Stratigraphic, Sedimentological, and Geochemical Variability in the Eagle Ford Group Across the Stuart City Paleo-Shelf Margin, South Texas

Abstract

This rock-based study integrates sedimentological, geochemical, paleontological, and petrophysical data of the Eagle Ford Group to establish stratigraphic and depositional relationships across the Albian Stuart City paleo-shelf margin in South Texas. This study will also refine the stratigraphic column of the Eagle Ford and underlying strata in South Texas. South Texas Eagle Ford facies include (1) massive-bedded argillaceous wackestones, (2) massive-bedded to laminated foraminiferal wackestones, (3) radiolarian and foraminiferal packstone, (4) laminated inoceramid and foraminiferal grainstone, (5) massive to bioturbated skeletal lime wackestones, (6) laminated foraminiferal packstone, and (7) massive to bioturbated illitic claystone. Significant stratigraphic changes occur across the shelf margin as documented by three cores in La Salle County, South Texas. Wireline log data suggest that the upper Eagle Ford section is not present across the margin (not deposited, eroded, or lateral facies change) while the lower Eagle Ford doubles in thickness basinward. The number of ash beds (massive to bioturbated illitic claystone facies) decreases from twelve on the shelf to seven on the shelf edge to none south of the margin. Another major change is that the basinward core comprises only one main lithofacies, massive-bedded to laminated foraminiferal wackestone, except for a few radiolarian and foraminiferal packstone beds. Therefore, the cyclic character of alternating limestone and marl beds of the lower Eagle Ford on the shelf is not observed south of the margin. Also, XRF and XRD data suggest that clay mineral content increases basinward accompanied by a decrease in calcite content. The abundance of the laminated inoceramid and foraminiferal grainstone facies also decreases across the paleomargin. These observations suggest that the basinward section is older than the shelf sections. Isotope and paleontological data suggest that the Eagle Ford lies disconformably on an upper Albian limestone. The integration of a highly detailed paleontological analysis with sedimentological, geochemical, and petrophysical data makes it possible to build a working stratigraphic framework that is time-constrained. Understanding the shelf-to-basin stratal relationships provides a new basis for future basin-wide chronostratigraphic correlation and for more accurate reservoir characterization instead of the commonly used lithostratigraphic correlation.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017