ABSTRACT

The Cenomanian-Turonian Eagle Ford Group, the likely source of oil for the East Texas field, is exposed along a broad belt in Central and northeastern Texas. Detailed analysis of Eagle Ford outcrops in Central Texas reveals considerable lithologic variability at several different scales. A lower section is interpreted to represent transgressive, in part "condensed" deposits, whereas the overlying section is interpreted as a highstand deposit. This sequence stratigraphic framework is suggested by stratigraphic and geochemical evidence. The lower portion of the measured Eagle Ford Group outcrops contain dark shales with a blocky character. These shales exhibit minor 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 unit. Above this interval, the Eagle Ford changes character dramatically, exhibiting a series of interbedded carbonate flagstones, recessive shales, and numerous bentonites. Several bentonite layers appear to be continuous regionally, and compositional analyses suggest that they may be useful in stratigraphic correlation. Detailed organic geochemical analyses clearly differentiate the upper and lower Eagle Ford. The shales in the lower transgressive unit are organically enriched, exhibit higher generation potentials, and are more oil-prone than shales in the overlying regressive interval. Even within the oil-prone intervals there is considerable variation in geochemical attributes. This work raises concerns about uncritically equating "condensed sections" with source rock potential.