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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 Mississippian Mission Canyon formation 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 relationship 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 relationship.
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) is well within the limit of massive anhydrite. Reservoir porosity consists of a thick sequence of intertidal-supratidal sucrosic dolomites which are sealed by 20 to 25 m (65 to 82 ft) of massive anhydrite. Reflux of magnesium-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 correlates directly with the development of good matrix porosity.
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