Depositional and diagenetic characteristics of Waulsortian-type buildups in the Lodgepole formation: Big Snowy Mountains, Montana, and Dickinson Field, North Dakota

Abstract

Three distinct lower Mississippian (Kinderhookian) carbonate buildups are found at the mouth of Swimming Woman Canyon, located on the southern flank of the Big Snowy Mountains, in central Montana. These buildups are composed of Lodgepole Limestone, which was deposited during a marine transgression. These buildups are similar to massive "Waulsortian'' mud mounds recognized worldwide, named after the original buildups discovered in Waulsort, Belgium. The Lodgepole buildups resemble subsurface mounds located in Dickinson Field, North Dakota. Mounds in Dickinson Field have similar depositional features, mound facies, and diagenetic histories compared with the mounds outcropping at Swimming Woman Canyon. In addition, the Dickinson mound facies are good hydrocarbon reservoirs. Initial production of a Lodgepole well is expected to be water-free and at a rate of 1000 BOPD or greater. Porosity in the mounds averages 5%, oil columns in the mounds average 180 feet, and the mound rock varies between 250 to 320 feet thick (Young et al., 1998). Both sets of mounds are composed primarily of early depositional submarine cements, clotted lime mud, and stromatactoid vugs. These vugs created depositional porosity while the mounds were forming, and contribute greatly to the total reservoir porosity in the subsurface Dickinson Field mounds. In addition, the biota from both localities is quite diverse, composed of macro and microorganisms found globally in Waulsortian fossil assemblages. Diagenetic features of the mounds - most notably the pore-filling blocky calcite cements are saddle dolomite cements- are seen in the Montana and North Dakota mounds. Early rim cements and replacement chalcedony are also present. Cathodoluminescence and isotope analysis also show correlation between the Mississippian mounds of Montana and North Dakota suggesting that both areas formed in related paleoenvironmental conditions and underwent similar diagenetic histories.