The main reservoir rocks in the upper and middle members of the Minnelusa Formation consist of windblown dunal sands in the area surrounding the Lusk embayment. Changes in the local depositional setting, tectonic framework, and eustatic sea level controlled the distribution and reservoir quality of these sandstones. An understanding of the lateral and vertical variations within and between these members explains the different production trends and may be utilized to formulate predictive models to aid in the development of future exploration programs.
The middle member exhibits two production trends. Age-equivalent Tensleep rocks deposited along the western margin of the embayment produce from sandstones accumulated in a sand sea paleoenvironment. Structure is necessary for trapping owing to permeability continuity. Along the eastern margin of the embayment, production comes from isolated accumulations of sandstone deposited as dunes on broad coastal sabkhas. Fields in these sandstones define a linear trend due to the coast-parallel alignment of these dunes.
Production from the upper member defines four major trends. Upper member sandstones in the southern part of the basin, similar to “Leo” reservoirs, produce from sediments deposited as coast-parallel dunes in a northwest-southeast alignment. In the northern portion of the basin, production is from sandstones deposited in broad, flat eolian sand seas. Because of the permeability continuity of these sandstones, structural closure is necessary for trapping hydrocarbons.
Upper member production has been influenced by the unconformity developed at the top of the Minnelusa. Movement along the Rosebud arch resulted in a southwest-northeast production trend apparent in each sandstone unit reflecting their northwestward erosional limits. The last, and most apparent, production trend, results from the Opeche Shale infilling of northwest-southeast-oriented stream valleys. Most production to date has been from sandstones following this alignment juxtaposed downdip of these impermeable shales.