The ability to map lithofacies trends suitable for hydrocarbon reservoirs is critical for a successful exploration program. In exploring basins with carbonate reservoirs, diagenetic alterations must also be understood in relation to porosity development. The Mission Canyon Formation (Mississippian) of the Williston basin provides an excellent example of the need to understand the lithofacies/diagenesis relation.

During the Mississippian, the Williston basin was the site of subtidal to supratidal carbonate deposition. In general, depositional environments became more restricted from Montana, eastward into North Dakota. Subsurface mapping suggests a strong relation between the degree of marine restriction and diagenesis and porosity development in carbonate sediments. Two fields that produce from the Mission Canyon interval illustrate this relation.

MonDak field, situated on the Montana-North Dakota border, lies west of the limit of massive Mission Canyon anhydrite, in a sequence of normal marine sediments. Reservoir porosity is due to fracturing of tight, fine-grained limestones. Low matrix porosity and sparse, erratic fracturing are responsible for low daily production rates.

The Billings Nose-Little Knife trend (Billings, Dunn, and McKenzie Counties, North Dakota) lies well within the limit of massive anhydrite. Reservoir porosity in this case consists of a thick sequence of intertidal-supratidal sucrosic dolomites which are sealed by 20 to 25 m of massive anhydrite. Reflux of Mg-rich brines is believed to be the process leading to dolomitization.

Good matrix porosity and permeability allow for higher daily production rates. Regional mapping indicates that the presence or absence of anhydrite is in direct correlation with the development of good matrix porosity.

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