Preferred Orientation of Phyllosilicates in Gulf Coast Mudstones and Relation to the Smectite-Illite Transition

Abstract

Abstract--Development of preferred orientations of illite-smectite (I-S) has been studied using X-ray diffraction (XRD) texture goniometry to produce pole figures for clay minerals of a suite of 16 mudstone samples from a core from the Gulf Coast. Samples represent a compaction-loading environment in which the smectite-to-illite (S-I) transition occurs. In five shallow, pre-transition samples, there is no significant preferred orientation for smectite rich I-S. Development of preferred orientation of I-S, although weak, was first detected at depths slightly less than that of the S-I transition. The degree of preferred orientation, which is always bedding-parallel, increases rather abruptly, but continuously, over a narrow interval corresponding to the onset of the S-I transition, then continues to strengthen only slightly with increasing depth. The degree of post-transition preferred orientation is also dependent on lithology, where the preferred orientation is less well-defined for quartz-rich samples.Previously obtained transmission electron microscope (TEM) data define textures consistent with the change in orientation over many crystallites. The smectite in pre-transition rocks consists largely of anastomosing, "wavy" layers with variable orientation and whose mean orientation is parallel to bedding, but which deviate continuously from that orientation. This results in broad, poorly defined peaks in pole figures. Post-transition illite, by contrast, consists of thin, straight packets, with most individual crystallites being parallel or nearly parallel to bedding. This results in pole figures with sharply defined maxima. By analogy with development of slaty cleavage in response to tectonic stress during metamorphism, the S-I transition is marked by dissolution of smectite and neocrystallization of illite or I-S locally within the continuous "megacrystals" of smectite. The transition is inferred to have some component of mechanical rotation of coherent illite crystals within a pliant matrix of smectite. The data suggest that change in orientation and coalescence of clay packets plays an important role in the formation of the hydraulic seal required for over-pressure generation.

Gas shale; Comparison between permeability anisotropy and elasticity anisotropy

“…According to our thinking, the preferred orientation of clay minerals which known for a long time to be the major source of shale anisotropy (e.g. Oertel and Curtis, 1972;Wenk, 1985;Ho et al, 1999 andDay-Stirrat et al, 2008) imparts a preferential direction for the pore-like cracks in between parallel to the bedding directions causes shale to behave like a TI media (Transversely Isotropic Media) where the bedding plane is the plane of isotropy.…”

Fabric Development and Pore-Throat Reduction in a Mass-Transport Deposit in the Jubilee Gas Field, Eastern Gulf of Mexico: Consequences for the Sealing Capacity of MTDs

“…As such, the extent of clay alignment can reflect the potential seal quality. Several studies on the Gulf of Mexico mudstones (Freed and Peacor, 1989;Ho et al, 1999;Aplin et al, 2003) show the concomitant relation between sealing effect and enhanced-clay fabric. Therefore, where diagenesis is not dominant, zones of enhanced microfabrics in mud-prone MTDs can play an important role in creating an effective sealing surface for fluids in the subsurface.…”

“…We propose that zones with enhanced-clay fabric in MTDs play a key role in creating effective hydrocarbon-sealing facies. Previous studies in the Gulf Coast mudstones (Freed and Peacor, 1989) and North Sea siltstones (Nadeau, personal communication in Ho et al, 1999) have shown the concomitant relationship between sealing effect and clay fabric development via smectite-illite (S-I) transformation.…”

Texture Development of Clay‐Rich Sediments Across the Costa Rica Subduction Zone

“…The microfabric anisotropy of phyllosilicate-rich sediments generally increases during burial and progressive compaction in marine environments (e.g., Bennett & Hulbert, 1986;Day-Stirrat et al, 2012;Ho et al, 1999), of course, also depending on mineral composition and distribution (e.g., Sintubin, 1994). This can be determined indirectly by, for example, AMS (anisotropy of the magnetic susceptibility) measurements, void ratios, or preferred alignment of larger grains, for example, illite flakes (e.g., Agar et al, 1989;Bowles et al, 1969;Kawamura & Ogawa, 2004;Kawamura, 2010;Maffione & Morris, 2017;Schumann, Stipp, Leiss, et al, 2014) or by analyzing crystallographic preferred orientations using diffraction techniques (e.g., Aplin et al, 2006;Day-Stirrat et al, 2012;Kopf & Behrmann, 1997;Schumann, Stipp, Leiss, et al, 2014).…”