Mudrock Chemostratigraphy: Its Role in Integrating Studies of Fine-Grained Sedimentary Successions

Rowe, Harry¹ and Ruppel, Stephen C.
¹[email protected]

Whereas the analysis, characterization, and interpretation of mudrock successions has been undertaken for decades, increased interest in petroleum resources generated by and preserved in these rocks makes their study significantly more important and challenging. Key to better understanding of processes of deposition, diagenesis, and maturation, as well as the effects of these processes on variations in hydrocarbon production response, is our ability to define, integrate, and interpret the best methodologies for characterizing these complex rocks. Wireline logs provide important information on the rocks but must be calibrated using integrated core studies to interpret key reservoir properties like TOC, porosity, permeability, rock strength, and continuity. However, conventional core description is insufficient for this purpose. In order to define the complex variations in attributes that typify these rocks, one must incorporate high-resolution measurements of elemental composition and mineralogy using x-ray fluorescence (XRF). This technique is based on the use of hand-held scanners calibrated to well-characterized reference materials. When integrated with more conventional macroscopic and microscopic studies of core attributes, these data provide a more robust record of variations in rock properties that may control such key reservoir attributes as organic matter type and abundance, pore types and abundance, hydrocarbon saturation, and brittleness.

Several examples of integrated results will be presented to demonstrate the role that XRF elemental analysis plays in defining mudrock facies, mineralogy, and the associated reservoir attributes. A primary example is high-resolution XRF (2-inch sample spacing) work undertaken on the Haynesville Formation of the East Texas Basin, which helped define depositional facies-successions that are otherwise invisible using traditional observational techniques. For example, high-amplitude stratigraphic shifts in the calcium, aluminum, and silicon in the Haynesville defined shifts in the deposition of calcite, clay minerals, and quartz, respectively-mineralogical variability that influences brittleness and other reservoir attributes. Furthermore, stratigraphic shifts in molybdenum, an element that is only elevated in euxinic facies, defined syndepositional changes in deep-basin water mass history that influence preservation of TOC-a direct linkage to organic matter, and a potential linkage to pore type and abundance.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013