Pore Systems of the B-chalk Zone in the Niobrara Formation, Denver-Julesburg Basin, Colorado

Abstract

Summary Horizontal drilling and artificial stimulation have made the Cretaceous Niobrara Formation, Denver-Julesburg Basin, a prolific, self-sourced, unconventional reservoir that yields both gas and liquids. As a carbonate-dominated reservoir, pore networks documented in most other unconventional shale reservoirs are not necessarily analogous. In this study pore system characterization was achieved using focused ion-beam scanning electron microscopy (FIBSEM) and Avizo Fire image segmenting software. We focused on samples from the B chalk, which is the primary landing zone for horizontal wells in the Niobrara. 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 >20,000). At the microfacies scale, samples consisted of chalks, marly chalks, chalky marls, and submillimeter laminations of marl in the chalk. Electron microprobe elemental mapping shows that organic macerals are concentrated in the marl interlaminations whereas finely disseminated organic matter is in peloids (particularly those that are black in thin section). Intercrystalline pores dominate all samples and occur primarily between fragments of coccolith debris and recrystallized calcite, and to a lesser degree between clay minerals. Median equivalent circular diameters of intercrystalline pores range from ~100 to 400 nm. Total image porosity averages 3.9% with a range of 1.4% to 10%. Highest porosity is in peloids within marly chalks. Residual hydrocarbon fills many former intercrystalline pores and organic macerals occur in marl lamina. In most cases, residual hydrocarbon-filled pores exceed the amount of open pores, and those filled pores have median equivalent diameters than range from 300 to 900 nm. Organic material exhibits intraparticle “bubble” pores with the abundance of bubble porosity in organics showing no increase with thermal maturity above ~0.7 Ro. The size of hydrocarbon-filled pores is greatest in the most thermally mature well, and greater than the size of open intercrystalline pores in all wells. This suggests pores that were not filled by organic matter underwent continued reduction due to diagenesis after hydrocarbon migration.