Analysis of Shallow Gas Development From Low-Permeability Reservoirs of Late Cretaceous Age, Bowdoin Dome Area

Natural gas is being developed and produced in the Bowdoin Dome area from shallow, low-pressure, low-permeability reservoirs that were previously noncommercial. Reservoirs are dominantly thin, discontinuous, shaly siltstone and sandstone laminae and beds which cannot be distinguished on conventional logs. They require stimulation to provide commercial flow rates and necessitate more sophisticated evaluation methods than do conventional reservoirs. Introduction This paper reviews the geologic, technological, and economic parameters that have contributed to the development of natural gas reserves in the recently expanded portions of the Bowdoin Dome area. The technology and methods discussed are those used by Midlands Gas Corp., a subsidiary of Kansas-Nebraska Natural Gas Co. Inc, and may differ from those of other operators in the area. The Bowdoin Dome is a very large structural uplift located in eastern Phillips and western Valley counties in north-central Montana (Fig. 1). Natural gas was first discovered on the structure in 1913 in a shallow well drilled for water. The central part of the structure was developed through the 1950's with natural openhole completions in wells drilled where the pay zones are shallower. With the advent of improved completion techniques and increased gas prices in the 1970's, the field was expanded in size to more than 600 sq miles (1550 km2). Natural gas is being produced from low-permeability, low-pressure reservoirs at depths less than 2,000 ft (610 m). The pay zones consist of thin, discontinuous laminae and beds of siltstone and sandstone, and a more persistent limestone bed. These beds are enclosed in a thick sequence of offshore marine shale of late Cretaceous age. The gas is of biogenic origin and was generated in the surrounding marine shales. One major problem existed with the most recent expansion. The development program was designed to be run concurrently with the construction of a large gathering facility. The purpose was to increase economic viability and to develop a more efficient optimization of stimulation techniques and spacing criteria. However, delays in the approval of the pipeline facility forced the operators to use technology evolved in other areas. Specifically, the development of similar shallow gas accumulations in southeastern Alberta was used as an analog for the original treatment designs and 640-acre (2.6 x 10(6) m2) spacing. Most of the development in the Bowdoin Dome area was completed before the final construction of a gathering facility. Therefore, information on the adequacy of the initial development design has become available only recently. Structure The Bowdoin Dome is a very large, nearly oval arch which has structural closure of about 700 ft (200 m) on top of the Upper Cretaceous Greenhorn lime. Most of the new development concentrates on a north/northwest-trending nose that extends across the Canadian border.