Tyler Formation

Geologic Summary

The Tyler Formation is predominantly comprised of alternating beds of offshore marine siliciclastic mudstones interbedded with paleosols (buried soil horizons) commonly capped by thin coals (~1-2 inches thick) (Nesheim and Nordeng, 2016). Depending upon location, interbedded sandstone and/or variable types of carbonates are also present. Early biostratigraphy studies concluded that the entire Tyler Formation was Pennsylvanian (Morrowan) in age (Ziebarth, 1962; and Grenda, 1977). However, a recently completed biostratigraphy study using palynology indicates that while most of the Tyler section is likely Pennsylvanian, the basal portions of the formation may be Late Mississippian (Chesterian) (Fig. 1).

Figure 1. Stratigraphic column of the Tyler Formation and bounding stratigraphic units with tentative correlations between central Montana and western North Dakota.

Globally, the Late Paleozoic Ice Age began during the Late Mississippian and extended through the Pennsylvanian, which was a phase in Earth’s history when relatively rapid glacial buildups and melting events led to pronounced cyclic fluctuations in global sea level. A result of the cyclic, glacial-induced sea level fluctuations commonly formed stacked, repeating cyclic sedimentary sequences of alternating paleosol (buried soil horizons) and marine mudstone deposits. The cycles of alternating marine mudstones and paleosol (terrestrial) deposits commonly comprising the Tyler Formation are believed to result from the Late Paleozoic Ice Age, glacio-induced, sea level fluctuations. Also, during this time, the Williston Basin connected to the open ocean through the Central Montana Trough, also referred to as the Big Snowy Trough (Fig. 2).

Figure 2. Paleogeographic map during deposition of the Tyler Formation, modified from Maughan (1984). Light grey areas depict the extent of Tyler source beds in the Williston Basin.

Petroleum Resources

A total of 293 productive oil and gas wells have been drilled in the Tyler Formation through the end of 2020, which have combined to produce more than 87 million barrels of oil (including the Heath, Tyler, and Tyler A oil pools). Most of the Tyler production has come from vertical wells targeting bar-type sandstone reservoir(s) along the Dickinson-Fryburg trend. The Rocky Ridge Field is thought to consist of channel-type sandstone bodies that have also yielded conventional, vertical well production (Fig. 3).

Figure 3. Map of western North Dakota showing the distribution of oil production from the Tyler Formation (black fill) along with the approximate distribution of organic-rich shales in the lower Tyler (green) and organic-rich limestones in the upper Tyler (blue).

Upton Resources drilled four horizontal wells during 2001-2007 targeting the conventional sandstone reservoir in the Tracy Mountain Field, where the sandstone reservoir thins to just a few feet thick and exhibits variable porosity/permeability (Nesheim, 2012). Two of those wells are still actively producing with cumulative production over 380 MBO and 558 MBO, a third well has yielded 103 MBO, but the fourth and final well drilled yielded mostly water.

The Tyler Formation has been understood to be a self-sourced petroleum system since the work of Dow (1974) and Williams (1974). More recent work has identified two sets of thermally mature petroleum source rock: 1) organic-rich shales within the lower Tyler within west-central North Dakota, and 2) organic-rich lime mudstones in the upper Tyler located within southwestern North Dakota (Nordeng and Nesheim, 2012a; Nesheim and Nordeng, 2016) (Fig. 4). These two sets of petroleum source rocks appear to form two separate, distinct petroleum systems. The organic-rich limestones of the upper Tyler have locally sourced most of the conventional sandstone reservoirs along the Dickinson-Fryburg Trend, which yields high-paraffin oil (Nesheim and Nordeng, 2016). Meanwhile, the lower Tyler organic-rich shales have generated low-paraffin oil (Nesheim and Nordeng, 2016).

Figure 4. Stratigraphic cross-section of the Tyler Formation highlighting the organic-rich black shales (shales “A” through “B” in grey) of the lower Tyler, the upper Tyler carbonates (lime “A” through “D” in blue), and an example localized sandstones (yellow). Most of the oil-productive conventional sandstone reservoirs are positioned just above the upper/lower Tyler contact (e.g. Medora and Fryburg fields) and/or just below (e.g. Dickinson Field).

The upper Tyler was drilled and tested as an unconventional resource play during 2013-14 in an area where the conventional sandstone reservoir is absent (Nesheim, 2017). Marathon Oil Company drilled and completed the Rundle Trust 21-29TH (NDIC: 26794, API: 33-087-00361-00-00) and the Powell 31-27TH (File: 26335, API: 33-087-00360-00-00) horizontal wells within the Moord Field targeting a series of organic-rich, low-permeability carbonate beds. Each well was drilled with an approximate two-mile lateral completed with multi-stage hydraulic fracturing. Both wells produced oil and gas but at low rates. Each well was plugged and abandoned within a year or less of completion. Fresh-water sensitive swelling clays present within portions of the upper Tyler are believed to have negatively interacted with the fresh-water-based hydraulic fracture fluid and impeded production results (Nesheim, 2017).

Last Updated: 01/12/2024.