Conventional and Low-Permeability Reservoirs of Shallow Gas in the Northern Great Plains
Authors: Gautier Donald L. , Rice Dudley D. 
Published: December 2007

Summary
Significant resources of natural gas occur at shallow depths [less than 4,000 ft (1200 m)] within a large area of the northern Great Plains. Reservoir properties are controlled mainly by the depositional environments in which source and reservoir rocks accumulated. Exploration and development of potential gas resources depends on successful prediction of the physical characteristics and distribution of gas-bearing rocks.

Introduction
Major resources of natural gas trapped in low-permeability, low-pressure reservoirs underlie large areas of the northern Great Plains at depths of less than 4,000 ft (1200 m). Rising gas prices and accelerated drilling suggest that the area may be of considerable economic importance in the near future. Effective exploration for low-permeability (unconventional) reservoirs depends on successful prediction of the distribution and physical characteristics of the gas-bearing rocks and on carefully managed completion and development techniques. The purpose of this paper is to compare. describe, and contrast conventional and low-permeability shallow gas reservoirs to provide geologic information useful to exploration and development of these resources. Potential unconventional reservoirs in the northern Great Plains occur within strata from the base of the Mowry Shale (Upper Cretaceous) to the top of the Judith River Formation (Upper Cretaceous). These reservoirs are thin. discontinuous siltstone and sandstone laminae enclosed in thick sequences of offshore marine shale. Gas occurs in a sedimentary section that is immature with respect to thermal oil and gas generation. The gas in both conventional and unconventional reservoirs was generated during the breakdown of organic matter by anaerobie microorganisms at shallow depths in accumulating sediments and is referred to as biogenic gas. The biogenic gas is composed predominantly of methane (C1/C1 through 5 greater than 0.99 enriched in the light isotope 12 C ( 13 C1 = -60 to -70 ppt). The occurrence of natural gas and the physical properties of the gas-bearing reservoirs are controlled chiefly by the depositional environments in which source and reservoir rocks accumulated. Examples for comparison have been selected from the Eagle Sandstone/Telegraph Creek Formation stratigraphic interval because equivalent rocks produce economic quantities of gas from three major reservoir types:conventional reservoirs in coastal deposits are illustrated with an example from the Eagle Sandstone in Tiger Ridge field. Bearpaw Mountains area, north-central Montana (Fig. 1 );shelf sandstone reservoirs of intermediate quality are exemplified by a coarsening-upward sequence in the Shannon Sandstone Member of the Gammon Shale from Bowdoin Dome, north central Montana (Fig. 1);a productive siltstone/shale reservoir in the Gammon Shale at Little Missouri field in southwestern North Dakota (Fig. 1) provides an example of a low-permeability shallow gas reservoir.
Comparable data for each reservoir type are listed in Table

Conventional Reservoirs in Coastal Sandstones
Conventional reservoirs in the Eagle/Telegraph Creek stratigraphic interval are confined to coastal sandstones of the Eagle Sandstone that occur in a north-south-trending zone through central Montana (Fig. 1). This zone approximates the position of the fluctuating shoreline during a major marine regression. Sandstones in this zone are widespread (blanketlike) bodies, more than 40 ft (12 m) thick, deposited under relatively high-energy conditions along the shoreline. The dominant transporting force was wave action that generated longshore and tip currents.