ABSTRACT

Lantry Field, located in Dewey County, South Dakota, has produced about 140,000 barrels of oil (140 MBO) from the "A" and "B" zones of the Upper Ordovician Red River Formation at a depth of about 1500 m (5000 ft). The field is situated about 160 km (100 mi) southeast of the nearest Red River production in the Buffalo Field of Harding County, South Dakota. The Lantry oil accumulation is an example of long-distance oil migration, probably through laminated dolomites in the Red River "B" zone, updip and out of the Williston Basin.

Lantry Field lies 8 km (5 mi) northwest of the subcrop of the uppermost Red River beds under a Siluro-Devonian unconformity on the southeastern flank of the Williston Basin. Wet wells located between Lantry Field and the updip Red River subcrop preclude a simple truncation trap hypothesis. Rather, examination of Lantry Field cores indicates that Red River oil is trapped near the diagenetic limit of porosity in the "B" laminated zone.

The Lantry producing beds tend to be highly permeable coarse-crystalline dolomite unlike the stratigraphically equivalent "B" zone reservoirs deeper in the basin which tend to be fine- to very fine-crystalline dolomites with much better preservation of depositional textures {e.g. microbial laminae, peloids etc.). The coarsely crystalline dolomites probably formed when Siluro-Devonian dolomitizing fluids migrated into the Red River, either through fractures in the overlying Stony Mountain Formation or, possibly, through the Red River subcrop. In contrast, the more typical finely crystalline "B" laminated dolomites deeper in the basin formed penecontemporaneously with Upper Ordovician precipitation of the capping "B" zone anhydrite beds.

Lantry Field consists of two productive areas, each covering over a hundred hectares. These areas are about a kilometre apart and separated by wet wells that yielded minor oil shows. The productive areas lack structural closure, but lie along a subtle structural nose. The Lantry oil accumulation also appears to be influenced by a local northwest-trending hydrodynamic gradient and the influx of fresh water from present-day outcrops in the Black Hills Uplift 160 km (100 mi) to the west.