Integrating Core, Wireline Log and Chemostratigraphic Data With Biostratigraphic Data and High Resolution U-Pb Zircon Geochronology to Determine Timing Constraints on the Eagle Ford Group Depositional Processes and Stratigraphy

Abstract

Calcareous nannofossils and foraminiferal biostratigraphic ages from the Cenomanian to Turonian Eagle Ford Group are integrated with published regional biostratigraphic data and constrained by U-Pb zircon ages from multiple bentonite beds to provide a comprehensive chronostratigraphic framework for this unit. Bulk rock organic and inorganic geochemistry, carbon stable isotope chemostratigraphy, petrophysics and lithology are included in this framework to better understand the timing of significant geologic processes and events. Data from wells in Webb, Mcmullen, Live Oak and Karnes Counties, Texas suggest that the Eagle Ford Group deposition is punctuated by subtle disconformities and hiatal surfaces that make unraveling the Eagle Ford’s depositional rock record problematic.

Detailed core description from the well in Karnes County shows that the Lower Eagle Ford Formation, characterized by higher values of total organic carbon (TOC), uranium (U), total gamma ray (GR), resistivity (R) and molybdenum (Mo), is composed of dark grey to black mudstone interbedded with light grey skeletal wackestone, foraminiferal packstone and grainstone that form marlstone–limestone couplets. Most of the foraminiferal grainstone and packstone beds are well cemented with a concave base and a convex top, suggesting they were differentially compacted with respect to the surrounding mudstone due to early cementation. The Upper Eagle Ford Formation, characterized by lower values of TOC, U, GR, R and Mo, is composed of light grey foraminiferal grainstone and packstone with few interbeds of skeletal wackestone and mudstone. Several limestone beds in this interval have scoured surfaces and soft-sediment deformation structures.

Whereas the bulk rock carbonate carbon isotope record is monotonously uniform, the organic carbon stable isotope curve delineates a carbon isotope excursion (CIE) of up to +4 ⁰/₀₀, coincident with the Cenomanian – Turonian stage boundary and correlates with oceanic anoxic event 2 (OAE2), that starts at the base of the upper Eagle Ford Formation in the W. archaeocretacea Zone. This CIE provides another stratigraphic marker for a regional correlation. Biostratigraphy also defines other disconformities within the correlations of the Eagle Ford group from Webb to Karnes County.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018