakken petroleum system are 7.375 billion barrels of oil, 6.7 tcf of gas, and 527 million barrels of natural gas liquids. The Bakken Formation regionally in the Williston Basin consists of four members: upper and lower organic-rich black shale, a middle member (silty dolostone or limestone to sandstone lithology), and a basal member recently named the Pronghorn. The Bakken Formation ranges in thickness from a wedge edge to over 140 ft (43 m) with the thickest area in the Bakken located in northwest North Dakota, east of the Nesson anticline. The Three Forks is a silty dolostone throughout much of its stratigraphic interval. The Three Forks ranges in thickness from less than 25 ft (8 m) to over 250 ft (76 m) in the mapped area. Thickness patterns are controlled by paleostructural features such as the Poplar Dome, Nesson, Antelope, Cedar Creek, and Bottineau anticlines. Thinning and/or truncation occurs over the crest of the highs, and thickening of strata occurs on the flanks of the highs. The Three Forks can be subdivided into three units (up to six by some authors; e.g., Webster, 1984 ; Gutierrez, 2014 ; Gantyno, 2011 ). Most of the development activity in the Three Forks targets the upper Three Forks. The upper Three Forks is dominated by silt-sized quartz and dolomite and some very fine-grained sandstones and has low permeabilities and porosities. The upper Three Forks ranges in thickness from a wedge edge to over 40 ft (12 m) in areas east of the Nesson anticline. The unit thins toward the margins of the depositional basin because of erosional truncation. The upper and lower shale members are potential source rocks and are lithologically similar throughout much of the basin. The shales are regarded as dominantly type II kerogens. The shales average 11 wt.% total organic carbon. Measured core porosity and permeability are very low in the Bakken, Sanish, and Three Forks reservoirs (<10% porosity and <0.1 md permeability) in the Williston Basin, so productivity is assumed to be due to natural and artificial fracturing. The reservoirs generally require advanced technology to get them to produce (fracture stimulation and horizontal stimulation). For this reason, they should be considered to be technology reservoirs. Natural fractures in some areas (e.g., Billings Nose area and Antelope field) are sufficient for vertical well production. Reservoir pressure in the Bakken is regarded as overpressured with pressure gradients exceeding 0.5 psi/ft. A new pressure map for the Bakken petroleum system was generated. The map is based on 92 BHP (bottom-hole pressure) and DFIT (diagnostic fracture injection test) data points, including six additional hydrostatic points at the eastern margin as well as six data points for the Sanish–Parshall area. High overpressures are found in large parts of the central basin and the Parshall area in the east, where gradients exceed 0.7 psi/ft. Elm Coulee has a pressure gradient around 0.55 psi/ft. Parshall is reported to have a gradient of 0.74 psi/ft. The area west of the Nesson anticline has pressure gradients of 0.6 to 0.7 psi/ft. Pressure gradients in Montana are generally in the 0.51 psi/ft range.

Abstract The most important mineral resource activity in Colorado during the past decade has been the discovery and development of the Wattenberg and adjacent petroleum fields. Located north of Denver across the axis of the Denver basin, the Wattenberg is estimated to have reserves of 1.3 trillion cubic feet (tcf) in the tight J (Muddy) Sandstone (delta front) reservoir over an area of 600,000 acres at depths of 7,600 to 8,400 ft (2,310 to 2,560 m). Net pay thickness varies from 10 to 50 ft (3 to 15 m), porosity ranges from 8 to 12%, and permeability varies from 0.05 to 0.005 millidarcys (md) (Matuszczak, 1973, 1976). Drilling for J gas has resulted in multiple pays in overlying strata. The Spindle field, situated in the southwest portion of the Wattenberg field, produces from two marine-bar complexes (Hygiene and Terry) in the middle portion of the Pierre Shale. In 1981 and 1982, the Cod ell Sandstone, approximately 500 ft (152 m) stratigraphically above the J, was developed as a new producing horizon of oil and gas. More than 100 discoveries have been made within and marginal to the outlined Wattenberg field area. The Codell is a tight bioturbated marine-shelf sandstone generally without a central-bar fades. Net pay thickness ranges from 3 to 2 5 ft (0.9 to 7.6 m). Porosities determined from logs range from 8 to 24%, but the average core porosity is from 10 to 12 % and permeabilities are less than 0.5 md. Because of rapid decline in production and economic uncertainties, potential reserves from the Codell are unknown. All petroleum accumulations in the Wattenberg area are regarded as stratigraphic traps, although unconformities and paleostructure have played a subtle but detectable role. Variation in thickness and reservoir quality is related to original environmental facies and paleostructure that locallyinfluenced unconformities, fracturing, and diagenesis.

