Abstract

In June 1984, Conoco Inc. drilled the discovery well at Gooseneck field in Divide County, North Dakota. This well flowed 200 bbl of oil and 20 bbl of water per day from the Devonian Duperow Formation. Subsequent drilling resulted in seven additional productive offsets, with initial potentials ranging from 22 to 400 bbl of oil per day.

Gooseneck field is the most recent significant oil discovery along a developing upper Duperow producing fairway in northern Divide County. An important regional control on production is the distribution of the Flat Lake Halite, a single bed of halite within the upper Duperow which extends across southeastern Saskatchewan, Canada for hundreds of square miles. Stratigraphically above and below the bedded salt and laterally equivalent to it are carbonate rocks plugged with halite. The feather-edge of the salt-plugged impermeable barrier in Divide County is generally oriented perpendicular to regional dip.

Within the upper Duperow are several shoaling and brining-upward carbonate cycles, two of which are productive at Gooseneck field. A typical sequence consists of a subtidal mixed skeletal to non-skeletal marine limestone, overlain by a restricted-marine non-skeletal dolomite. Above this is dolomitized algal stromatolite, which contributes to the indigenous hydrocarbon source. Capping the sequence is anhydrite and siliciclastic dolomitic mudstone, which provide the top seal. Production at Gooseneck field occurs from porous dolomites, which grade laterally and updip into tight, often salt-plugged carbonates. Maximum dolomitization and porosity development is coincident with subtle structural nosing. Isopach mapping suggests dolomitization occurred during early structural growth on a Devonian paleo-high.