PRELIMINARY CHEMOSTRATIGRAPHIC RESULTS FROM THE EAGLE FORD FORMATION, SOUTH TEXAS: IMPLICATIONS FOR REGIONAL CORRELATION AND LATE CRETACEOUS OCEAN ANOXIA

Deposition of the Late Cretaceous (Cenomanian-Turonian) Eagle Ford Formation represents a brief period of mixed siliciclastic-carbonate mud accumulation during an otherwise carbonate-dominated period of deposition on the southern end of the Western Interior Seaway (WIS) of North America. Furthermore, strata within the Eagle Ford Formation records organic carbon-rich accumulation during Oceanic Anoxia Event #2 (OAE-2). Finally, the Eagle Ford Formation represents a growing focus of oil, condensate, and gas recovery. The ultimate goals of the present study are to 1) enhance our understanding of the environmental conditions during deposition and their role in organic enrichment of the strata, and 2) develop the use of chemostratigraphy as a means of regional correlation.

The study incorporates analysis of seven drill cores from La Salle, Zavala, Frio, De Witt, Gonzalez, Bee, and Wilson Counties, with each core preserving all or a portion of the organic-rich unit within the Eagle Ford. Each core was scanned at a ~1-foot interval using a Bruker handheld x-ray fluorescence instrument, providing rapid, quantitative analysis of the elemental concentrations: Mg, Al, S, Si, P, K, Ti, Ca, Mn, Fe, Mo, Cr, Co, Ni, Cu, Zn, Th, Rb, U, Sr, Zr, and V. Total organic carbon (TOC), total inorganic carbon (TIC), %N, and the d¹⁵N and d¹³C_TOC have been measured on three of the cores. Petrographic studies are forthcoming on the same three cores.

Preliminary evaluation of core chemostratigraphies reveals that, on average, the Eagle Ford Formation is rich in calcium carbonate (and correspondingly low in siliciclastics), relative to other hydrocarbon-yielding mudrock targets in the region (e.g., Barnett, Bossier/Haynesville). Molybdenum and uranium, two redox-sensitive trace elements that strongly accumulate in anoxic, sulfide-rich (euxinic) bottom waters, are highly elevated in much of the organic-rich strata, indicating prolonged ocean anoxia in the region during Eagle Ford deposition. TOC values from one core from De Witt County are, in some intervals exceed 6% and average 3.6%, suggesting that significant organic accumulation occurred during OAE-2 at that location. Further evaluation and integration of the core datasets will focus on inter-core correlation and developing the paleoceanographic response to OAE-2 in the region.