Abstract The Williston Basin Bakken petroleum system is a giant continuous hydrocarbon accumulation. The petroleum system consists of source beds in the upper and lower Bakken shales and reservoirs in the middle and upper Three Forks, the Pronghorn member of the Bakken, and the middle Bakken. The petroleum system is characterized generally by low-porosity and permeability reservoirs, organic-rich source rocks, and regional hydrocarbon charge. The USGS (2013) mean technologically recoverable resource estimates for the Bakken Petroleum System is 7.375 billion barrels oil, 6.7 TCF gas, and 527 million barrels of natural gas liquids ( Gaswirth et al., 2013 ). In the western US, relative sea level changes may be a combination of glaciation in the southern hemisphere, regional flexural tectonics related to the Antler orogeny, epeirogenic uplift, and/or localized structural movement ( Cole et al., 2015 ). The controls are not fully or clearly differentiated in the rock record. The Three Forks is a silty dolostone throughout much of its stratigraphic interval. The Three Forks ranges in thickness from less than 25 ft to over 250 ft in the mapped area. Thickness patterns are controlled by paleostructural features such as the Poplar dome and the Nesson, Antelope, and Cedar Creek anticlines. Thinning and/or truncation occurs over the crest of the highs and thickening of strata occurs on the flanks of the highs. The Three Forks unconformably (?) overlies the Birdbear in the Williston Basin and in turn is unconformably overlain by the Bakken Formation. The Three Forks consists of one overall deepening upward third-order sequence consisting of continental sabkha dolostones and anhydrites at the base changing to supratidal dolostones in the middle part to intertidal dolostones and mudstones in the upper part. The unit is subdivided into six shallowing upward parasequences by various authors. For mapping purposes, a three subdivision scheme has been adopted for this paper (i.e., upper, middle, lower). Most of the development activity in the Three Forks targets the upper Three Forks. The Bakken Formation in the Williston Basin consists of four members: a basal member (dolostone, limestone, and siltstone) recently named the Pronghorn; a lower organic-rich black shale; a middle member (silty dolostone or limestone to sandstone lithology); and an upper organic-rich shale member. The Bakken Formation ranges in thickness from a wedge edge to over 140 ft with the thickest area in the Bakken located in northwest North Dakota, east of the Nesson anticline. The Bakken appears to be composed of one complete third order sequence and part of a second third order sequence. The basal Pronghorn to middle part of the Middle Bakken represents one complete sequence (lowstand to transgressive to highstand system tracts). The Pronghorn to Lower Bakken Shale represent lowstand to transgressive system tracts. The lower Middle Bakken (facies A-C) represents a highstand system tract (falling stage system tract in upper part). The upper Middle Bakken (facies D-F) through the Upper Bakken Shale represents part of another third order sequence (lowstand to transgressive system tract). The Middle Bakken has an oolitic, bioclastic, sandy middle facies (facies D) which represents a lowstand deposit. This is overlain by the upper Middle Bakken (facies E-F) and the Upper Bakken Shale which is a transgressive system tract. Part of the overlying Lodgepole represents the highstand part of the second sequence. Sharp downlap surfaces are noted at the base of the Middle Bakken and the base of the Lodgepole. The downlap surfaces represent the transition from transgressive system tracts to highstand system tracts. Maximum flooding surfaces are found in the middle and upper portions of the upper and lower Bakken shales.