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Variability in Organic-Matter Pore Development Over Depth: A Case Study From the Devonian-Mississippian Woodford Formation, Delaware Basin, West Texas


The Devonian-Mississippian Woodford Formation of west Texas is an organic-rich mudrock unit with varying development of nanometer-scale pores at different depths. Samples for this study are from the Pioneer Reliance Triple Crown #1 well, Pecos Co. Texas located in the southern part of the Delaware Basin. Fourteen samples were chosen based on facies and dispersed throughout the Woodford Fm. core. Sample depths range from 12,759 ft. to 13,097 ft. In this well, the unit contains a variety of mudrock lithologies ranging from siliceous mudstones to argillaceous mudstones to dolomitic siltstones. Lithologies are notably calcite-poor. Total organic carbon values for the core range from 0.6 to 8.6%. Thermal maturity from estimated vitrinite reflectance is in the range 1.2 to 1.4%. Samples were broad-ion-beam (BIB) milled to produce a flat surface for observation using a field-emission scanning electron microscope. Where present, pores are primarily developed in organic matter with only minor interparticle pores or intraparticle pores in nonorganic grains. Of the fourteen samples chosen for this study, seven contained organic matter with abundant pores, seven contained organic matter with few or no pores. Organic-matter pores in these samples are generally less than 100 nm in diameter with most being considerably smaller, down to the 5 nm resolution limit. Average organic-matter pore diameter can vary between adjacent areas. The less common organic-matter pores are in a sample, the smaller their diameters tend to be. Pore shape is highly variable, with the smaller pore outlines tending to be more circular and the larger pore outlines being more irregular and elongate. Organic matter rarely takes grain-like form (equant shape, diameters larger than a micrometer) but is mostly dispersed as sub-micrometer irregular pieces between mineral grains, representing either solid bitumen or very ductile kerogen. Pore development does not correlate with depth; porous samples are found at intermediate depths with nonporous samples occurring on both ends of the core. Pore development does not appear to correlate with amount of organic matter present or the distribution of organic matter. Pore development does not correlate with lithology. The most likely explanation for the variation in pore development is a difference in the type of organic matter. Terrestrial organic matter is thought not to develop pores during thermal maturation, whereas marine organic matter does.