Tuesday, July 14, 2015

Highlights—Mudstone Description Tools

Much of the sedimentary record comprises mudstone, and recent interest is driven by recognition of hydrocarbon sources, reservoirs, and seals in deposits of mudstone. Beyond this importance, they also influence groundwater dynamics and provide important perspectives on the global carbon cycle and climate and oceanography through geologic time. Yet, traditional mudstone descriptors neither capture the variability in their physical, biogenic, and chemical attributes, nor facilitate appropriate characterization and interpretation of controlling processes on their formation. Here, Lazar et al. discuss the utility of existing mudstone nomenclature, provide context for the information required to fully characterize mudstone, and recommend guidelines that facilitate consistent, repeatable, and efficient capture of key attributes to evaluate and compare fine-grained rocks. The application of their simple, integrated descriptive scheme to the Cretaceous Eagle Ford Shale demonstrates the utility of the technique for assessing and predicting rock properties.

Capturing keyattributes of fine-grained sedimentary rocks in outcrops, cores, and thinsections: nomenclature and description guidelines by O. Remus Lazar, Kevin M. Bohacs, Joe H.S. Macquaker, Juergen Schieber, and Timothy M. Demko

Tuesday, June 30, 2015

Highlights—Clinoform Geometries and Sediment

Clinoforms are fundamental sequence and seismic stratigraphic elements, reflecting information on sediment distribution in space and time to and across shelf margins. In this contribution, Gong et al. quantitatively explore relationships between clinoform-growth styles and sand- and sediment-budget partitioning across shelf margins of late Miocene-Quaternary clinoforms of the northwestern South China Sea. The results illustrate that clinoform-growth styles represent distinct stratal stacking patterns and sediment distribution. For example, downward-prograding shelf-margin clinoforms with low angles of growth trajectories (Gct) and low clinoform height (Hc) favor partitioning volumes of sediment into deep-water areas, thus are fronted by sand-rich submarine fan systems. In contrast, steeply upward aggrading shelf-margin clinoforms with high Gct and high Hc favor storage of volumes of sediment on shelf margins, and hence include mud-dominated mass-transport systems downdip of shelf breaks. Gct and Hc therefore increase linearly with sediment budget stored on shelf margins, but decrease linearly with sand- and sediment-budget partitioning into deep-water areas, provided similar sediment-supply conditions through time. The results suggest that clinoform-growth styles are thus good predictors of sand- and sediment-volume partitioning across shelf margins.

Growth styles of shelf-margin clinoforms: prediction of sand- and sediment-budget partitioning into and across the shelf by Chenglin Gong, Yingmin Wang, Ronald J. Steel, Cornel Olariu, Qiang Xu, Xiangnan Liu, and Qianhui Zhao

Monday, June 8, 2015

Highlights—Dueling pore systems from bioturbation

Bioturbation is an important post-depositional process that can alter sedimentary textures, porosity, and permeability. Here, Baniak et al. examine the influence of burrow geometry and connectivity on porosity and permeability within the Upper Jurassic Ula Formation of the Norwegian Central Graben. Spot permeametry data and numerical modeling of this shoreface succession illuminate relationships among burrow morphology, bioturbated volume, and burrow connectivity. These data provide insights for a conceptual framework for assigning bulk permeability to reservoir media, useful to better characterize hydrocarbon deliverance through bioturbated sandstone reservoirs with dual porosity and dual permeabeability systems.

Thursday, May 28, 2015

Highlights—Thriving Devonian microbialite reefs

The abundance of reefs and the biota that they include vary through geologic time, punctuated by periods of rapid change. To understand the dynamics of such change, Matysik et al. document the structure, dimensions, and spatial patterns of microbial-dominated patch reefs in mixed carbonate-siliciclastic environments on the northern margin of Gondwana during the Early Devonian “crisis” in metazoan reef development. Despite different composition, these reefs show close similarities in shape and spatial arrangement with present-day coral patch reefs, suggesting comparable controls. Additionally, the strata show a close relation between metazoans and microbialites in reefs and reef-associated strata, illustrating that microbialites can compete successfully, even in the presence of metazoans, given elevated nutrient supply and elevated temperatures.

Friday, May 22, 2015

Highlights—Why is the Holder tight? The impact of early diagenesis

Diagenesis represents the progressive alteration of sediment and rock; it is never simple. To examine diagenesis, many studies of carbonate successions focus on detailed petrographic study and bulk geochemical analyses. To test the hypothesis that multiple episodes of early diagenesis (subaerial exposure) are recorded as multi-phase calcite cements, Wasson and Lohmann examine petrographic and geochemical character of the Holder Formation (Pennsylvanian, New Mexico, USA). This study integrates field observations of the phylloid-algal and microbial mounds with microsampled geochemical data from some of the key features, and clarifies the detailed diagenetic and developmental history of the unit. The results illustrate that most primary and secondary porosity of the units was occluded within the first 500 m of burial, by Early Permian time, and highlight how early diagenesis can markedly impact carbonate strata. [Ed. Note: the Osmonds knew this in 1972.)