Abstract

The concept of "deep basin gas" was first used and promoted for the huge gas accumulations at Elmworth Field, Alberta. In this area, gas occurs in lower Cretaceous sandstones within the deep basin. A regional trap, produced by the diagenetic reduction of pore throats, permits gas to exist structurally below water, a reversal of ordinary reservoir fluid dynamics. In the deep basin, porous intervals are gas saturated.

A review of other North American basins reveals that similar habitats exist for natural gas. In the Rocky Mountain area of the United States, large, basin-centered gas accumulations occur in Cretaceous and Tertiary rocks. The San Juan, Denver, Piceance, Uinta, Wind River, and Greater Green River Basins contain large gas resources and major gas fields. Rocky Mountain basins contain similar components that control and characterize gas accumulations. Likewise, individual basins have their own set of variables that control production.

In the Great Divide, Washakie, and Green River Basins, the major components of basin-centered gas are:

1. Thick accumulations of sandstones, shales, and locally coal (potential source and reservoir rocks) exist in these basins.

2. Burial and thermal histories promoted the development and preservation of diagenetic pore throat traps and extensive gas generation.

3. Although the centers of basins are completely gas saturated, production is controlled by stratigraphy. Both basin-wide and local stratigraphic variations are important in creating traps and reservoirs (local compartments).

4. Structure also plays a role in localizing gas accumulations, especially when coupled with stratigraphy.

5. Pressure regimes, ranging from slightly under-pressured to highly over-pressured, are important. In areas of abnormally high pressures, productive capacity can be greatly increased. Over-pressuring also creates problems in drilling and completion, increasing the costs of both.

6. The presence of fractures, both tectonic and produced by gas generation, are important to overall productivity.

7. Secondary porosity, produced by the dissolution of unstable grains and rock fragments, is important in both basin-wide and local accumulations.

In the Greater Green River Basin, the future of basin-centered gas exploration lies in a better understanding of the following components:

1. By utilizing high frequency stratigraphic evaluations of reservoirs, the presence and extent of individual stratigraphic compartments will be better understood. Outcrop and subsurface core examinations are necessary in completing this task.

2. More complete petrophysical evaluations of reservoirs should include the recognition and extent of reservoir compartments by wireline log evaluations.

3. A better understanding of secondary porosity formation, cement paragenesis, and clay authigenesis is needed. There are sparse data compiled on these components, yet they exert primary controls on production.

4. The local and basin-wide trends, extent, and degree of fracturing in gas reservoirs is important. Like secondary porosity, the data compiled on this facet of basin-centered gas production are sparse.