Abstract

Silo field is located in the northern part of the Denver basin. Production is from the fractured Niobrara Formation at depths ranging from 7,600 to 8,500 ft (2,318 to 2,593 m). Cumulative production from 40 vertical wells at Silo is in excess of 1.3 million BO. Two wells combined have produced 466,000 BO. Recent drilling success with horizontal wells suggests much greater future production. Initial potentials from the new horizontal wells range from 216 to 2,026 BOPD. Ultimate production for horizontal completions is difficult to define by “early-life” decline curves because many of the wells are produced at low rates (choked back) to prevent formation damage. Cumulative production from vertical and horizontal wells through June 1992 is 2,008,146 BO and 1,366,975 MCFG and 221,589 BW.

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 [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 serve as source beds.

Open fractures systems are essential to Niobrara production because little matrix porosity or permeability exists in the limestones. Open fractures may be created by: folding; basement or listric faulting; solution of Permian evaporites; high fluid pressures associated with oil maturation; regional stress field; or a combination of the above.

High resistivities are observed in limestone beds at Silo. These resisitivity anomalies appear to be related to the presence of a 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: mature source rocks interbedded with brittle limestone; open fractures to form the reservoir; resistivity anomalies indicating accumulation; and technology to efficiently produce the reservoir.