The Weak Textural and Compositional Control on Rock Properties in the Barnett Shale (Mississippian), Fort Worth Basin, Texas, USA

Porosity, permeability, and TOC in high-maturity samples (R_o 1.52 - 2.15) from the Barnett Shale in the eastern Fort Worth Basin display few correlations with parameters of primary rock texture, fabric, or inorganic composition. Compaction and cementation have largely destroyed primary intergranular porosity. Porosity (0.9 to 3.8 percent) and pore size are reduced to a degree such that pores are difficult to assess even by imaging Ar-ion milled surfaces with a field-emission SEM. Pores that can be imaged are mostly secondary and localized dominantly within organic particulate debris and pyrobitumen.

The ratio of extrabasinal to intrabasinal sources of siliciclastic debris has a weak correlation with bulk properties. Higher porosity, permeability, and TOC are observed in samples representing the extreme end-members of mixing between extrabasinal siliciclastic sediment and intrabasinal biosiliceous debris. Reservoir quality in these rocks is no longer strongly related to primary texture and composition because the conventional interrelationships between texture and composition and a primary pore system have been destroyed by the intense diagenetic overprint. Pore system properties and hence reservoir quality now have more affinity with the porous particulate organic matter and pyrobitumen occupying inter-mineral pore space within these samples. The correlation between organic matter and porosity is most evident in samples from the shallower of the two cores investigated in this study where an R² of 0.75 is observed.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California