Mudstone Microfabrics and Pore Networks: Gulf Coast Analogs
William C. Dawson, William R. Almon, and Laura M. Vestal
Chevron ETC, 3901 Briarpark Dr., Houston, Texas 77042
The intrinsic heterogeneity of mudstones contributes to substantial variability in rock properties and reservoir attributes. A major factor that contributes to this variability is microfabric (i.e., grain alignment, packing, and sorting) of the matrix, which, inturn, influences the character of the pore network. Mudstone microfabric is the endresult of both sedimentologic and diagenetic processes; bioturbation, permeable silt laminae, compaction, and cementation-replacement can induce heterogeneities, and thereby enhance or degrade mudstone reservoir quality. Pore types, pore sizes, and pore connectivity vary with differences in mudstone fabric and diagenetic/burial history. Rather than studying individual nanometer-size pores, this study focuses on the larger-scale aspects of mudstone pore networks. Shale microfabric variability can be understood in the context of shale microfacies analysis and well-log scale sequence stratigraphy. Each shale microfacies has limited stratigraphic occurrence where considered within a highresolution (log-scale) sequence-stratigraphic framework.
High-pressure mercury injection capillary pressure (MICP) analysis offers a consistent methodology for determining porosity and permeability in mudstones. Mudstones exhibiting different thin section- and scanning electron microscopy (SEM)-scale microfabrics have distinctive MICP profiles that are related to pore system connectivity as controlled by differences in sorting of pore throat sizes and matrix elements. MICP analyses permits quantification of shale pore networks and, when integrated with results of thin section and SEM studies, allows establishment of relationships between fabric and a variety of other geologic variables (e.g., mineralogy, total organic content, grain size, sorting, and fossil content). Pores in mudstones occur within both inorganic matrix and scattered organic particles. Our studies of Gulf Coast Tertiary- and Mesozoic-aged shales show that intercrystalline (i.e., phyllosilicate) micro- and nannopores and interparticle micropores are abundant, and often, are the dominant elements of mudstone pore networks. Intercrystalline pores, where interconnected, appear to form SEM-scale micro-channels. Intracrystalline and intra-organic pores are present, but are not the dominant pore types. Integrated SEM and MICP analyses indicate that bimodal pore networks are a common aspect of silt-rich mudstones. Silt-sized siliciclastic and bioclastic components are seen at thin section- and SEM-scales to disrupt compaction, thus allowing preservation of interparticle micro- to mesopores. Secondary (dissolution) micropores also occur in some mudstones. Although their volumetric contribution appears relatively minor, such relatively large matrix pores probably improve mudstone reservoir potential. Maximum effectiveness of mudstone pore networks is observed in mudstones having total silt contents exceeding 20 percent. Where organized as laminations or burrow networks, silt-sized components can significantly enhance horizontal permeability. Silt-rich mudstones and argillaceous siltstones typify upper parts of coarsening upward mudstone depositional cycles.
Pore types, pore sizes, pore connectivity, and organic content are known to vary with mudstone fabric and ultimately, are related to mudstone lithofacies. Mudstone microfacies exhibit strong statistical correlations between microfabric and MICP data. Consequently, micro-textural (thin section and SEM) observations, correlated to well-log character/rock-typing, where integrated with porosity-permeability data, and image analysis result in the recognition of ‘mudstone petrotypes,’ which appear to link with reservoir performance. These analyses provide a geological basis for understanding and mapping vertical and lateral differences in observed mudstone properties, and ultimately, development of models for unconventional reservoirs. Ultimately, failure to recognize, document and understand microfabric heterogeneities could contribute to erroneous economic assessment of unconventional gas plays.
AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012