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Pore Networks in Niobrara, Piceance Basin, Western Colorado Exhibit Minimal Regional Variability as a Function of Thermal History

Abstract

The Niobrara member of the Mancos Shale is an unconventional gas reservoir in the Piceance basin, western Colorado. In general, burial and thermal histories were less on the western side of the basin and greater in the northeastern side of the basin. As a result, vitrinite reflectance values for the full Niobrara interval range from ~0.5 Ro to ~1.2 Ro. This study investigates how nano- and micron-scale pore systems differ as a result of this variability in thermal histories. Core material was sampled from 6 wells that cover a 6,750 ft range in subsea burial depths and Ro values of 0.5 to 0.9. To control for lithologic heterogeneity, all samples were taken from the same ~50 ft-thick stratigraphic interval that comprises the primary horizontal landing zone across the basin. Lithologies in that zone are marls to marly shales with reasonably high (up to ~55%) carbonate content. The primary constituents are quartz silt, peloids, and argillaceous clays. Elemental mapping shows that peloids are almost exclusively calcite and contain disseminated carbon. Detrital carbon (kerogen) and siliciclastics are in the matrix. Pore networks were identified, characterized, and analyzed using focused ion-beam scanning electron microscopy (FIB-SEM) and Avizo Firetm image analysis software. A variety of pore types were observed, with the dominant forms being intercrystalline pores between recrystallized calcite in the peloids and clay-related plus interparticle pores in the intervening argillaceous matrix. At one point in time the average amount of porosity in peloids (13%) was 6 times greater than the average amount in matrix (2.2%). But many of those pores (70% in peloids, 60% in matrix) are now filled with residual migrated hydrocarbons that contain organic matter bubble pores (~7% of all residual hydrocarbon is now bubble pore). As a result, total imaged porosity now averages only 3.5% in peloids and 0.9% in matrix with OM pores comprising 10% to 20% of those values. Cumulative area and size distributions (equivalent circular diameters average 257.6 nm in peloids and 266.2 nm in matrix) for peloids and matrix pore networks show no trend related to burial depth or thermal history. The lowest maturity well (~0.5 % Ro) shows a much lower abundance of organic matter porosity relative to all other wells, but there is no trend in organic matter porosity above a Ro of ~0.7.