Abstract

The Antelope Field produces from both conventional and unconventional traps. The traps in the Bakken petroleum system (Bakken, Sanish, and upper Three Forks) are thought to be unconventional because of low reservoir quality, natural fractures, overpressuring, general lack of water production, and production coming from the flank of the Antelope Anticline. Traps in Cambrian Deadwood, Ordovician Winnipeg, Silurian Interlake, Devonian Duperow and Nisku, and Mississippian Madison (Nesson) are related to structural closure and are conventional with down-dip water.

Fractures play a key role in the unconventional accumulations because they enhance an otherwise very tight reservoir system. The fractures are both horizontal and vertical and result from a variety of causes. Fractures in the Bakken shales and the Bakken, Sanish, and upper Three Forks reservoirs are created by hydrocarbon overpressuring, tectonic folding, recurrent movement on basement fault (Antelope Fault), and regional fractures.

Fracturing can be recognized on well logs using four different criteria: 1) separation between dual laterolog resistivity (DLL) and microspherically focused resistivity (MSFL) (invasion profile, salt-based mud); 2) abnormally high sonic porosities (compared to neutron and density porosities) indicating signal attenuation due to microfractures and oil saturation; 3) caliper logs showing borehole rugosity or enlargement (caliper wobble); 4) significant changes in density correction curve (Δρ curve).

Drill-stem tests (DSTs) of the fractured Bakken and Three Forks show unique Horner pressure build-up plots. Analysis of build-up data illustrates that significant damage of the reservoir can result from drilling fluids.

The Antelope Anticline is a paleostructural feature which has had recurrent structural movement. Thickness variations in the Bakken and Three Forks illustrate that the structure was active in Late Devonian and Early Mississippian.