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Regional correlation and mapping of Niobrara in the Powder River Basin, Wyoming

Abstract

In Powder River Basin, the Upper Cretaceous Niobrara Formation has potential of becoming another giant unconventional play, with horizontal drilling and hydraulic fracturing. It is crucial to understand regional facies distribution to aid hydrocarbon exploration and production. This study performs basin wide regional correlation of the internal Niobrara stratigraphy by using wireline logs from 920 wells. These wells are distributed across the whole basin. From integrated core and log analyses, Niobrara was subdivided into chalky intervals interbedded with marl intervals, consisting of A, B, C chalky benches, interbedded with A, B, and C marls. Chalky facies consist of 50% to 90% carbonate. Carbonate grains include peloids and foraminifera tests. The entire Niobrara Formation thickness ranges from 182 to 614 feet with a general westward thickening trend. The Niobrara A Bench ranges from 0 to 46 feet. It is present in the southern part of PRB, and pinches out northward. The Niobrara Upper B Bench varies from 9 to 98 feet in thickness, and thickens westward. The Niobrara Lower B Bench is the cleanest and most regionally continuous chalk bench within the Niobrara. Its recorded thickness ranges from 10 to 107 feet, and it hosts the same westward thickening trend. Niobrara B marl underlies the Niobrara Lower B Bench and overlies Niobrara C Bench stratigraphically. The average thickness of the interval is approximately 90 feet, but it varies from 30 to 200 feet. Similar to other Niobrara intervals, this interval thickens westward. The Niobrara C Bench varies from 10 to 68 feet in thickness. The Niobrara C Marl is the most variable interval in the Niobrara with regards to thickness. It varies from zero to 94 feet, and is present along an apparent erosional scouring channel. This channel runs west-east across Johnson and Campbell counties, and then turns in a north-south orientation across Campbell and Converse counties. Log Facies modelling from wireline logs was completed by applying cut off values on gamma ray and resistivity measurements. From this, two facies were identified, TOC-rich marl and Hydrocarbon-prone Chalk. TOC-rich marl varies from 0 to 339 feet in thickness, whereas hydrocarbon-prone chalk varies from 0 to 150 feet. Both TOC-rich marls and hydrocarbon-prone chalks show the greatest thickness along basin axis. In addition to occurrence along basin axis, they occur in the Southern part of Converse County. These areas show the greatest potential.