SEQUENCE STRATIGRAPHIC ANALYSIS OF THE TYLER FORMATION (MORROWAN; LOWER PENNSYLVANIAN) IN WESTERN NORTH DAKOTA

A better understanding of the Tyler Formation in terms of temporal and spatial relationships could lead to improved insights into climate change during ice house conditions, and an improved application of EOR to this formation. The Tyler Formation (Upper Carboniferous: Morrowan) occurs in the subsurface of the Williston Basin of Western North Dakota. Carboniferous rocks like the Tyler in North America that are commonly cyclical which result from sea level changes caused by changing climate conditions in an ice house world. The Tyler contains meter-scale cycles of repeating lithofacies, which can be used to create a sequence stratigraphic framework.

This group has previously focused on the lower part of Tyler, whereas this study examines the entire formation. Examining core samples and well logs of the Tyler Formation along multiple transects from the basin center to the edges has shown cyclical lithofacies patterns repeating throughout the formation. Gamma ray markers in the well logs allows for correlation of cycles in the lower Tyler across much of the basin. There is some uncertainty correlating the lower Tyler to the basin edges because the sandstone bodies near the edge accumulated in possible incised valleys and the gamma ray markers disappear. Correlation of the upper Tyler is also accomplished using gamma ray markers, although cores containing the upper Tyler are less frequent, which also adds a degree of uncertainty to the correlation. Cycles in the lower part of the Tyler are relatively thick (7 to 20 ft.) and begin end with fossiliferous marine shale overlying a paleosol (ancient soil). The cycles in the upper part are thin (3 to 6 feet) and have more limestone and evaporate minerals. We currently interpret this vertical pattern in the Tyler as the result of a long-term climate change in the basin from more humid to drier conditions.

Continued refinement the stratigraphic framework will further aid in future studies of fossil distribution patterns of the Tyler Formation, and will provide insight into the climatic conditions of the Carboniferous in the Williston Basin, and will improve the economic development of this formation.