Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale

Abstract

Research on mudrock attributes has increased dramatically since shale-gas systems have become commercial hydrocarbon production targets. One of the most significant research questions now being asked focuses on the nature of the pore system in these mudrocks. Our work on siliceous mudstones from the Mississippian Barnett Shale of the Fort Worth Basin, Texas, shows that the pores in these rocks are dominantly nanometer in scale (nanopores). We used scanning electron microscopy to characterize Barnett pores from a number of cores and have imaged pores as small as 5 nm. Key to our success in imaging these nanopores is the use of Ar-ion-beam milling; this methodology provides flat surfaces that lack topography related to differential hardness and are fundamental for high-magnification imaging. Nanopores are observed in three main modes of occurrence. Most pores are found in grains of organic matter as intraparticle pores; many of these grains contain hundreds of pores. Intraparticle organic nanopores most commonly have irregular, bubblelike, elliptical cross sections and range between 5 and 750 nm with the median nanopore size for all grains being approximately 100 nm. Internal porosities of up to 20.2% have been measured for whole grains of organic matter based on point-count data from scanning electron microscopy analysis. These nanopores in the organic matter are the predominant pore type in the Barnett mudstones and they are related to thermal maturation. Nanopores are also found in bedding-parallel, wispy, organic-rich laminae as intraparticle pores in organic grains and as interparticle pores between organic matter, but this mode is not common. Although less abundant, nanopores are also locally present in fine-grained matrix areas unassociated with organic matter and as nano-to microintercrystalline pores in pyrite framboids. Intraparticle organic nanopores and pyrite-framboid intercrystalline pores contribute to gas storage in Barnett mudstones. We postulate that permeability pathways within the Barnett mudstones are along bedding-parallel layers of organic matter or a mesh network of organic matter flakes because this material contains the most pores.

Pore structure and tracer migration behavior of typical American and Chinese shales

“…Shales contain distinct phases of oil-wetting organic matter (e.g., kerogen) and water-wetting minerals, and tracers in two fluids (API brine and n-decane) are used to interrogate the wettability and connectivity of organic matter and mineral pore spaces. The organic fluid (n-decane) is expected to be preferentially attracted to the hydrophobic component (e.g., organic particles) of the shale matrix, with reported organic (kerogen) particle sizes ranging from less than 1 lm to tens of lm (Loucks et al 2009;Curtis et al 2012). Organic grains are found to be dispersed through the Barnett shale matrix (Hu and Ewing 2014), with their connection to mineral phases unknown.…”

“…While the presence of nanopores in shales has been well recognized since the first application of Ar-ion milling and field-emission SEM imaging (Loucks et al 2009) to indicate the dominant organic nanopores in Barnett shales, larger pores cannot be discounted in their roles of mass transport and connecting nanopore regions. Pores less than 3 nm can be quantified with approaches such as lowpressure gas sorption isotherm, but such small pores probably do not play critical roles in hydrocarbon movement (Javadpour et al 2007).…”

High-pressure adsorption of methane on montmorillonite, kaolinite and illite

“…The experimental evaluation showed that the CH 4 adsorption capacity of shale increases with the increment of pressure (Cheng and Huang, 2004;Loucks et al, 2009;Lu et al, 1995;Bustin, 2007, 2008), implying that the high pressures in the actual shale gas reservoir might result in a large CH 4 adsorption amount. In contrast, high reservoir temperatures are unfavorable with respect to CH 4 adsorption and decrease the adsorption capacity of shale (Ross and Bustin, 2008;Zhang et al, 2012).…”

“…Shale gas, which is derived from organic matters in shale and stored in shale deposits, is an important unconventional gas resource and has recently attracted attention for its promising exploitation (Chalmers and Bustin, 2008;Curtis, 2002;Jing et al, 2011;Loucks et al, 2009;Bustin, 2007, 2009;Zhang et al, 2012). Methane (CH 4 ) sourced from thermogenesis and/or biogenesis of organisms is the dominant component of shale gas (Hill et al, 2007;Strapoc et al, 2010;Zhang et al, 2012).…”

“…The mechanism of CH 4 adsorption in organic matters has been well documented, illustrating that the surface functional groups and micropores provided the adsorption sites for CH 4 (Loucks et al, 2009;Ross and Bustin, 2007Zhang et al, 2012). The effects of the characteristics of organic matters on CH 4 adsorption, including the total organic carbon (TOC) content (Cluff and Dickerson, 1982;Harris et al, 1970), kerogen type (Zhang et al, 2012) and thermal maturity (Loucks et al, 2009), have also been investigated.…”