Examination of Source Rocks within the Tyler Formation (Pennsylvanian), North Dakota
Timothy Nesheim and Stephen Nordeng
The Tyler Formation is a clastic dominated sedimentary sequence present in western North Dakota that has produced over 80 million barrels of oil using vertical wells. The North Dakota Geological Survey is re-examining the Tyler Formation's potential as a resource play. The Tyler Formation contains two sets of regionally extensive source rocks. In west-central North Dakota, the lower Tyler contains three organic-rich shale intervals (offshore marine deposits). The bottom two intervals are 4-12 ft. thick, 10-20 wt. % TOC, and extend for approximately 5,000 square miles. The third interval is more erratic and irregular. These three shale intervals can be identified in wirelogs by gamma ray spikes, high resistivity, low sonic velocity, and high neutron and density porosity values. The upper Tyler in southwestern North Dakota contains another potential source rock interval, which consists of organic-rich limestone interbedded with shale (fresh water, lacustrine deposits). The gamma ray signature of this interval is often low (<50 API). However, this interval averages 3-6 wt. % TOC measured from drill cuttings and commonly exhibits gas kicks of 10-100 units during drilling. This interval can be identified in wirelogs using resistivity-porosity log TOC calculations (Passey et al., 1990). High resistivity (100-1000+ ohm-m), Tmax values above 435, and fluid overpressure suggest these source rocks are mature in parts of western North Dakota.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012