New Understanding of Niobrara Reservoir Characteristics Based on Horizontal Drilling in Silo Field, Southeastern Wyoming

CAMPBELL, MICHAEL J., and ROC E. SAINT, Union Pacific Resources Company, Fort Worth, TX

The Niobrara Formation was deposited in deep, open-marine environments of the Cretaceous Interior Seaway. This broad depocenter enabled the accumulation of a regionally correlative sequence of alternating limestones and calcareous shales covering much of the Rocky Mountain area. In southeastern Wyoming, the limestones were deposited as very porous chalks rich in coccolith, inoceramus, and foraminifera remains. Subsequent burial-related compaction and cementation transformed the chalks into dense, brittle limestones. Although porosities range from 6 to 16%, core analyses indicate matrix permeabilities to be less than 0.01 md. The Niobrara is its own source of hydrocarbons, with total organic carbon content averaging 3.2 wt.% in the Silo field area.

Silo field in Laramie County, Wyoming, produces oil from vertical fractures within the limestones of the Niobrara. Previously developed by vertical wells, Silo field is now being redeveloped by horizontal drilling and completion techniques intended to better define the fracture geometries and to maximize per-well productivity.

Horizontal logging of the new wells reveals the fractures to be essentially parallel, striking to the northwest. Fracture apertures range from 0 to over 3 mm, often occluded by secondary calcite mineralization. Well control and seismic data indicate causes of natural fracturing to include late-stage, tectonically controlled dissolution of deeper Permian salts, as well as folding and faulting of the Niobrara. High resistivity trends of the limestone in Silo field correlate strongly to production trends, possibly reflecting formation maturity and/or the presence of oil-filled fractures.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)