Grain assemblages and strong diagenetic overprinting in siliceous mudrocks, Barnett Shale (Mississippian), Fort Worth Basin, Texas

Porosity, permeability, and total organic carbon (TOC) in a heterogeneous suite of 21 high-maturity samples (vitrinite reflectance 1.52-2.15%) from the Barnett Shale in the eastern Fort Worth Basin display few correlations with parameters of rock texture, fabric, and composition, these factors being mostly obscured by the effects of a protracted history of diagenesis. Diagenesis in these rocks includes mechanical and chemical modifications that occurred across a wide range of burial conditions. Compaction and cementation have mostly destroyed primary intergranular porosity. The porosity (average <5 vol. % by Gas Research Institute helium porosimetry) and pore size (<8 nm median pore-throat diameter) are reduced to a degree such that pores are difficult to detect even by imaging Ar ion-milled surfaces with a field-emission scanning electron microscope. The existing porosity that can be imaged is mostly secondary and is localized dominantly within organic particulate debris and solid bitumen. The grain assemblage is highly modified by replacement. A weak pattern of correlation survives between bulk rock properties and the ratio of extrabasinal to intrabasinal sources of siliciclastic debris. Higher porosity, permeability, and TOC are observed in samples representing the extreme end members of mixing between extrabasinal siliciclastic sediment and intrabasinal-derived biosiliceous debris. Reservoir quality in these rocks is neither more strongly nor more simply related to variations in primary texture and composition because the interrelationships between texture and composition are complex and, importantly, the diagenetic overprint is too strong.