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Petroleum Geology of the Parshall Field Area, Mountrail County, North Dakota Andrea Simenson and Stephen A. Sonnenberg 1,2 1 1 2 Department of Geology, Colorado School of Mines, Golden, CO The Discovery Group Inc, Denver, CO ABSTRACT The Parshall Field of the Williston Basin was discovered in 2006 by EOG Resources. This Devonian-Mississippian middle Bakken Formation resource play covers some 40 townships in Mountrail County, North Dakota and is still expanding. The development of horizontal drilling and modern fract techniques has made this field possible. The Bakken Formation in the field area consists of three members: (1) upper shale, (2) middle dolomitic sandstone, (3) lower shale. The total Bakken interval ranges in thickness from 80 to 130 ft over the field area. The upper shale is dark-brown to black organic rich mudstone and ranges in thickness from 10 to 18 ft over the field area. The middle member ranges in lithology from bioturbated, argillaceous, calcareous very fine grained sandstones to cross- bedded fine to very fine-grained sandstone to very fine-grained sandstone with cm scale shale laminations. The middle member ranges in thickness from 30 to 70 ft. The lower shale member is also a dark-brown to black organic rich mudstone and ranges in thickness from 20 to 45 ft. The main reservoir in the Parshall Field is the middle member which has low matrix porosity and permeability and is found at depths of 9000 to 10500 ft. The middle Bakken porosities range from 2 to 12% and permeabilities average 0.02 mD. Some key factors that contribute to the success of this play include good stratigraphic trapping conditions, generation of hydrocarbons from the organic rich mudstones, and fractures. Facies F – Patterned pyritic dolostones and calcitic, whole fossil, dolo-to lime wackestones: fossil-rich beds Fossil beds may represent storm deposits Absence of waves and currents, suspension deposition ? ? Facies E – Thin-bedded dolo- mud/wackestone Can be rhythmically bedded Offshore setting Rhythmically interlaminated sandstone and muddy siltstone may represent tidal processes Thin sandstone beds interpreted as tempesites strongly reworked by biogenic action ? ? ? ? Facies D – Highest energy, coarsest grained alternating cross-bedded bioclast, very fine grained sandstone Interpreted as having been deposited in a high-energy region: channel fill to shoal ? Facies C – Rhythmic, varve-like, mm to cm laminated, well sorted, very fine grained sandstone and siltstone with calcite cement, hummocks and wave ripples Rhythmically interlaminated sandstone and muddy siltstone may represent tidal processes ? Facies B – Bioturbated, argillaceous, calcareous, poorly sorted, very fine grained sandstone/siltstone with Open-marine, below storm wave base, reworking by deposit feeding ichnofauna helminthopsis/sclarituba ? Facies A – Intraclastic-skeletal lime wackestone, 1-4 ft thick Absence of waves and currents, suspension deposition, deposit-feeder traces High index of bioturbation and common shell fragments ? ? Upper Bakken Shale - Black, massive to locally parallel-laminated shale Lower Bakken Shale - Black, massive to locally parallel-laminated shale Late Devonian - Early Mississippian Bakken Petroleum System including the Lower Lodgepole Formation, Bakken Formation, and the Upper Three Forks Formation. Late Devonian - Early Mississippian black shales paleogeography distribution map. The Bakken Formation is located in the northern United States and southern Canada. Location of the Parshall Field in Mountrail County North Dakota. The study area is highlighted in red. Selected wells used in the study. Wells were chosen based on penetration of the entire Bakken Formation, well logs availability, and core availability. Unconventional, Continuous Tight Oil Accumulations Prevasive petroleum saturation Mature source rock Abnormally pressure Generally lacks down-dip water Up-dip water saturation Low porosity and permeability reservoirs Enhanced by fracturing and partings ? ? ? ? ? ? ? From LeFever, 2008 From Smith and Bustin, 1996 Facies based on work done by Cantor et al, 2009 GR RES GR DEN NEU
