AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Recognition of Mudrock Types from Integration and Upscaling of Geologic, Petrophysical and Geochemical Data Examples from Haynesville, Woodford and Marcellus Shales
(1) WWE Unconventional New Ventures, Marathon Oil Company, Houston, TX.
(2) Upstream Technology, Marathon Oil Company, Houston, TX.
Integrated petrologic analyses are used to develop a proprietary gas shale classification that captures the textural and compositional variability of very fine-grained rocks and provides a scheme to more precisely recognize shale types and their associated petrophysical parameters. The classification incorporates lithology based on bulk composition as determined by XRD analyses, and thin section petrographic recognition of microfabrics. Mudrock types are differentiated from Haynesville, Marcellus and Woodford cores at macro-, micro- and nano-scales by combining new technologies with standard petrographic techniques. Integration of compositional variations from XRD, TOC analyses and thin section petrography clarify of how and where minerals occur. Intra-basinal versus extra-basinal sediments, quartz origins, and diagenetic cements can be interpreted. Whole thin sections are examined using high resolution digital scans under white and reflected light to distinguish sedimentary features (laminae, micro-erosion surfaces, trace fossils, textures) and identify skeletal and other grain types. Contrast enhancement by digital image analysis of thin section photomicrographs reveals sedimentary features and compositional variations previously masked by the dark coloration of organic-rich mudrocks. Whole core CT scans provide 3D approximations of relative bulk density and views of fractures, cements, carbonate-rich and organic-rich intervals, bedding contacts, skeletal fragments, and ichnofacies distributions. Nano-scale pore systems are identified in argon ion-milled samples using FE-SEM with low-vacuum back-scattered electron imaging capabilities. Petrophysical model guide sample selections that show interconnected systems with a mixture of sub-resolution pore and organic material. Nanopores within the organic material appear to enhance permeability.
Boundaries in the shale classification relating to petrophysical properties can be defined by upscaling features observed from FE-SEM to thin sections to those revealed by the CT scans, and by incorporating biostratigraphy, petrophysics, geochemistry and rock mechanics data. Controls on porosity and permeability and tendencies for the occurrence of natural and hydraulic fractures can be interpreted. Mudrock types have been differentiated that can be used to model potential completion zones, recognize high-quality reservoir intervals, and to extrapolate localized knowledge into regional geologic models.