DIAGENESIS OF PROMINENT NORTH AMERICAN MUDROCKS DOCUMENTED BY PALEOMAGNETISM, MAGNETIC FABRICS AND PARAGENESIS

Diagenetic alteration of mudrocks is a function of several variables such as temperature, pressure and, fluid chemistry operating across non-discrete spatial-temporal boundaries. The end-products of these interactions include complex paragenesis, chemical remagnetizations, and anomalous magnetic fabrics. Integrated magnetic and diagenetic studies were performed on cores from the Marcellus, Haynesville, Wolfcamp, Woodford, and Barnett shales. The paragenetic sequences for each shale show authigenesis of pyrite, calcite, dolomite, quartz, barite, celestine, anhydrite, sphalerite, and albite. The relative timing of mineralization events is variable, for example, barite in the Woodford, Marcellus, and Wolfcamp shales formed during early to middle diagenesis and during late diagenesis as fracture fill in all five shales. Mineralized fractures are common however, marked differences in frequency, mineralogy and, morphology are evident. An example of basin-specific diagenetic features include hydrothermal minerals such as witherite and monazite in the Woodford. Paleomagnetic studies show chemical remanent magnetizations (CRM) in the Woodford, Marcellus, Haynesville, and Barnett. These CRMs reside primarily in magnetite and are interpreted to be caused by fluid-flow or burial events. The Wolfcamp core experienced lower burial temperatures and does not contain a stable CRM. Anisotropy of magnetic susceptibility data suggests that magnetic fabrics in these mudrocks are generally shaped by compaction. Vertical and sub-vertical fabrics are attributed to inverse magnetic minerals (e.g. ferroan dolomite), flowage microstructures and mineralized fractures. The nature of such fractures were investigated using x-ray computed tomography which reveals complex fracture geometries and fracture wall porosity networks in the Woodford. Similarly, the Wolfcamp also shows complex celestine/barite fracture networks. The primary objective of this research is to characterize the diagenetic complexity of mudrocks and to better understand if they behave as closed or open geochemical systems. Similarities in early mineral authigenesis followed by highly variable middle and late authigenesis suggests that most mudrocks evolve from relatively closed systems to more open systems during burial.