ABSTRACT

A distinct relation exists between the location of faults and the development of reservoirs in many Devonian Dawson Bay and Duperow Formation pools in North Dakota. Hydrocarbons produced from the Duperow Formation on the Billings nose and from the Dawson Bay Formation in central and northwestern North Dakota are from dolomitic reservoir rocks with intercrystalline and vugular porosity. Porosity in these reservoirs is enhanced on both regional and local scales by dedolomitization and the dissolution of pore-plugging evaporites. These zones of diagenesis commonly occur along fracture/fault zones inferred from isopach maps and remote sensing data (satellite imagery; aeromagnetics). Dedolomite fabrics that enhance porosity show extensive corrosion and dissolution of dolomite rhombs along fractures and in alteration rinds surrounding dissolution vugs formerly filled with replacive anhydrite.

Duperow reservoirs are developed along the flanks of paleohighs that appear to be bounded by north- and northeast-trending fault zones. Intermediate and late stage diagenesis, including de-dolomitization (e.g. calcitization and dolomite dissolution) and anhydrite/calcite cementation and replacement, has played an important role in reservoir and trap formation. Fluid movement along postulated faults and fractures is believed to have localized some intermediate and late stage diagenetic processes. This localization may account for the pod-like geometry and small areal extent of lower Duperow reservoirs. Dissolution of anhydrite and dolomite precedes or is contemporaneous with two phase fluid inclusions associated with carbonate and silicate cements. The fluids responsible for the cements with two phase fluid inclusions appear to be associated with hydrocarbon migration coeval with Laramide tectonism.

The Dawson Bay Formation in central and northwestern North Dakota is primarily dolomite, with halite plugging most of the matrix porosity. This mixture of mineralogies confounds log analysis which suggests that the Dawson Bay is dominantly limestone. Well samples indicate that limestone only occurs in the lower portion of the formation. The origin and distribution of most open porosity in Dawson Bay dolomite is controlled primarily by salt dissolution and dedolomitization along major lineament trends. These diagenetic effects are especially prevalent along a northeast-trending fault zone through Dolphin and Moraine fields, along the northwest-trending Bismarck lineament and along the northeast-trending Yellowstone lineament in the central Williston Basin. Isosalinity maps suggest that fresh water influx along the Yellowstone lineament zone may account for the development of high porosity zones in Dawson bay reservoirs, whereas reservoirs to the north may have formed by processes similar to those for the Duperow.

Fluid movement along faults/fractures appears to have been a factor in localizing the areas of dissolution of dolomite and evaporite because porosity trends in the Dawson Bay and some Duperow Formation reservoirs are geometrically related to inferred fault/fracture trends or intersections.