The Role of Muddy Hyperpycnites in Shelfal Mudstones and Their Effect on Reservoir Quality: Examples From the Geneseo Formation of New York, USA

Abstract

Unconventional reservoir character varies at the mm- to km-scale vertically and laterally. This variability occurs in systematic ways that can be deciphered utilizing process-based models within a genetic framework. Variations in mudstone properties have a dramatic effect on prodicibility in shale reservoirs, however, the relative controls are not well understood. Detailed facies analysis, geochemistry, and petrography of the lower Genesee Group in the Northern Appalachian Basin (NAB) shows a wealth of sedimentary textures and fabrics that indicate mud deposition by lateral transport across and along the shelf under energetic conditions. Intervals of silt-rich mudstones and muddy siltstones with internal scours, diffuse stratification, soft-sediment deformation, normal and inverse lamina-set grading, and a reduced intensity and diversity of bioturbation occur in multiple facies types and “interrupt” what appears to be the overall background sedimentation. These intervals and their sedimentary features are interpreted as products of high-density fluvial discharge events, which generated turbulent flows that carried fine-grained clastics several tens of kilometers offshore from the paleoshoreline. Recognizing these sediments as products of river-flood- and storm-wave-generated offshore-directed underflows challenges previous depositional models for organic-rich mudstones in the lower Genesee succession, which call for clastic starvation and suspension settling of clay and silt in a deep stratified basin. In the Genesee Group, these observations imply rapid deposition of fine-grained intervals from hyperpycnal plumes in a setting favoring preservation of organic-rich mudstones, and suggest that similar reappraisals of depositional setting are necessary for comparable mudstone successions elsewhere in the Appalachian Basin. The described strata are yet another example for a carbonaceous mudstone succession that was deposited under comparatively energetic conditions, reflects multiple modes of sediment transport and deposition, and records significant carbon burial without a need for anoxic bottom waters. Through understanding the dynamic nature of mudstone depositional environments, process-based modeling can be conducted much more accurately at the reservoir scale, and can account for subtle changes in composition, cementation, porosity/permeability, as well as organic-matter type.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015