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Controls on Mudrock Pore System Development in the Upper Mississippian Barnett Shale, Fort Worth Basin, Wise County, Texas


An investigation of the nanopore systems in the Barnett Shalefound that the controls on pore development are in part related to lithofacies, but are also related to organic-matter properties.Sixteen Ar-ion milled samples from core in the Devon Adams No. 7 well in the Newark East Field were analyzed using scanning electron microcopy. This core begins in the upper Barnett and provides a complete section of the lower Barnett. Samples were selected based on core lithofacies descriptions and XRF chemostratigraphic analysis to ensure a complete range of lithofacies was analyzed. Quartz content averages 42% with a high of 73%. Clay-mineral content is generally low, averaging 15%. Four of the samples are relatively calcareous including two from the Forestburg limestone member. TOC averages 3.66% with a range from 0.54% to 7.42%. Siliceous lithofacies have higher TOC values than the carbonate-rich lithofacies. Calculated Roaverages 1.04%. Pore types consist of interparticle, intraparticle, and organic-matter (OM) pores. Pore type and development varies between lithofacies and to a lesser extent between spots on the same sample surface. Based on visual estimates, OM pores are commonly the dominant pore type in siliceous and argillaceous mudstones. OM pore morphology varies between kerogen and solid bitumen. Kerogen pores are generally larger, some greater than 1 µm in length, and more elongate. Bitumen pores are generally smaller and more equant. Organic matter in association with phosphatic grains appears especially porous.The calcareous lithofacies, which contain less organic matter, have fewer OM pores. Intraparticle pores are present in all lithofacies, but are not common. Interparticle pores are rare and are generally less than 1 µm in size. The lack of interparticle pores in most lithofacies is interpreted to be the result of spread of ductile kerogen and occlusion by solid bitumen. Two organic-rich siliceous mudstone samples, one from the lower Barnett and one from the basal Mo-enriched mudstone, show few pores in their OM. These samples suggest that OM properties rather than simple TOC content controls pore development. Even using XRF, one nonporous siliceous mudstone sample was not differentiated as a separate lithofacies in the initial core analysis.The conclusion that both lithofacies and organic-matter properties have an influence on the generation of mudrock pores is important for trying to predict pore development and distribution in mudrock plays.