ABSTRACT

The Dawson Bay is one of several formations in the central part of the Williston Basin which has potential for petroleum production that has been generally overlooked. The depositional environment of the Dawson Bay in this area was a restricted platform exhibiting subtidal to supratidal conditions. Much of the original depositional texture was obliterated by subsequent diagenesis, especially dolomitization.

Oil production was first established from the Dawson Bay Formation in North Dakota at Temple Field in Williams County. Production is controlled by an updip pinchout of porosity on an anticlinal nose which parallels the Nesson Anticline.

The Dawson Bay presents some unusual log interpretation problems. Various combinations of six matrix minerals are usually present. They are calcite, dolomite, anhydrite, halite, gypsum and quartz. Porosity is a heterogeneous combination of vuggy and intercrystalline types. Complete halite infill of porosity can be detected on logs. Partial infill of porosity is difficult to determine and may lead to falsely optimistic porosity and water saturation calculations. The most effective tool for evaluation of potential Dawson Bay reservoirs is drillstem or production testing.