ABSTRACT

Stanley Field provides a model for a new exploration play in the Mission Canyon Formation of the Williston Basin. Equally important, it establishes the economic significance of early mechanical compaction of shallow-water marine carbonates.

Stanley Field, discovered in 1977, is a stratigraphically-trapped oil accumulation situated midway between two older established production trends: one lies along the Nesson Anticline in the center of the basin, the other lies along the northeast side of the basin in fields created by updip anhydrite sealed stratigraphic traps. Nearest current production is 22 to 28 miles away in any direction. In Stanley Field, 20 wells currently produce from the Bluell and Sherwood zones of the Mission Canyon Formation with estimated recoverable reserves of at least 1.6 to 2.0 million barrels. The pay interval (maximum thickness 120 feet) was cored in 19 wells in or near the field.

The distribution of different depositional facies and their subsequent diagenesis provides the trap and controls the quality of the reservoir at Stanley Field. During late Mission Canyon time, low intertidal-supratidal barrier banks and islands developed in the area of the field. Islands were separated by marine channels. A shallow-water lagoon existed shoreward (east) of the island-bank barrier. Where channels entered the lagoon, tidal delta sand bodies accumulated. Island sediments were syndepositionally cemented by both marine and meteoric cement, but marine sediments escaped early cementation.

The reservoir is a porous (primary interparticle porosity) marine grainstone-packstone and fractured island sediments that locally retain open fenestral porosity. Trapping is accomplished by a combination of overlying massive anhydrite and lateral updip compacted (lagoonal) mudstone and wackestone.

Mechanical compaction of carbonates at Stanley Field is significant for the following reasons:

1. Only marine sediments were mechanically compacted, thus isopach maps of the upper Mission Canyon and overlying Charles Formations reflect distribution of depositional facies in the upper Mission Canyon.

2. Compaction implies lack of early cementation, thus suggesting why primary porosity is preserved in the reservoir facies.

3. Compaction reduced interparticle pore volume in marine sections, thus reducing porosity in marine reservoir facies and improving the effectiveness of the updip trap.