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Hiatuses in the Eagle Ford Unconventional Resource Play, Recognizing Them and Determining How They Form


Recent research in the Eagle Ford Group of west and south Texas indicates this unit contains prolonged (a few 100's of thousands to millions of years duration) hiatuses that may be regionally correlative. Many of these hiatuses are cryptic surfaces or zones that commonly have little evidence of physical erosion or non-deposition. However, distinct petrophysical, chemostratigraphic, and biostratigraphic breaks, as well as interpreted changes in the sequence stratigraphic stacking patterns (progradational to retrogradational) occur at these surfaces, suggesting that distinct gaps in the stratigraphic record can occur across these significant, but cryptic surfaces Throughout the study area, one such cryptic surface occurs within the Turonian. In multiple outcrops and cores of the Eagle Ford Group in west Texas the +2 to +4‰ positive stable carbon fluctuation associated with OAE2, that formed over 0.5 – 1 million years, near the Cenomanian-Turonian boundary is only partially recorded or is missing altogether. In outcrops of the Eagle Ford Group in Antonio and Lozier Canyons this hiatus is a sharp, planar surface with bioturbated skeletal wackestone and interbedded calcareous mudstone below overlain by burrow mottled skeletal wackestone-packstone with multiple intraclast horizons. Stable isotope chemostratigraphy and biostratigraphy of this outcrop indicates the OAE2 is only partially recorded here. In the Hot Springs section of the Eagle Ford/Boquillas in Big Bend National Park this hiatus occurs at a sharp surface marking a lithologic change from interbedded skeletal packstone and calcareous shale with numerous bentonite beds to thicker bedded skeletal packstone with less calcareous shale and no bentonite beds. Stable isotope chemostratigraphy and biostratigraphy indicate the entire OAE2 is missing at this section. Cores of the Eagle Ford Group in Karnes and Live Oak counties also do not record the OAE2 carbon isotope excursion, instead indicating a hiatus across the Cenomanian-Turonian boundary of about a million years. Our research to date suggests that these surfaces are sequence boundaries, that may be tectonically enhanced. However, no matter how these hiatuses form, understanding their presence is important for regional correlations, as well as explaining and predicting the distribution and thickness variations of the unconventional source rock reservoirs which they effect.