Depositional environments and diagenesis, Winnipegosis Formation (Middle Devonian), Williston Basin, North Dakota

In the Williston Basin, the Winnipegosis Formation is the major carbonate unit of the initial transgressive-regressive pulse of the Kaskaskia sequence. Twenty-two lithofacies were identified by well-log, core, and thin-section studies of Winnipegosis rocks; these belong to seven environments of deposition which include deep basin, deep shelf, shallow shelf, reef, lagoon, tidal flat, and evaporite basin. The deposition of the Winnipegosis and Prairie Formations were inter-related and occurred during six episodes. Following a brief hiatus separating the underlying Ashern Formation from the Winnipegosis, a clear, quiet, shallow-marine environment became established in the North Dakota portion of the Elk Point Basin. Beds deposited during this First Episode are found in the central, deepest part of the Basin and pinch out laterally. Beds of the Second Episode overstep those below indicating the extent of maximum, marine transgression. At this time, the basin differentialted into two distinct regions due to rapid sea-level rise accompanied by varying amounts of carbonate production: 1) a deep basin where slow deposition gave rise to a deep-marine environment with scattered, atoll-like, pinnacle reefs, and 2) a peripheral, carbonate shelf where, proceeding landward, there developed shallow-marine, patch-reef, lagoon, and tidal-flat environments. Evaporite beds that formed during the Third (dolostone and anhydrite) and Fourth (Prairie; halite) Episodes were restricted to the initially deep, inter-reef basin, filling it to the level of the top of the pinnacle reefs. In this evaporite basin, there was increase in salinity and decrease in water depth due to evaporative drawdown. Beds formed during the Fifth Episode were tidal-flat deposits that accumulated over sites of former, pinnacle reefs and shelves with continued deposition of halite elsewhere. Additional Prairie halite (Sixth Episode) filled the remainder of the Basin. Diagenesis of Winnipegosis rocks took place during six stages of diagenesis: syndepositional 1, eogenetic 1, telogenetic, syndepositional 2, eogenetic 2, and mesogenetic stages of diagenesis. Dolomitization is the major diagenetic process in Winnipegosis rocks. It has altered rocks from the pinnacle reefs and shelf-margin, patch reefs producing intercrystal porosity that, along with intraparticle, moldic, growth-framework, and solution-vug types of porosity produced rocks with the greatest oil potential