Petrophysical Modeling Approach toward Assessing the Chalk Purity and Mechanical Deformation of the Niobrara Formation, Southern Powder River Basin, Wyoming

Matthew M. Morton
South Dakota School of Mines and Technology, Rapid City, SD

Due to the enhancement of hydraulic fracturing and horizontal drilling, the self-sourced Niobrara Formation has recently become an intriguing unit among petroleum geologists and the oil and gas industry. This carbonate reservoir exhibits a unique lithofacies pattern by displaying higher siliciclastic content toward the Cordilleran geosyncline to the west. The Niobrara Formation is composed of two members – the basal chalk-rich interval which makes up the Fort Hays Limestone Member and the inter-bedded marl-rich and chalk-rich benches of the Smokey Hill Member. The total Niobrara Petroleum System reaches hydrocarbon maturation at depths greater than 8,000 feet within the southern Powder River basin to form Type II (sapropelic) source rocks within the marl-rich intervals.

Various petrophysical models were created within ESRI’s ArcMap and Schlumberger’s Petrel application suite in order to assess the effect that chalk purity (geochemistry) has on the structural deformation of the Niobrara Formation of the southern Powder River basin. These models were based upon the neutron-porosity, gamma-ray, and resistivity geophysical well logs. Structure contour maps and isochore maps were also implemented within these software packages to envision the geologic distribution of the Niobrara Formation. Further description of the Niobrara chalk intervals was performed from a petrographic analysis of the lithology, diagenetic fabric, micro-facies configurations, and micro-fractures. From these models and interpretations it becomes ostensible that the chalk purity obtained from the gamma-ray model, coupled with the neutron-porosity and resistivity models, gives rise to the indication that the enhanced porosity and permeability is a result of regional tectonism.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013