Dual Mineral Matrix and Organic Pore Textures in Mature Niobrara Formation, Rocky Mountains, USA – Implications for Tight-Oil Carbonate Reservoir Modeling

Prospective tight-oil reservoirs in the Upper Cretaceous Niobrara Formation B Chalk in the northern Denver basin include elements of hybrid/interbedded and porous mudstone fine-grained systems. We recognize three reservoir/source rock lithologies: 1) argillaceous, organic-rich, pelletal impure chalk; 2) organic-rich pelletal chalk; and, 3) highly organic-rich pelletal impure chalk. Geochemical parameters indicate the rocks belong to Organic Facies B (oil-prone), and are thermally mature with respect to oil generation. We recognize both mineral matrix pores and organic-matter pores in the Niobrara reservoirs. Mineral matrix pores include interparticle and intraparticle voids. Intraparticle pores dominate storage capacity in the rocks and consist of nano- to micro-scale fabric-selective and not-fabric-selective carbonate pore textures preserved within compacted pelletal allochems. Organic porosity is well developed in alginite and liptodetrinite, but is absent in bituminite, and thus may be related to the original kerogen structure. Complex intraparticle pore geometry is readily modeled by 3D FIB/SEM segmentation. Laboratory NMR measurements of the modeled samples appear to discriminate discrete pore and fluid distributions in the tight reservoirs.