Integrated analysis of Waulsortian-type bioherms of the Lodgepole Formation: Stark County, North Dakota and Bridger Range, Montana

Abstract

Significant amounts of hydrocarbons have been produced in Stark County, North Dakota from Mississippian-aged carbonate mounds or bioherms within the Lodgepole Formation. These carbonate mounds have been compared and contrasted to Waulsortian mounds, which are found in the Lower Carboniferous (Tournaisian-Visean) throughout the northern hemisphere (Wilson 1975). While Waulsortian or Waulsortian-type mounds are characterized by their mud-rich and massive nature, the mounds in Stark County are noticeably different. Initial petrographic study of the Patterson #1-24 well (NESE Sec. 24 T. 139N R. 97W) in Stark County has shown that major constituents of the mounds include crinoids, brachiopods, and ostracods among other minor constituents. Porosity is minimal and is generally limited to intergranular and intragranular pore types. Outcrops of these mounds have been identified in two different mountain ranges in Montana. Two mounds are present in Swimming Woman Canyon of the Big Snowy Mountains in central Montana. Up to five mounds are located at two different locations within the Bridger Range of southwestern Montana. Field work was done on two mounds that had been un-studied due to their remoteness at the southernmost tip of the Bridger Range Mountains. Data collected included dimensions of mounds, stratigraphic sections, facies associations, and systematic sampling both laterally and vertically. Data collected from this outcrop was used to compare and contrast observations made previously on outcrops that had been studied in great detail. In addition, these outcrop data were used to compare the Montana outcrop mounds to those that are producing significant amounts of hydrocarbons from the subsurface in Stark County, North Dakota. Rocks from both the subsurface and outcrops were studied extensively (via core and hand samples) followed by petrographic thin section analysis. For each thin section acquired, grains present, texture, cavity structures, cements, replacements, and porosity were noted. Of particular importance was developing a diagenetic history of these mounds. As such, the variety of cements present were noted and studied. Furthermore, cathodoluminescence microscopy was used to aid in distinguishing the different episodes of cementation that comprise the paragenetic history of these mounds. A total of five distinct facies were observed at the Bridger Range outcrops. They are (1) a crinoid and fenestrate bryozoan mottle wackestone basal biostrome facies; (2) a skeletal wackestone to mud-rich packstone to grainstone to marine cemented boundstone inner core facies; (3) a crinoidal and bryozoan to mud-rich packstone to grainstone outer core facies; (4) a bryozoan and crinoidal mud-rich packstone to grainstone; and (5) an argillaceous mudstone to skeletal wackestone of the regional Lodgepole facies. Comparing the outcropping mounds to the subsurface mounds, the previously studied mounds at Sacagawea Peak and Saddle Peak are dwarfed by the large mound complexes in Stark County. This difference in size is attributed to a smaller initial area upon which the Bridger Range mounds nucleated followed by a much earlier cessation of mound development (i.e., earlier drowning event) compared to the Stark County mounds. The locations of mounds found in the subsurface coincide with areas that have abnormally thick Upper Bakken Shale. These areas of thick Upper Bakken Shale have been attributed to two-stage dissolution of the underlying Prairie salt, which ultimately may have allowed for the development of the subsurface Lodgepole mounds. Initial dissolution resulted in a paleotopographic low, where additional Upper Bakken Shale was deposited. The second stage of dissolution resulted in a paleotopographic high of the Upper Bakken Shale, forming an ideal site for nucleation of the Lodgepole carbonate mounds. Based on petrographic studies, the Bridger Range outcropping mounds and the Stark County subsurface mounds were cemented early with pseudo-peloidal, radial-fibrous, and equant calcite cements. These cements allowed for minimal compaction and preservation of the mound core facies.