The upper Tyler Formation (Pennsylvanian) contains four distinct limestone beds that can be individually traced across southwestern North Dakota. Up to three of the limestone beds can be classified as organic-rich, excellent quality source rock within a given area averaging 4–6% TOC and >25 mg/g S2 based on extensive geochemical data sets from two spatially dispersed cores located ∼40 miles apart. Generally, there is 10–15 ft. net thickness of organic-rich limestone within most of the study area. Hydrogen versus oxygen index values indicate these organic-rich limestone beds contain primarily Type I/II, oil-prone kerogen, and measured Tmax values tend to be >440°, indicating the Tyler Formation is thermally mature. Examining the average sonic travel time across each limestone bed appears to be a quick and effective method to map out where each limestone bed is organic-rich versus organic-lean. Textural changes can also be observed in core that correlate with variations in organic-richness.

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