Silo Field, Laramie County, Wyoming: A Guide for Future Fractured Niobrara Production

SONNENBERG, S. A., Consultant, Denver, CO, and R. J. WEIMER, Consultant, Golden, CO

The Silo field is located in the northern part of the Denver basin. Production is from the fractured Niobrara Formation at depths ranging from 7600 to 8500 ft (2318 to 2593 m). Cumulative production from 40 vertical wells at Silo is in excess of 1.3 million bbl of oil. Two wells have produced 437,000 bbl of oil. Recent drilling success with horizontal wells suggests much greater future production. Initial potentials from the new horizontal wells range from 500 to 2000 bbl of oil/day.

The dominant lithologies of the Niobrara are limestones (chalks) and interbedded calcareous and organic-rich shales. Niobrara thickness ranges from 280 to 300 ft (85 to 92 m). Four limestone intervals, averaging 30 ft (9.2 m), and three intervening shale intervals (averaging 47 ft or 14.3 m) occur regionally and are easily recognized on geophysical logs. The lower limestone is named the Fort Hays, and the overlying units are grouped together as the Smoky Hill member. The fractures are concentrated in the more brittle limestones. The main production is from the middle limestones of the Smoky Hill. The intervening shales have high organic matter content and served as source beds.

Open fracture systems are essential to Niobrara production because little matrix porosity exists in the limestones. Open fractures may be created by (1) solution of Permian evaporites; (2) recurrent movement on basement fault systems associated with regional paleotectonic features; and (3) other uncertain causes. Additional data are needed to resolve the origin of the fractures.

High resistivities are observed in limestone beds at Silo. These resistivity anomalies appear to be related to the presence of a large hydrocarbon accumulation delineated by isoresistivity mapping.

Factors present at Silo will serve as a model for future Niobrara production in the Rocky Mountain region. These factors include (1) mature source rocks interbedded with brittle limestone; (2) open fractures to form the reservoir; (3) resistivity anomalies indicating accumulation; and (4) technology to efficiently produce the reservoir.

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