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Defining the Sequence Stratigraphic Framework, Organic Richness, and Hydrocarbon Storage of the Cenomanian/Turonian Eagle Ford Formation, South Texas

John Guthrie, Ting C. Huang, Robert Handford, Randy Mitchell, Steve Crews, Rick Beaubouef, Andy Pepper, and Jim Halgas
Hess Corporation, Houston, Texas, USA

Predicted total organic carbon (TOC) content derived from DLogR analysis, measured geochemical data (TOC and Rock-Eval analyses), biostratigraphy, and carbon isotopes of TOC are used to define a detailed sequence stratigraphic framework for core from a well in the Cenomanian/Turonian Eagle Ford Formation, South Texas. Variations in the geochemical data are interpreted within the context of the stratigraphic framework to help define hydrocarbon storage and relate it to the mechanical properties in this unconventional resource.

Recurring patterns of TOC derived from DLogR analysis help to subdivide the Eagle Ford Formation into four third order sequences that have been correlated to the global sea level chart using detailed biostratigraphy and the globally recognized Cenomanian/Turonian carbon isotopic excursion (OAE2) in organic carbon (Figure 1). The maximum TOC in a sequence correlates to the maximum flooding surface (MFS) and serves to further subdivide the sequences into transgressive (TST) and highstand (HST) systems tracts. The biostratigraphic and carbon isotope data help reveal stratigraphic complexity defined by the removal of portions of each sequence at major sequence boundary (SB) surfaces.

Geochemical data from the core show that the lowest sequence (Ce3) contains the highest TOC in the TST with maximum TOC associated with the MFS in the sequence. The best hydrocarbon storage, calculated as oil yield in barrels per acre foot using the S1 (mg hydrocarbons/g rock) from Rock-Eval pyrolysis, occurs directly above the MFS in the HST of sequence Ce3. These units of better hydrocarbon storage contain relatively higher amounts of mineral matrix and intra-granular porosity associated with increasing interbedded carbonate lithologies in the HST. The mechanical properties of these HST units indicate a more brittle character as compared to the more ductile, high TOC intervals associated with the MFS. The unconventional resource in the Eagle Ford Formation is therefore described as the juxtaposition of the more organic-rich (higher TOC) MFS intervals with the overlying more brittle, carbonate-rich, hydrocarbon-bearing intervals of the HST. The detailed sequence stratigraphy helps to define the stratigraphic occurrence of this unconventional hydrocarbon resource and allows for its regional correlation.

AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012