Unification of Compositional Data and High-Resolution Facies Analysis in the Union Springs Formation of New York

Abstract

Compositional data and facies analysis are commonly employed to evaluate organicrich mudstones within both the academic and industrial realms. Compositional analysis of these rocks, commonly performed using handheld x-ray fluorescence on drillcore or well cuttings, supplies workers with relative abundances of volumetrically major and trace elements within a rock. Often supported through the incorporation of total organic carbon data, these data are utilized to draw conclusions regarding paleodepositional environment, resource potential, and mechanical behavior. Facies analysis classifies rocks based upon a suite of visually-identifiable characteristics including mineral content, bedding character, fossil abundance, and other properties related to their paleodepositional setting. This research examines the geochemical variability within and between facies categories, leading to a more comprehensive depositional assessment and a test of the predictive power of compositional data at the microfacies scale.

For the Union Springs Formation of central New York, microscopy and hand sample analysis reveal that 15 microfacies comprise three broad lithofacies groups in three complete sections at locations spanning a 38.6 km distance. Rocks of each microfacies were compositionally analyzed using handheld x-ray fluorescence in order to obtain major and trace elemental composition, and LECO carbon analysis in order to measure total organic carbon content. Inter-facies and intra-facies variability was statistically assessed to determine the relationships between the physically-defined facies categories and major element, trace element, and total organic carbon abundances.

This study establishes the geochemical significance of physically-defined facies categories. As expected, compositional analysis quickly provided valuable paleodepositional insight, and most microfacies display a compositional signal matching the mineralogic signals identified in thin section. However, major and trace element variability is not constant across microfacies, and some facies possess a narrow compositional range while others are variable. Geochemical variability between microfacies is strongly correlated with depositional features such as bedding character, and diagenetic features such as cementation. The incorporation of petrographic analysis, which supplies information about heterogeneity generated by cementation, bioturbation, and organic matter morphology, strengthens interpretations that are commonly based on composition alone. Assessments of paleoenvironments, resource potential, and mechanical behavior all improve through the combination of compositional and microfacies information.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016