STRATIGRAPHIC AND GEOCHEMICAL VARIABILITY: EAGLE FORD GROUP, EAST-CENTRAL TEXAS
Published:December 01, 1993
William C. Dawson, Barry J. Katz, Louis M. Liro, Vaughn D. Robison, 1993. "STRATIGRAPHIC AND GEOCHEMICAL VARIABILITY: EAGLE FORD GROUP, EAST-CENTRAL TEXAS", Rates of Geologic Processes, Tectonics, Sedimentation, Eustasy and Climate–Implications for Hydrocarbon Exploration, John M. Armentrout, Roger Bloch, Hilary C. Olson, Bob F. Perkins
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The Cenomanian-Turonian Eagle Ford Group crops out along a broad belt in central and northern Texas. It has been suggested that this unit was a primary source for the 6 billion barrels of oil in East Texas Field. The unit was deposited during a major shoreline transgression. Although the unit may be viewed as representing a single “condensed” section, it displays considerable variability at several different scales.
This study is based on detailed descriptions and geochemical analyses of Eagle Ford strata at two outcrop localities in central Texas, West Bouldin Creek (Austin) and Midway Park (Waco). The lower portion (South Bosque Member) of the measured sections of Eagle Ford at both localities is dominated by dark shales with a massive character. On detailed examination, these shales exhibit little bioturbation and are well-laminated. A few bentonites are evident in this portion of the section but are not as significant as in the overlying section. Above this interval, the Eagle Ford changes character dramatically. The upper section (Bouldin Member) is composed of a series of interbedded carbonate resistive layers and recessive shales with numerous bentonites.
Geochemical analyses clearly establish differences in source rock quality between localities and within individual localities. If either locality was examined independently or as a representative sampling of the Eagle Ford there would be substantial differences in the estimated amounts of generated hydrocarbons as well as differences in the gas/oil ratios even if thermal maturity histories were considered comparable and differences in stratigraphic thickness were removed.
Three basic organ ic-lithologic facies were determined at the Waco locality, based on the stratigraphy observed, the amount of organic carbon present, and expected products (oil and/or gas). At the Austin locality, two organic-lithologic facies were observed. An attempt is made to place the Austin outcrop in a sequence stratigraphic framework, although correlation to the Waco outcrop is ambiguous.