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

Laughrey, Christopher D.; Ruble, Tim E.; Purrazzella, Peter; Hooghan, Kultaransingh; Beuthin, Jack; Washburn, Kathryn; Dorsey, William

Prospective tight-oil carbonate reservoirs in the Upper Cretaceous Niobrara Formation in the northern Denver basin include elements of both hybrid/interbedded and porous mudstone fine-grained reservoir systems. We recognize three reservoir/source rock lithologies: 1) porous and permeable, argillaceous, moderately organic-rich, pelletal impure chalk; 2) relatively porous, low permeability, moderately organic-rich pelletal chalk; and, 3) relatively porous, low permeability, very organic-rich pelletal impure chalk. Geochemical parameters indicate the rocks belong to Organic Facies B (oil-prone) and B-C (oil/gas-prone), and have reached early to peak thermal maturity. Depositional fabric, mineralogy, compaction, cementation, and petroleum generation control the measured porosity, permeability, and relative fluid saturations in these rocks.

Excluding fractures, we recognize two mudstone pore types in the Niobrara reservoirs - mineral matrix pores and organic-matter pores. Mineral matrix pores include interparticle voids and intraparticle pore textures. Intraparticle pores dominate void space in the rocks and consist of effective nano- to micro-scale interparticle and intraparticle carbonate pore textures preserved within compacted pelletal allochems.

Organic porosity is related to original kerogen structure and to organic matter volume changes during petroleum generation and expulsion. Organic porosity within kerogens in the rock matrix is rare and poorly connected. Organic pores in kerogens preserved within individual pellets are common and interconnected.

Complex intraparticle pore geometry is readily modeled by 3-D FIB/SEM segmentation. Modeling petrophysical calculations correlate poorly with laboratory analysis of crushed core samples. Laboratory NMR measurements of samples modeled by FIB/SEM analysis appear to discriminate discrete pore and fluid distributions in the tight Niobrara Formation reservoirs.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013