Date of Award
Master of Science (MS)
David A. Budd
The Niobrara Formation, Denver-Julesburg (DJ) and Piceance basins, Colorado, is one of the most active unconventional petroleum plays in the United States. This study characterizes those pore systems in one well in the DJ Basin and two wells in the Piceance Basin in order to isolate the impact of lithologic variability on pore characteristics. This objective is accomplished through the combination of scanning electron microscopy of Ar-milled rock surfaces, image analysis with Avizio 9 software, and mineralogical characterization by X-ray fluorescence (XRF) and electron microprobe mapping.
The twenty-one imaged samples span the chalk to marly shale lithologies of the Niobrara Interval. This study focuses on the pre-hydrocarbon migration pore systems in order to minimize the influences of differential hydrocarbon saturations, surface wettability, and post-migration thermal maturation. The goal of this project is to understand the basic lithologic controls on the pore systems. Data on size, shape, and orientation of individual pores were collected and compared to lithology (weight percent calcium) and grouped by fabric element to establish trends that can be related to well-scale observations.
Total pre-migration porosity positively correlates to lithology (r = 0.93) with a range of 3.4% to 11.7%, with the chalkiest samples showing the greatest porosity. Median pore size, width, anisotropy, and degree of horizontal orientation also all correlate, to varying degrees, with lithology. Overall, pores associated with peloids (always calcitic) and calcitic matrices (chalk and marly chalk) consistently display the largest sizes (medians of ~175 nm), most equant shapes, and smallest horizontal preference. In contrast, pores in clay-rich, calcite-poor matrices (marl and shaley marl) are the smallest (medians of ~150 nm), most elongate, and most horizontal.
Petroleum explorationists should guide drilling toward zones of the highest calcite and peloid concentrations in order to find the largest and best connected pore systems. Pairing this information with an understanding of the thermal maturity and hydrocarbon saturations will help identify the most favorable petroleum resources in the Niobrara Formation.
Ball, Wesley Evans, "Influence of Lithologic Variability on Nanopore Systems, Niobrara Interval, Denver-Julesburg and Piceance Basins, Colorado, USA" (2015). Geological Sciences Graduate Theses & Dissertations. 104.