Pore Systems of the B Chalk and Lower A Marl Zones of the Niobrara Formation, Denver-Julesburg Basin, Colorado

摘要

This study identifies, characterizes, and analyzes the pore systems of the Niobrara Formation, and explores the linkage between measurable rock properties and the observed pore system. Pore system characterization was achieved using focused ion-beam scanning electron microscopy (FIB-SEM) and Avizo Fire™ image segmenting software. The study focuses on the B Chalk, the primary landing zone for horizontal wells, and overlying A Marl units.Material was examined from four cores representing thermal maturities ranging from the oil window (Ro≈ 0.7, GOR≈ 1,000) to the dry-gas window (Ro≈ 1.2, GOR u3e20,000). Total image porosity averages 4.3% with a range of 1.4% to 10%, with the highest porosity in peloids. Relationships between lithology, geochemistry, thermal maturity, and microfacies were tested for attributes such as shape, size, and abundance of mineral pores, residual hydrocarbon-filled pores, and pores within organic matter (OM pores). OM pores occur in residual hydrocarbon-filled pores in all wells, but not in all filled pores. These pores range in size from tens of nanometers to microns in diameter. Abundance of OM porosity shows no increase with thermal maturity above ~0.7 Ro. OM pores are generally larger in the higher thermal maturity wells.The abundance and size of mineral and organic pore systems does not vary significantly with respect to lithologies, microfacies and geochemical facies defined by redox and nutrient proxies. Predicting the nature of the pore systems is not possible from these larger scale rock attributes. Paucity of pores in the matrix of samples implies that matrix material may be a fluid flow barrier. Therefore, nearly all storage potential and flow capacity exists within peloid-rich laminae. Conversely, the amount of residual hydrocarbons within original mineral pores raises concerns about how much fluid may flow through the pore system.