Variability of Nanopore Systems In The Lincoln Limestone, Denver-Julesburg Basin, Colorado, USA

With a shift to exploiting unconventional petroleum plays in the United States to support energy consumption, understanding the nanopore systems in those unconventional resources becomes important for future exploitation. The goal of this study is to characterize the pore systems of the Cretaceous Lincoln Limestone found in the Denver-Julesburg (DJ) Basin in order to test the hypothesis that lithologic variability has a control on pore characteristics and total porosity. The Lincoln Limestone represents a possible reservoir interval in close proximity to a source interval, the Hartland Shale, which directly overlies it, thus making it a potentially profitable future exploration target. Pore networks in the Lincoln were characterized using AR-milled rock surfaces run through a scanning electron microscope (SEM) for image capture, image analysis with Avizio 9 software, and mineralogical characterization by XRay fluorescence (XRF). Seven samples were chosen for analysis from a single core taken from one well in the DJ basin. Five of the samples span the roughly 70-90 ft thick Lincoln Limestone and the other two come from right above and right below the Lincoln. The seven samples span the range of lithologic and porosity variability in the Lincoln Limestone. They have normalized volumes of carbonate that range from 6.0% to 70.6% (mostly as calcite); clay content ranges from 14.8% to 55.2%. The samples are thus marls (n=5), marly shale (n=1) and shale (n=1). Total organic carbon (TOC) contents range from 1.5% to 7.8% and porosity ranges from 5.0% to 10.4%. Pores that were present prior to hydrocarbon maturation are now generally infilled with migrated hydrocarbons. Total imaged porosity of the pre-migration pore system ranges from 5.1% to 7.8% in the seven samples. In individual samples, median equivalent circular diameters of those pores range from 64.2 nm to 91.5 nm, median pore elongations (anisotropy) range from .77 to .81, and median pore width ranges from 51.0 nm to 71.3 nm. The data show that median pore width and median pore size increases with increasing volume of carbonate and decreases with increasing volume of clay, although the trends are not strong. Pores associated with calcite are bigger and less elongated because they occur between relatively equidimensional rhombs of calcite. Where calcite is not present, pores are shorter and more elongate because they are associated with clay minerals. Total imaged porosity also increases with increasing volume of carbonate, and decreases with increasing volume of clay. These trends with total imaged porosity, however, may be misleading. Calcite occurs in two basic habits. One is as compacted but still porous peloids of cocolith and pelagic foraminifera debris and the other is as nonporous, whole pelagic foraminifera shells whose original internal void space is completely cemented, often with calcite. In the latter case, there is calcite, but no pores. Imaged areas were chosen to avoid concentrations of calcite-cemented, non-porous foraminifera in order to document the actual pore systems. But doing so may have biased the amount of imaged porosity with respect to calcite content. A major implication of this work is that the Lincoln Limestone’s pore system is not analogous to the overlying Niobrara Formation. Attributes of the Niobrara’s pore system is more strongly related to total calcite because most of that calcite is found in microporous peloids (Ball, 2015). In contrast peloids are relatively rare in the lower half of the Lincoln, and are not particularly common until near the contact with the overlying Hartland Shale.