Low-Thermal-Maturity (<0.7% VR) Mudrock Pore Systems: Mississippian Barnett Shale, Southern Fort Worth Basin

Abstract

Limited scanning electron microscope (SEM) analysis has been done previously on Ar-ion-milled samples of low-thermal-maturity, organic-rich mudrocks transitioning into the oil window. Using 23 samples from 12 wells, this study examines pore systems and characterizes organic matter from such mudrocks in the Mississippian Barnett Shale of the southern Fort Worth Basin. Samples, prepared using broad ion-beam (BIB) milling with Ar ions, are at relatively low thermal maturities of 0.4 to 0.6% vitrinite reflectance (VR). Lithologies are variable, but are generally more argillaceous and less siliceous than those in the northeastern Fort Worth Basin.

Pores in mudrocks can be classified into three types: organic-matter, interparticle, and intraparticle. Variable development of all three types has been observed in studied samples. Organic matter occurs primarily as a fine mesh surrounding mineral grains. Few or no organic-matter pores are found in the study samples. Interparticle pores are present in some samples in areas where organic matter is absent or limited. Some interparticle pores are also found in the stress shadows around rigid grains. Intraparticle pores after dissolved grains are common in the study samples. Most of these are related to feldspars, particularly albite feldspars. Intraparticle-pore diameters range from nanometers to micrometers.

Low-thermal-maturity Barnett mudrocks have less porosity than originally expected because of the extreme ductility of kerogen, which has flowed into interparticle-pore spaces during compaction. Most of the pore volume is represented by intraparticle pores (dissolved grains), but there are some interparticle pores present, particularly between clay mineral grains. Organic-matter pores are very rare to absent at vitrinite reflectance less than 0.75% in the Barnett Shale.

AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015